Patent Application
20060246555
The present invention relates to degradation of nitramines, and in particular to the degradation of explosives RDX, HMX, CL20 and tetryl through chemical, biochemical, or biological attack.
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are widely used explosives recognized to severely contaminate soil and groundwater. The large scale manufacturing, use, and disposal of the cyclic nitramine explosives RDX, HMX, and CL-20), commonly used for military and commercial purposes, has resulted in severe contamination of soil and groundwater. These chemicals are toxic, mutagenic and carcinogenic to human and other biological systems.
Presently, little information is available on the initial processes involved in degradation of cyclic nitramine explosive compounds. Such information would be beneficial to elucidating the degradation pathways of these compounds, and thus to develop and optimize processes for their degradation.
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is widely used for military and commercial purposes due to its superior explosive properties. The large scale manufacturing, use and disposal of RDX has resulted in elevated levels of soil and groundwater contamination. RDX and its degradation products are toxic, mutagenic and carcinogenic to humans and other biological systems. Hence there is an urgent need to remove it from the contaminated sites. Literature is available regarding the transformation of RDX under anaerobic and aerobic conditions, but little information is available regarding enzymatic degradation. Nitroreductases, largely from bacterial strains belonging to the enterobacteriacae family, may play a role in the anaerobic bacterial transformation of RDX. Until now, little has been known about the initial enzymatic processes involved in the transformafion of RDX and its products.
RDX is a highly oxidized compound. A successful initial enzymatic attack on its nitro group(s) destabilizes the inner C—N bonds and leads to ring cleavage and spontaneous decomposition, as has been demonstrated for its chemical decomposition. Methylenedinitramine has been considered a key RDX ring cleavage product in anaerobic sludge, and may have been produced by three different routes, for example initial hydrolytic cleavage of a C—N bond, hydroxylation at C-H bond and/or via formation of hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX). However, the nature of enzyme(s) involved is not clear.
It is therefore necessary to develop effective routes for degradation RDX, HMX, and other such explosives, and to optimize procedures for their degradation and mineralization, so as to optimize the bioremediation processes for the treatment of contaminated soil and groundwater.
It is an object of the present invention to provide processes for degradation of nitramines. These processes employ chemical, biochemical, or biological means to degrade or mineralize nitramine explosives.
According to the invention there is provided a process for degradation of cyclic nitramines comprising the steps of: attacking a first N—NO2 group in the presence of water resulting in denitration of the first N—NO2 group; and subsequently effecting ring cleavage. Optionally, a second N—NO2 group may be attacked after denitration of the first N—NO2 group, resulting in a second denitration prior to effecting ring cleavage. The nitramines may be monocyclic or polycyclic nitramines.
The invention also relates to a process to detect decomposition of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) comprising detection of 4-nitro-2,4-diaza-butanal (4-NDAB) as a decomposition product.
The invention further relates to a process for preventing sequential reduction of cyclic nitramine explosives to nitroso-derivatives comprising the steps of: attacking a first N—NO2 group in the presence of water resulting in denitration of the first N—NO2 group; and subsequently effecting ring cleavage.
Further, according to the invention there is provided a process for degradation of cyclic nitramines comprising the steps of: effecting α-hydroxylation of a —CH2 bond to form unstable carbinol; and subsequently effecting ring cleavage. The α-hydroxylation may be effected by exposure to diaphorase. Subsequently, this reaction may be followed by reduction of an —NO2 group to —NO, and denitration prior to ring cleavage.
The invention further relates to a process for preventing sequential reduction of cyclic nitramine explosives to nitroso-derivatives comprising the steps of α-hydroxylation of a —CH2 bond to form unstable carbinol; and subsequently effecting ring cleavage.
The invention additionally provides a process for degradation of 4-nitro-2,4-diazabutanal comprising exposure to an attack selected from the group consisting of anaerobic microbial attack, anaerobic enzymatic conditions, and chemical hydrolysis. In a preferred embodiment said attack comprises an attack selected from the group consisting of exposure to one or more of diaphorase, cytochrome P450, xanthine oxidase and nitrate reductase; exposure to a microbe selected from the group consisting of Clostridium kluyveri, Rhodococcus sp., and Aspergillus niger; and alkaline hydrolysis.
This invention has environmental applications for the optimization and control of treatment processes, whether chemical, biochemical, or biological means, developed for the degradation of explosive compounds. Advantageously, any institute, government agency, or private company that is concerned about the contamination of soil, subsurface soil, sediment and ground-water with toxic and/or mutagenic explosive compounds will benefit from this invention.
Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures.
The invention provides chemical, biochemical, and biological processes for RDX, HMX and CL-20 and tetryl transformation, leading to complete mineralization of these explosives. The invention involves either chemical processes which may be chemical, for example photolysis or alkaline hydrolysis, biochemical, for example using specific enzymes such as diaphorase, cytochrome P450, xanthine oxidase, nitrate reductase, and nitrate oxidoreductase or may be biological, for example using specific microbes intermixed with a sample containing the nitramines explosives. For example, Clostridium kluyveri, Rhodococcus sp. (various strains, including DN22, A-BRI, and 11Y), and Aspergillus niger may be used.
According to the invention, processes are used on cyclic nitramine explosives (e.g., RDX, HMX, and CL-20) to stop the explosives from undergoing sequential reduction to the corresponding di-nitroso, tri-nitroso, and in the case of HMX, tetra-nitroso derivatives. An initial denitration step (enzymatic, microbial or chemical) on RDX and HMX decreases or eliminates sequential reduction of the nitro groups to the corresponding nitroso derivatives in this family of cyclic nitramine explosives. This step leads to spontaneous decomposition in water or wet soil. The conditions required to selectively facilitate the desired decomposition reactions can be created by varying parameters including: types of enzyme used, the physiological conditions of the microbes employed, the chemical reagents utilized, photodegradation conditions, and the redox potential of the system.
The invention allows for custom design of remediation strategies revolving around the selected method of initial attack to yield a rapid, efficient, and spontaneous decomposition of RDX and HMX in water or in wet soils. The process of RDX and HMX transformation according to the invention shows that the intermediate metabolites and end products of degradation will vary depending upon the nature of initial attack on these molecules.
The invention further relates to a system for preventing the sequential reduction of these explosives to their nitroso-derivatives, but instead selectively promotes a mechanism of initial denitration attack that rapidly, efficiently and spontaneously produces a well defined set of harmless end products. Advantageously, this results in safe conversion of these highly toxic cyclic nitramines to gaseous products.
The invention allows for degradation of 4-nitro-2,4-diazabutanal by exposure to an attack such as anaerobic microbial or enzymatic attack, or by chemical hydrolysis. This could be accomplished, for example, by moving a reacted mixture of anaerobic sludge to conditions used to degrade RDX in sludge. Alternatively, the reacted mixture can be hydrolysed under relatively high pH conditions.
Processes are described herein for selective degradation of nitramine explosives. The invention is based on the discovery that nitramine explosives, for example, RDX, HMX, CL20 and tetryl can be both enzymatically and chemically attacked at several bonds. The outcome of the attack may depend on the initial point (bond) of attack on these molecules. This observation was used in this invention to develop chemical, biochemical, and biological reaction on these compounds to obtain effective mineralization to nitrous oxide and carbon dioxide. The invention has important implications in the field of remediation of soil and water contaminated with these explosives.
Cyclic nitramines have distinctive bonds (functional groups) for possible chemical and enzymatic attack. These bonds include —NO2, —N—NO2, —CH2—N and -CH2. According to the invention, among the possible sites of attack, two particular reactions are responsible for the rapid and effective ring cleavage, which in the presence of water can lead to effective hydrolytic decomposition and mineralization. The invention allows control of such reactions, to prevent functional groups from undergoing sequential nitrosation (reduction of —NO2 to —NO), but to instead enhance attack on either the —N—NO2 (denitration followed by hydration) or the CH2 (α-hydroxylation) to form and unstable carbinol prior to cleavage and decomposition. These latter reactions are important for effective mineralization of nitramine based explosives. The invention allows the initial reaction to be conducted under both anaerobic and aerobic conditions in soil and in water biochemically (enzymatically), biologically (microbially) and chemically (photochemically or with alkaline hydrolysis). As far as low water solubility is concerned, cosolvents may be added to the reaction conditions to enhance solubitity and to catalyze degradation. Such co-solvents include additives such as cyclodextrins to enhance solubility and also to catalyze degradation.
As used herein, the term “chemical” means any chemical reaction, whether caused by photolytic, hydrolytic, inorganic chemical catalysis or other chemical reactions. The term “biochemical” as used herein refers to a reaction caused by any biochemical mechanism, for example thorough the use of an enzyme, enzyme equivalent, or an organic catalyst. The term “biological” as used herein refers to a reaction caused by the use of a biological entity, such as by a microbe or other lower life form, or a portion thereof.
The invention is a model system for understanding the mechanism of initial enzymatic attack on cyclic nitramines such as RDX, and its subsequent degradation by an enzyme.
Nitrate reductases are ubiquitous enzymes in diverse groups of microorganisms, especially denitrifying bacteria, and their physiological role is to reduce nitrate to nitrite via a two electron transfer. Hence there is a great possibility that a nitrate reductase, a nitroreductase, or a similar enzyme may be one of the key enzymes responsible for RDX transformation in anaerobic sludge. No information existed until now regarding nitrate reductase reaction with cyclic nitramine compounds such as RDX.
In this invention, a variety of enzymes may be used. For example, a nitrate reductase (EC 1.6.6.2) from a fungus Aspergillus niger may be used to transform RDX under anaerobic conditions. Other similar nitrate reductases or nitroreductases from other microorganisms would also work.
The invention involves the detailed enzymatic mechanisms for transformation of RDX, HMX and CL-20, leading to their substantially complete mineralization, based on results obtained through experiments involving microbial enzymes, such as nitrate reductase. Reactions with cyclic nitramine explosives (e.g., RDX, Mx, and CL-20) may be carried out to prevent the compounds from undergoing sequential reduction to the corresponding di-, tri-, and in the case of HMX, tetra-nitroso derivatives. These derivatives are extremely toxic and persist in the environment for longer periods.
Biotransformation of one nitro group is sufficient for ring cleavage of cyclic nitramine compounds and ensues in subsequent decomposition. Experimental conditions required to selectively facilitate the desired decomposition reactions may be created by varying parameters including: the type(s) of enzyme used, the physiological conditions of the microbes employed and the redox potential of the systems.
The present invention serves to safely convert these highly toxic cyclic nitramines to harmless products and is useful for designing remediation strategies revolving around the selected method of initial attack to yield a rapid, efficient, and spontaneous decomposition of RDX and HMX in water or in wet soils.
The detailed mechanisms of RDX and HMX transformation show that the intermediate metabolites and end products of the degradation processes vary depending upon the nature of initial attack on these molecules. The invention encompasses a system to prevent the sequential reduction of cyclic nitramines to their highly toxic, recalcitrant, multi-nitroso derivatives. Instead, the route of initial attack according to the invention produces harmless end products with relative rapidity, efficiency and spontaneity.
The processes according to the invention may be used to degrade cyclic nitramine explosives present in a natural environment, such as in water or soil. In order to conduct such remediation activities, soil samples or water samples can be removed from the site for treatment, or they may be treated on site. In the case of treating soil, an area may be flooded, to form a soil slurry or a flooded biopile. Further, for long-term remediation projects, plants may be introduced into a contaminated area in which microbial species have been added to root nodules, or in which modified bacteria (such as Rhizobium species) containing appropriate enzymes to conduct the inventive processes has been added. In this way, phytoremediation of a region may be effected by enzymatic conversion of cyclic nitramines in the soil by the root tips of such plants. All such remediation strategies fall within the scope of the invention.
In this Example, biotransformation of RDX with nitrate reductase derived from Aspergillus niger is exemplified.
Chemicals. Commercial grade RDX (purity >99%) was provided by Defence Research Establishment, Valcartier, Quebec, Canada MNX of either 95% or 99% purity was used. For the experiment describing MNX biotransformation with nitrate reductase, MNX (>99% purity) was obtained from SRI International (Menlo Park, Calif.). Aspergillus niger NAD(P)H: Nitrate oxidoreductase (EC 1.6.6.2), NADPH and formaldehyde were obtained from Sigma Chemicals, Canada. Methylenedinitramine was obtained from the rare chemical department of Aldrich, Oakville, ON, Canada. Hydrazine and formamide were purchased from Aldrich, Canada. Standard nitrous oxide (N2O, 980 ppm by mole) was obtained from Scott Specialty Gases, Sarnia, ON, Canada.
Enzyme preparation and assays. Lyophilized enzyme is suspended in potassium phosphate buffer (50 mM) at pH 7.0 and washed thrice with 2.5 ml of buffer using Biomax-5K membrane centrifuge filter units (Sigma chemicals). The washed enzyme is then suspended in 0.5 ml of buffer and the protein concentration is measured by bicinchoninic acid (BCA) kit (Sigma chemicals) using bovine serum albumin as standard. The native enzyme activity is estimated spectrophotometrically at 340 nm as the rate of oxidation of NADPH in the presence of nitrate.
Nitrate reductase catalyzed RDX transformation is performed under anaerobic conditions in 6 ml glass vials containing one ml of reaction mixture sealed under an atmosphere of argon. The above assay mixture contained RDX (or MNX) (100 μM), NADPH (300 μM) and one mg enzyme (equivalent to 0.25 native units) in potassium phosphate buffer (50 mM) at pH 7.0 and 30° C. Three different controls are prepared as follows: 1) RDX and NADPH in buffer (i.e. without enzyme); 2) RDX and enzyme in buffer (i.e. in the absence of NADPH); and, 3) RDX only in buffer. The samples from liquid and gas phases are withdrawn periodically to analyze for RDX and transformation products as described below. The enzymatic transformation rate of RDX is expressed as μmoles RDX transformed h−1mg−1protein.
To study the fate of formamide and hydrazine in the reaction mixture, each chemical (50 μM) is incubated separately with nitrate reductase and NADPH for one hour at pH 7.0 and 30° C. under the same conditions as described above. The residual compounds and their products are analyzed as stated below.
Analytical procedures. RDX is analyzed by a reversed phase-high pressure liquid chromatograph (RP-HPLC) connected to photodiode array (PDA) detector (λ254 nm). Samples (50 μl) are injected into a Supelcosil LC-CN column (4.6 mm ID×25 cm) (Supelco, Bellafonte, Pa.) and the analytes are eluted using methanol/water gradient at a flow rate of 1.5 ml/min. The MNX and methylenedinitramine are analyzed by a Micromass bench-top single quadrupole mass detector attached to a Hewlett Packard 1100 series HPLC system equipped with a photodiode array detector. Ionization is carried out in a negative electrospray ionization mode ES(−). Their identities are confirmed by comparison with a reference standard.
Hydrazine is analyzed by a HPLC system equipped with a Waters model 600 pump (Waters Associates, Milford, MA), a 717 plus autosampler, a Hamilton RPX-X200 analytical cation exchange column (250 mm×4.1 mm), a Waters post column reaction module with a Waters reagent manager pump, a Waters model 464 electrochemical detector with a gold-working electrode and a base resistant Ag/AgCl reference electrode.
The eluent is 6% v/v acetonitrile in 0.005 M KH2PO4 solution in deionized water. The eluent is degassed by continuous helium sparging before and during use. The post-column reaction solution is 0.1 M NaOH solution in deionized water. The operating parameters for the system are: eluent flow rate, 1.0 ml/min.; temperature, 30° C.; injection volume, 25 μl; flow-rate of post-column reaction solution, 250 μl/min.; working electrode cleaning potential, 500 mV (0.333 sec); pretreatment potential, −350 mV (0.333 sec) and measuring potential, 100 mV in DC mode.
Formamide is analyzed by a Micromass bench-top single quadrupole mass detector attached to an HPLC system equipped with a photodiode array detector and a synergi polar-RP column (4.6 mm ID×15 cm) (Phenomenex, Torrance, Calif.) at 25° C. The solvent system is a methanol/water gradient (10-90% v/v) at a flow rate of 0.75 ml/min. For mass analysis, the ionization is carried out in a positive electrospray ionization mode ES+ producing mainly the [M+H] mass ions. The electrospray probe tip potential is set at 3.5 kV with a cone voltage of 35 V at an ion source temperature of 150° C.
A. niger nitrate reductase catalyzes the transformation of RDX in the presence of NADPH as electron donor. One embodiment for the transformation is under anaerobic conditions at pH 7.0 and 30° C. (data not shown). The disappearance of RDX is accompanied by the formation of MNX and the ring cleavage product methylenedinitramine (
In control experiments, no degradation is observed except in control number 1 which contained NADPH and RDX (see experimental section) where a negligible RDX loss (<3% of total RDX degradation) is observed in one hour of reaction time.
To further prove that RDX is first reduced to MNX and the latter undergoes further reduction to finally produce methylene dinitrarnine, (O2NNH)2CH2, the standard MNX is incubated with nitrate reductase under similar reaction conditions as those of RDX biotransformation (see above). As a result, we observed the formation of MNX forms (O2NNH)2CH2MNX (
Methylenedinitramine exhibits a characteristic mass data at 61 and 135 Da, representing NHNO2 group and the [M−H] mass ion of the parent compound.
Interestingly, in
Stoichiometry and Pathway. In the time course study, methylenedinitramine is observed as the prime intermediate and N2O and HCHO are the major end products (
Table 1 illustrates carbon and nitrogen mass balance and stoichiometry of metabolites produced during RDX transformation catalyzed by nitrate reductase from Aspergillus niger after 150 minutes of reaction. Table 1 shows that 86 μmoles of RDX produces 148 μmoles N2O and 10 μmoles methylenedinitramine (equivalent to 20 μmoles N2O). This demonstrates that for each reacted molecule of RDX only one molecule of methylenedinitramine, containing two N—NO2 groups, is produced.
*Calculated from the total carbon and nitrogen mass in the transformed RDX (86 μmole);
NA, not applicable;
Standard deviations are within 7% of the mean absolute value (n = 3).
A part of the remaining N—NO2 group in RDX leads to the formation of ammonium ion (88 μmoles). The total nitrogen mass balance is 82.10% and is distributed between N2O (57.37%), methylenedinitramine (7.75%), and ammonium ion (16.98%) (Table 1). The carbon mass balance is 96.13% and is distributed between HCHO (92.27%) and methylenedinitramine (3.87%) which indicates that all the recovered carbon is in the form of HCHO. Hence, one RDX molecule produces three HCHO molecules (
It is difficult to quantify MNX since it exists only in trace amount throughout the RDX reaction time course. It is believed that only MNX (with two N—NO2) produces methylenedinitramine (with two N—NO2) and that neither DNX (with only one N—NO2) nor TNX (with no N—NO2) can act as a precursor to methylenedinitramine. In addition, as aforementioned, neither of the two compounds is detected. Finally, for each reacted RDX molecule, roughly three molecules of NADPH, representing 6 electrons, are consumed (Table 1).
Based on this stoichiometry, we propose a degradation pathway for RDX as shown in
The latter, being a form of a-hydroxylamine should also decompose in water since it is not possible to detect it. The decomposition of VI is presumed to proceed by two possible routes to produce HCHO and NH4+ i.e. via formation of formamide and/or hydrazine (
82.10% of the total nitrogen mass content of RDX is accounted for, and it is believed that the missing 18% nitrogen mass exists in the form of an unidentified compound that might react with other components, as aforementioned. In particular, hydrazine is a very reactive compound and it is known to polymerize with HCHO. DNX and TNX are excluded from the RDX biotransformation pathway (
In contrast, the stoichiometric formation of methylenedinitramine (one μmole of RDX produces one μmole of methylenedinitramine) confirms that ring cleavage proceeds via MNX (
Role of Nitrate Reductase in the Initial Transformation of RDX. Eukaryotic nitrate reductases (EC 1.6.6.1-3) contain three redox cofactors i.e. FAD, heme-Fe and molybdopterin (Mo-MPT). FAD is the oxidation site for NAD(P)H and the subsequent electron transfer proceeds through a heme-Fe center to the molybdopterin which is a reduction site for nitrate. During nitrate reduction, the molybdenum (Mo) present in the Mo-MPT cofactor frequently changes its oxidation state from oxidized (MoVI) to reduced (MoIV) and vice-versa by receiving two electrons from the heme-Fe center and transferring them to nitrate. Part of the invention resides in showing that nitrate is a competitive inhibitor of RDX biotransformation, as evidenced by the Lineweaver-Burk's plots in the presence of increasing concentration of nitrate in the reaction mixture (
The transformation of 86 μmoles of RDX is accompanied by the consumption of 280 μmoles of NADPH. This indicates that roughly three NADPH molecules are consumed during transformation of one RDX molecule under anaerobic conditions (Table 1). Therefore, a total of six electrons (or 3 NADPH) participate in the complete transformation of RDX. Since RDX requires an obligatory two electrons to be transformed to MNX, it is concluded that nitrate reductase catalyzes two-electron transfer to the RDX in the first reduction step to produce MNX. Thereafter, the remaining two pairs of electrons are presumably involved in the sequential reduction of MNX to RDX-NHOH (I) (1 NADPH) and RDX-NH2 (II) (1 NADPH) (
However, it is also possible that RDX-NHOH, being unstable, undergoes ring cleavage, using the last pair of electrons (1 NADPH) after the cleavage.
The transformation of cyclic nitramine compound(s) with nitrate reductase(s) has not yet been characterized elsewhere in the literature.
The results clearly demonstrate that MNX is the first reduced product of RDX formed by a two-electron reduction catalyzed by nitrate reductase. MNX undergoes further reduction followed by the ring cleavage and subsequent decomposition to produce HCHO, NH4+, and N2O.
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) can be biotransformed by anaerobic sludge via three different routes, 1: direct ring cleavage via a-hydroxylation of a —CH2 group, 2: reduction of one of the —NO2 groups to —NO, and 3: N-denitration prior to ring cleavage. The present Example describes biotransformation of RDX via route 3 by a diaphorase (EC 1.8.1.4) from Clostridium kluyveri using NADH as electron donor. The removal of RDX was accompanied by the formation and accumulation of nitrite ion (NO2—), formaldehyde (HCHO), ammonium (NH4+) and nitrous oxide (N2O). None of the RDX-nitroso products were detected. The ring cleavage product methylenedinitramine was detected as the transient intermediate. Product stoichiometry showed that each reacted RDX molecule produced one nitrite ion and the product distribution gave a carbon (C) and nitrogen (N) mass balance of 91% and 92%, respectively, supporting the occurrence of a mono-denitration step prior to the ring cleavage and decomposition. Severe oxygen mediated inhibition (92% inhibition) of RDX biotransformation and superoxide dismutase-sensitive cytochrome c reduction indicated the potential involvement of an anion radical RDX●− prior to denitration. A comparative study between native- and apo- enzymes showed the possible involvement of flavin mononucleotide (FMN) in catalyzing the transfer of a redox equivalent (e/H+) from NADH to RDX in order to produce RDX●− responsible for secondary decomposition.
Introduction. It has been recently reported that RDX can be easily degraded with anaerobic sludge to produce HCHO, CO2, N2O and NH3 (Hawari et al., 2000), but there was no elaboration on the enzymes responsible for initiating RDX degradation. Kitts et al. (2000) have reported the reduction of RDX by a type 1 (2 electron transfer process) oxygen-insensitive nitroreductase without providing details on either products or degradation pathways. Whereas Shah and Spain et al. (1996) reported N-denitrohydrogenation of tetryl by a ferredoxin-NADP oxidoreductase (EC 1.18.1.2) using NADPH as electron donor. Example 1 shows that RDX can be transformed by a nitrate reductase from Aspergillus niger to produce MNX via a 2 e- transfer process prior to its decomposition to HCHO and N2O.
In this Example, a flavoenzyme, diaphorase, was selected from an anaerobic bacterium Clostridium kluyveri to biotransform RDX. Clostridium species are common inhabitants of anaerobic environments and thus it is expected that diaphorase is one of the key enzymes responsible for RDX degradation by the anaerobic sludge. Previously, Clostridium species have been known to biotransform RDX and nitroaromatic explosive, 2,4,6-trinitrotoluene (TNT) (Regan et al., 1994). A carbon monoxide dehydrogenase from Clostridium thermoacticum reportedly biotransformed TNT (Huang et al., 2000). However, no detailed study has been reported so far in regard to biotransformation of RDX by a purified enzyme from Clostridium species. The diaphorase (EC 1.8.1.4) from Clostridium kluyveri is a 24 kDa flavoenzyme containing 1 mole of FMN per mole of enzyme, and it catalyzes the pyridine nucleotide dependent reduction of dyes. This enzyme catalyzed an oxygen-sensitive 1 e-reduction of nitrofluorenes (Ritter et al., 1998). The present Example illustrates the metabolites and the mechanism of RDX biotransformation catalyzed by the diaphorase (EC 1.8.1.4) from Clostridium kluyveri. This process is useful for achieving optimal mineralization of this energetic chemical under field conditions.
Chemicals. Commercial grade RDX (purity>99%) was provided by Defense Research and Development, Quebec, Canada. NADH, flavin monononucleotide (FMN). 2,6-dichlorophenol-indophenol (DPIP), superoxide dismutase (SOD, EC 1.15.1.1, from bovine erythrocytes), cytochrome c (from horse heart, MW 12,384 Da) and formaldehyde were purchased from Sigma Chemicals, Canada. Methylenedinitramine was obtained from the rare chemical department of Aldrich, Canada. Standard nitrous oxide (N2O, 980 ppm by mole) was obtained from Scott specialty gases, Sarnia, ON, Canada. All other chemicals were of highest purity grade available.
Enzyme preparation. Diaphorase (BC 1.8.1.4) from Clostridium kluyveri was obtained from Sigma Chemicals, Canada, as a lyophilized powder. The enzyme was suspended in 50 mM potassium phosphate buffer (pH 7.0) and filtered through a Biomax-SK membrane (Sigma Chemicals) before resuspension in the same buffer. The protein concentration was measured by bicinchoninic acid (BCA) kit (Sigma Chemicals) using bovine serum albumin as standard. The native enzyme activity was estimated (as per company guidelines) spectrophotometrically at 340 nn as the rate of oxidation of NADH using 2,6-dichlorophenol-indophenol as the electron acceptor.
Biotransformation Assays. RDX biotransformation assays with diaphorase were performed in 6 ml glass vials under anaerobic conditions (with an atmosphere of argon) at pH 7.0 and 27° C. Each vial contained RDX (100 μM), NADH (150 μM) and 50 μl of the enzyme (0.5 native units) in a final volume of one ml potassium phosphate buffer (50 mM, pH 7.0). Three different controls were prepared. The first control contained RDX, NADH and buffer without enzyme; the second contained RDX, enzyme and buffer without NADH, and the third contained only RDX and buffer. The reaction time was one hour unless stated otherwise. Samples from the liquid and gas phases in the vials were withdrawn periodically to analyze for RDX and the transformed products as described below. RDX transformation activity of enzyme was expressed as μmoles of RDX transformed h−1mg−1 protein.
Inhibition with Oxygen. The effect of oxygen on RDX transformation activity of diaphorase was studied by performing the assays under aerobic and anaerobic conditions at pH 7.0 and 27° C. In another experiment, we determined the role of O2 transformation to superoxide radical anion (O2●−) in the inhibition of RDX biotransformation. This was done by incubating the RDX with diaphorase in the presence of NADH, cytochrome c (100 μM and superoxide dismutase (SOD) (150 μg/ml) as described previously (Anusevicius et al., 1998). The absorption of reduced cytochrome c was monitored at 550 nm.
Preparation ofapoenzyme (Deflavo Enzyme) and its Reconstitution. The deflavo form of diaphorase was prepared as described before (Madigan et al., 1993 and Tedeschi et al., 1995) with some modifications. The holoenzyme was dialyzed for 48 hours at 4° C. against a dialysis solution composed of 100 mM potassium phosphate buffer (pH 7.0), EDTA (0.1 mM), glycerol (10% v/v) and KBr (3 M). The dialysis buffer was changed every 6 hours. The reconstitution of apoenzyme was carried out in ice cold potassium phosphate buffer (pH 7.0) in the presence of glycerol (10% v/v). FMN was added at variable concentrations (0-250 μM) to the apoenzyme preparation. The unbound FMN was removed by washing the enzyme with the same buffer using Biomax-5K membrane centrifuge filter units. The enzyme activity was assayed after each addition of FMN to the apoenzyme in order to determine the concentration-dependent reconstitution of apoenzyme by the FMN.
Analytical Procedures. RDX was analyzed by a reversed phase-high performance liquid chromatograph (RP-HPLC) connected to photodiode array (PDA) detector (λ254 nm). Samples (50 μl) were injected into a Supelcosil LC-CN column (4.6 mm ID×25 cm) (Supelco, Bellafonte, Pa.) and the analytes were eluted using methanol/water gradient at a flow rate of 1.5 ml/min (Hawari et al., 2000). Methylenedinitramine was analyzed by a Micromass™ bench-top single quadrupole mass detector attached to a Hewlett Packard 1100 series HPLC system equipped with a photodiode array detector. Ionization was carried out in a negative electrospray ionization mode ES(−) producing mainly the deprotonated molecular mass ions [M−H]. Methylenedinitramine was detected as its deprotonated molecular mass ion [M−H] at 135 Da and confirmed by comparison with a reference standard (Halasz et al., 2002). Nitroso-derivatives of RDX (i.e. hexahydro-1-nitroso-3,5-dinitro-1,3,5,-triazine, MNX; hexahydro-1,3-dinitroso-5-nitro- 1,3,5-triazine, DNX and hexahydro-1,3,5-trinitroso-1,3,5-triazine, TNX), nitrite (NO2), ammonium (NH4+), formaldehyde (HCHO) and nitrous oxide (N2O) were analyzed as reported (Hawari et al., 2000 and Halasz et al., 2002).
Results and Discussion
RDX Biotransformation and Product Identification. Clostridium kluyveri diaphorase was found to transform RDX at pH 7.0 and 27° C. under anaerobic conditions using NADH as the electron donor.
RDX transformation with diaphorase showed the production bf methylenedinitramine as a key ring cleavage metabolite at a retention time of 4.2 minutes. None of the nitroso-RDX intermediates, particularly hexahydro-1-nitroso-3,5-diflitro-1,3,5-triazine (MNX), were detected, although such initial RDX reduced products are frequently observed during RDX degradation by anaerobic sludge. No biotransformation of RDX was observed in the control experiments that did not contain enzyme and/or NADH. The disappearance of RDX and NADH were accompanied by the formation and accumulation of HCHO, N2O and NO2—.
Inhibition by Oxygen. It was found that the presence of oxygen (O2) inhibited (92%) the transformation of RDX, thereby supporting the involvement of an oxygen-sensitive step during the initial enzymatic attack on RDX. A negligible oxidation of NADH under aerobic conditions was observed within the assay time of one hour, indicating that O2 inhibition of RDX biotransformation was not due to the depletion of NADH. It was observed that diaphorase did not catalyze the electron transfer from NADH to O2 and this observation was similar to the one reported by Kaplan et al. (1969) using the same enzyme. Based on these observations, inhibition of RDX transformation by O2 is most probably due to quenching of an unpaired electron from the RDX anion radical (RDX●−) by O2. Such a reaction would revert RDX●−, an initial transformation product, to its original structure RDX producing, instead, O2 and thus preventing the molecule from ftirther decomposition as was the case with tetryl (Shah et al., 1996 and Anusevicius et al., 1998). RDX●− appears to be formed via an initial electron transfer process catalyzed by diaphorase. Ritter et al. (1998) also reported the reduction of a nitro group in nitrofluorene compounds via an O2-sensitive one electron transfer process catalyzed by a diaphorase from Clostridium kluyveri. In order to test the hypothesis that O2 inhibited the transformation of RDX by quenching an electron from RDX, RDX was incubated with diaphorase in the presence of NADH, superoxide dismutase (SOD) and cytochrome c. It was observed that SOD inhibited the reduction of cytochrome c by 37% thus providing evidence for the potential involvement of redox cycling between RDX●− and RDX through which O2 was converted to O2●−. Control experiments without diaphorase showed that ROX can neither autooxidized nor can it reduce cytochrome c.
C— and N— Mass-Balance and Stoichiometry of the Reaction. Table 2 illustrates carbon and nitrogen mass balance and stoichiometry of reactants consumed and metabolites produced during RDX transformation catalyzed by a diaphorase from Clostridium kluyveri at pH 7.0 and 27° C.
*Calculated from the total carbon and nitrogen mass in the transformed RDX (78 μmole);
Initial RDX and NADH concentrations were 100 and 150 μmoles, respectively;
NA, not applicable;
Standard deviations are within 6% of the mean absolute values (n = 3).
The reaction stoichiometry (Table 2) indicates that one nitrite molecule was produced per reacted RDX molecule. On the other hand, 78 μmoles of the reacted RDX produced 129 μmoles N2O and 9.2 μmoles of methylenedinitramine. The latter is unstable in water at pH 7.0 and 27° C. and decomposes quantitatively to produce stoichiometric amounts of N2O (two molecules) and HCHO (one molecule). Considering that all of the N2O produced during the reaction comes from the decomposition of methylenedinitramine, it was concluded that one molecule of methylenedinitramine was produced for each reacted RDX molecule. The total nitrogen mass recovery was 91% and was distributed as N2O (55%), nitrite (15%), ammonium (13%) and methylenedinitramine (8%). The N mass balance data revealed that of the six nitrogen atoms in one RDX molecule, four atoms were finally recovered as nitrous oxide (2 N2O molecules) whereas the fifth and sixth atoms were present in nitrite (NO2●) and ammonium (NH4+), respectively.
The total carbon mass recovery was 92% and it was distributed as formaldehyde (88%) and methylenedinitramine (4%). The latter, as mentioned above, decomposes quantitatively to HCHO and N2O, indicating that all the three carbon atoms in RDX were finally recovered in the form of HCHO.
Numerically, 1.4 μmoles of NADH were consumed in order to biotransform 1.0 μmole of RDX (Table 2). The stoichiometry of NADH consumption vs RDX transformation supports that 1 molecule of NADH (2e−/2H+) was utilized per RDX molecule for its initial denitration. The remaining 0.4 μmoles were either utilized by other transient intermediates and/or end-products following the ring cleavage or bound to protein and thus prevented their detection. The other possibility is that some NADH may be consumed during the futile redox cycling in the presence of traces of oxygen in the reaction medium.
Mechanism of Biotransformation. Without being limiting to the invention in any way, the following mechanism of biotransformation is now put forth for this Example. Based on the oxygen sensitivity, product identification, C— and N— mass-balance, and stoichiometry of the reaction, RDX appears to undergo a mono-denitration process which was responsible. and sufficient for the ring cleavage and secondary decomposition. The absence of nitroso products such as hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX) and/or hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX) also supported the denitration of RDX as the main reaction step responsible for the ring cleavage.
The oxygen inhibition (92%) of RDX transformation and SOD-sensitive cytochrome c reduction experiments suggests that the transfer of a net two redox equivalents (2 e/H+) to the RDX, as determined by stoichiometry of NADH, occurred in a stepwise manner. First step produce RDX●− whose spontaneous denitration would generate nitrite and the free radical RDX. In the second step, RDX● either undergoes H— abstraction to form the amine derivative by acquiring a second redox equivalent (e−/H+) or can lose a hydrogen atom to form the cyclohexene derivative.
The stoichiometry of NADH consumption favors the formation of amine derivative (III). The formation of amine derivative (III) via a H-abstraction by the RDX● (II) during photolysis of RDX has been reported previously (Peyton et al., 1999), but no similar biological reactions are known. However, Anusevicius et al. (1998) reported two single e-transfer steps during N-denitration of tetryl catalyzed by a mammalian DT-diaphorase (EC 1.6.99.2).
The proposed pathway in
Involvement of FMN in RDX Transformation. FMN is a redox cofactor of Clostridium kluyveri diaphorase and it is present in the ratio of 1 mole per mole of enzyme (Kaplan et al., 1969). In the present Example, the deflavo enzyme (apoenzyme) lost 87.5% of the RDX transformation activity, suggesting that FMN is an active redox center that possibly mediates the transfer of electrons from NADH to RDX. The remaining 12.5% enzymatic activity may be due to the incomplete removal of FMN from the native enzyme. It was found that when the deflavo enzyme was reconstituted with increasing concentration of FMN, there was a gradual increase in RDX transformation activity up to a maximum of 85% at the FMN concentration of 200 μM. Further, it was found that a commercially available FMN in free form can transform RDX using NADH as electron donor. However, the transformation rate of the free FMN was comparatively lower than the diaphorase-bound FMN. These results indicate that FMN plays an important role in the reduction of RDX by acting as a possible site of electron transfer reaction. In this context, Khan et al. (2002), showed that reduced FMN in pentaerythritol tetranitrate reductase caused the reduction of nitroester explosives (glycerol trinitrate and pentaerythritol tetranitrate) and nitroaromatic explosives (TNT and picric acid). Other reports also showed that the flavin moieties (FAD and FMN) in the flavoenzymes play a key role in substrate reduction Madigan et al., 1993; Lindsay et al., 2000; Xia et al., 2001).
In conclusion, the present Example shows that diaphorase catalyzed N-denitration of RDX is an oxygen-sensitive reaction which possibly requires a net two redox equivalents. Based on this Example, the first redox equivalent appears to lead to the transformation of RDX to RDX●− whose subsequent denitration leads to RDX●. The second redox equivalent possibly causes hydrogenation of RDX leading to the formation of corresponding amine (III). The latter undergoes spontaneous hydrolytic decomposition in water to produce the transient intermediates and end-products. FMN seems to play a key role in transferring the redox equivalents from NADH (donor) to RDX (acceptor).
One of the significant points of the present Example is that Clostridium kluyveri diaphorase catalyzed the RDX transformation at the expense of a net two redox equivalents per RDX molecule which is clearly different and more economical than the earlier reported conventional RDX biotransformation route: via MNX formation (Halasz et al., 2002; Kitts et al., 2000).
Rhodococcus sp., of a variety of strains, degrades hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) (1) aerobically via initial denitration followed by ring cleavage. Using UL 14C-[RDX] and ring labeled 15N-[RDX] approximately 30% of the energetic chemical mineralized (one C atom) and 64% converted to a dead end product that was tentatively identified as 4-nitro-2,4-diaza-butanal (OHCHNCH2NHNO2). To have further insight into the role of initial denitration on RDX decomposition, the energetic chemical was photolyzed at 350 nm and pH 5.5 and monitored the reaction using a combination of analytical techniques. GC/MS-PCI showed a product with a [M+H] at 176 Da matching a molecular formula of C3H5N5O4 that was tentatively identified as the initially denitrated RDX product hexahydro-3,5-dinitro-1,3,5-triazacyclohex-1-ene (II, shown in
Introduction. Both RDX and HMX are toxic and mutagenic and have the tendency to persist in contaminated environment. Rhodococcus strain DN22 can degrade RDX effectively. Other strains show effective degradation, such as Strain A (BRI, Montreal, Canada), strain 11Y (Dr. N. Bruce, Cambridge University, UK). As these various strains of Rhodococcus work under similar conditions as DN22, and yield the same or a similar product distribution, kinetics and stoichiometry as DN22, only strain DN22 is discussed herein in detail. One of skill in the art could easily determine the efficacy of other strains. In this Example, 30% mineralization was achieved, accounting for only one of the three C atoms in RDX leaving the remaining two carbons in a dead end product that was identified as 4-nitro-2,4-diaza-butanal, O2NNHCH2NHCHO (
Since initial enzymatic denitration of RDX was found sufficient to cause spontaneous decomposition, photodenitration of the compound in water should also lead to spontaneous decomposition. Denitration of RDX by photolysis is presumed to be more rapid than enzymatic denitration and therefore might lead to the formation of sufficient amounts of intermediate products to allow detection.
Several studies report the successful photodegradation of RDX via the homolysis of the N—NO2 bond, under several oxidation conditions. In these cases, photodecomposition of RDX produces common secondary products including HCHO, HCOOH, NH2CHO, NH3, NO2—, NO3—, and N2O. Interestingly, most of these secondary photoproducts resembled those observed during aerobic degradation of RDX with strain DN22 (
This Example employs photodenitration of RDX in deionized water at 350 nm in an attempt to generate sufficient amounts of initial intermediate(s) that are not detectable using previous technologies with Rhodococcus sp. strain DN22. The photolysis experiments were conducted in non degassed aqueous solutions at pH 5.5 and room temperature to mimic the conditions employed during RDX degradation with Strain DN22. Low energy wavelength light (350 nm) at neutral conditions was used to minimize the formation of secondary photoproducts whose presence might interfere with other prime products considered important to the understanding of the degradation process. Some of the photolysis experiments were conducted in the presence of acetone in an attempt to enhance denitration through the energetic acetone triplet (T1(n,π*)) whose excitation energy equals to 326.04 KJmole−1.
Chemicals. Commercial grade RDX (purity >99%) was provided by the Defence Research Establishment Valcartier, Quebec, Canada (DREV). Acetone, formamide and formaldehyde were from Aldrich, Calif. Methylenedinitramine was from the rare chemical department of Aldrich, Canada. MNX with 98% purity was obtained from R. Spanggord (SRI, Menlo Park, Calif.) and TNX (99%) was obtained from G. Ampleman (DRDC, Quebec, Canada). All other chemicals used were of reagent grade.
Irradiation Experiments. A Rayonet photoreactor RPR-100 fitted with a merry-go-round apparatus (Southern New England Co, Hamden, Conn.) equipped with sixteen 350 nm lamps were used as light sources. Photolysis was conducted in 20 ml tubes made of quartz. Each tube was charged with a 10 ml of an aqueous solution of RDX (10 mg/L) in the presence and absence of acetone (250 ppm, v/v). Photolysis was carried out in non degassed quartz tubes sealed with teflon coated mininert valves. Controls containing RDX were kept in the dark during the course of the experiment. The temperature of the reactor was maintained at 25° C. by maintaining the apparatus in a cold room at 10° C. during irradiation. Light intensities, λ 350=3.0 10−6 einstein ml−1s−1 (RPR-350 nm lamp tubes emitting >90% of their energy at 350 nm) were measured using ferrioxalate actinometry.
Degradation of RDX with Rhodococcus sp. strain DN22. Experimental details on the biodegradation of RDX with strain DN22 can be found in Fournier et al. (2002).
Analytical Procedures. Analysis of 3,5-dinitro- 1,3,5-triaza-cyclohex-1 -ene (II) was carried out on a Hewlett Packard 6890 gas chromatograph coupled to a 5973 quadrupole mass spectrometer in the positive chemical ionization (PCI) mode using methane. Five μl of ethyl acetate extracts of the photolyzed mixture were injected in the solvent vent mode i.e. at 10° C. maintained 0.15 min. then fast heated up to 200° C. under splitless condition on a 15 m×250 um×0.5 um RTX-5amine capillary column from Restek. Helium was used as the carrier gas under an average velocity of 27 cm/sec. The column is heated at 55° C. for 3 minute then raised to 200° C. at a rate of 10° C./min, which was held 2 min. The detector interface was maintained at 200° C. The quadrupole and the source are held at 106 and 150° C., respectively. The product was detected as its protonated molecular mass ion [M+H].
The cyclic carbinol intermediate (I), its ring cleavage product (IV) and their subsequent hydrolyzed products including methylenedinitramine (VI) and 4-nitro-2,4-diaza-butanal (V) were analyzed as described previously using LC/MS (ES−). Briefly, a MicromassTm bench-top single quadrupole mass detector attached to a Hewlett Packard 1100 Series HPLC system and equipped with a photodiode array detector was used. The samples were injected into Supelcosil LC-CN column (4.6 mm ID×25 cm) (Supelco, Oakville, ON) at 35° C. The solvent system consisted of acetonitrile (20%) in water at a flow rate of 1 ml/min. Ionization was done in a negative electrospray ionization mode ES(−) producing mainly the [M−H] mass ions. The mass range was scanned from 40 to 400 Da with a cycle time of 1.6 s and the resolution was set to 1 Da (width at half-height) (Hawari et al., 2000).
Formamide was derivatized with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine.HCl (PFBHA) for 30 min. at 80° C., and analyzed by LC/MS (ES−) using a 5 μm Supelcosil LC-8 column (4.6 mm ID×25 cm) (Supelco, Oakville, ON) maintained at 35° C. The solvent system consisted of acetonitrile/water gradient (50 to 90% v/v) at a flow rate of 1 ml/min. Detection was done by monitoring its [M−H] mass ions.
Formic acid, NO2— and NO3— were measured using a Hp3D CE instrument model 1600 equipped with a diode array detector. The system was fitted with a Agilent G16006132 fused silica capillary with a total length of 64.5 cm (56 cm effective) and an internal diameter of 50 μm. The voltage was set at 25 kV (negative polarity) and the temperature at 25° C. Samples were injected by applying 50 mbar pressure to the capillary inlet for 50 s. The electrolyte solution was prefiltered and buffered with triethanolamine at pH 7.7. Hexamethonium hydroxide is added as a cation flow modifier. Separation time was 15 minutes. Indirect detection at 254 nm was used with pyromellitic acid as the background absorbing ion. Detection limit was 50 ppb.
Ammonium cation was analyzed using an SP 8100 HPLC system equipped with a Waters 431 conductivity detector and a Hamilton PRP-X200 (250 mm×4.6 mm×10 μm) analytical cation exchange column as described by Hawari et al. (2001). For trace analysis concentration of NH4+ was measured by capillary electrophoresis using a HP3D CE instrument model 1600 equipped with a diode array detector. The system was fitted with a Agilent G16006132 fused silica capillary with a total length of 64.5 cm (56 cm effective) and an internal diameter of 50 um. The voltage was set at 15 kV (positive polarity) and the temperature at 25° C. Samples were injected by applying 50 mbar pressure to the capillary inlet for 50 s. The electrolyte solution was prefiltered and buffered with lactic acid at pH 3.5. 18-crown 6 is added as a anion flow modifier. Separation time was 15 minutes. Indirect detection at 350 nm was used with imidazole as the background absorbing ion. Detection limit was 50 ppb.
Analysis of N2O was carried out with a GC/ECD system as previously described (Hawari et al., 2000). Formaldehyde (HCHO) was analyzed as described by Summers (1990) with minor modifications. Samples were derivatized with 2,4-pentanedione in the presence of ammonium acetate and glacial acetic acid for one hour at pH 6.0, 40° C. The derivatives were then analyzed by HPLC using a 5 μm Supelcosil LC-8 column (4.6 mm ID×25 cm) (Supelco, Oakville, ON) maintained at 40° C. The mobile phase consisted of an acetonitrile gradient of 15% to 27%, at a flow rate of 1.5 ml/min for 6 min. Detection and quantification were carried out using a fluorescence detector (excitation at 430 nm and emission at 520 nm). Measurement of methanol is made on a Hewlett Packard 6890 gas chromatograph coupled to an FID. 1 μL of water sample is injected on a 2 m×0.03 mm Hayesep Q micropacked column from Supelco. The column is heated at 60° C. one minute then raised to 180° C. at a rate of 20° C./minute. Helium is used as carrier gas. The injector and detector are maintained at 150° C. and 250° C. respectively. Standards are prepared from neat compounds from JT Baker. Detection limit is 0.25 ppm.
Results and Discussion
Photodegradation of RDX in aqueous solution at 350 nm.
The present RDX photo product was tentatively identified as hexahydro-3,5-dinitro-1,3,5-triazacyclohex-1-ene. However, when a standard material of RDX was injected on the same GC, a peak was observed that had the same r.t. and [M+H] as that obtained during RDX photolysis. RDX is thermally unstable and decomposes via HNO2 elimination (Zhao et al., 1988), indicating that the energetic chemical might also undergo thermal decomposition at least partially during GC analysis. However, the GC area of II generated during RDX photolysis was found to be approximately twice as large as the area obtained from dark controls, suggesting its potential formation as a photo product. Previous reports indicated the formation of II during photolysis, thermolysis and hydrolysis of RDX, but product II has never been isolated. Product II is an enamine with a reactive —C═N bond that is expected to undergo rapid hydrolysis in water more likely to be unstable in water.
When photolysis was carried out in the presence of acetone, two new LC/MS peaks marked III and IV were detected at 6.8 and 18.2 min respectively as shown in
Two other familiar intermediates, V at a r.t of 2.9 min with a [M−H] at 118 Da, matching a molecular formula of C2H5N3O3 and VI at a r. t. of 2.3 min with a [M−H] at 135 Da matching a molecular formula of CH4N4O4 were observed (
Interestingly, the LC/MS(ES−) spectrum of IV was found to contain a strong mass ion at 118 Da (
The present mild photolysis (350 nm) of RDX in non degassed aqueous solution did not produce any of the RDX nitroso derivatives such as MNX, DNX and TNX. Interestingly none of the nitroso products was detected during RDX aerobic degradation with strain DN22. Fenton reagent (Fe+2/H2O2), which is known to be a powerful generator of the highly reactive hydroxyl radicals, OH, mineralized both RDX and HMX to CO2 and NO3— without producing the nitroso compounds (Zoh et al., 2000). However, photolysis of RDX at short wavelengths (higher energy) produced the nitroso derivatives (Peyton et al., 1999). Nitroso products were frequently detected during biodegradation of RDX under anaerobic conditions. Interestingly when RDX was photolyzed under a blanket of argon (anaerobic conditions) at shorter wavelengths (254 nm) all three nitroso products were detected.
At the end of photolysis, which lasted 16 h the accumulation of the products NO2−, NO3−, NH2CHO, HCHO, HCOOH, N2O, NO2NHCH2NHCHO and NO2NHCH2NHNO2 was observed. Most of these products were also detected during RDX incubation with the aerobic degrader DN22, emphasizing that once a successful initial attack takes place on RDX the molecule undergoes spontaneous decomposition in water.
The presence of acetone during RDX photolysis did not seem to drastically change product distribution. For instance, all products that were detected during RDX photolysis in the absence of acetone were also detected during its presence. Some slight variation in the relative distribution of these products was noted. Preliminary investigation with other photo sensitizes such, as phenothiazine and benzyl viologen also did not seem to drastically affect RDX product distribution. No RDX degradation was observed in dark aqueous controls with and without the sensitizer.
Kinetics and Stoichiometry—A Comparative Study with Strain DN22. The removal of RDX during photolysis was accompanied by the concurrent formation of nitrite NO2− and nitrate NO3− ions (
Photodegradation of RDX was also accompanied by the accumulation of NH2CHO, HCHO, and HCOOH (
Table 3 shows carbon and nitrogen mass balances of RDX (0.46 μmole) after 16 h of photolysis at 350 nm in water. These data illustrate that the N mass balance of RDX removal is 73% distributed as follows O2NNHCH2NHCHO (V) (22.2%), NO2− and NO3− (21.4%), NH2CHO (15.7), N2O (11.6%) and CH2(NHNO2)2 (VI) (1.9%) and the C mass balance is 93% distributed as follows: NH2CHO (31.9%), O2NNHCH2NHCHO (V) (31.5%), HCHO (22.0%), HCOOH (9.1%), and CH2(NHNO2)2 (VI) (0.9%). The nitrogen content of nitramide (NH2NO2) is not included in the N mass balance because of the absence of a standard material for quantification. In comparison, N— and C— mass balances of RDX biodegradation with strain DN22 were calculated as 90 and 94%, respectively. In the latter case approximately 30% of the C-content (1 atom) of RDX were found in 14CO2 and 64% (2 C atoms) were incorporated in the dead end product V. The slight differences in product distribution and N and C mass balances between photolysis and biodegradation experiments was possibly caused by some variations in the relative reactivities of certain intermediates toward light and enzymes.
(a) indicates a single measurement.
Insights into Photodecomposition Mechanism of RDX: Parallels with Strain DN22: Initial Denitration.
During RDX photolysis in the presence of acetone the ketone should produce a triplet T1(n,π*) with excitation energy equals to 326.0 KJmole−1 (Turro 1978). Subsequent hydrogen atom abstraction from one of RDX methylene groups by acetone T1(n,π*) would generate an RDX radical RDX●. Elimination of NO2 from RDX● should lead to the formation of the enamine II. This reaction is exothermic because the bond dissociation energies (BDE) of the cleaved C—H (in RDX) and the generated O—H (in acetone) bonds are 376.2 and 418,0 KJmole−1, respectively. However, the energy associated with acetone triplet T1(n,π*) is high (326.0 KJmole−1) and should be sufficient to cleave the N—NO2 bond (BDE 204.9 KJmole−1). Alternatively, direct homolytic cleavage of the N—NO2 bond in RDX should not be excluded. For instance the energy associated with λ 350 nm is 342 KJmole−1 and therefore should be sufficient to cleave the N—NO2 bond (BDE 204.9 KJmole−1) (Behrens et al., 1991). However, a synchronous bimolecular elimination (E2) of one molecule of HNO2 during RDX photolysis has been suggested to occur (Peyton et al., 1999; Melius et al., 1990). Nonetheless such elimination would also lead to the formation of the enamine II (
Secondary Decomposition. The potential involvement of the enamine II in the initial decomposition of RDX is supported by our observation of its carbinol derivative (III) and its subsequent ring cleavage product (IV) (
Intermediate IV with several labile C—NNO2 bonds is thus expected to decompose in water to give several products depending on which bond is hydrolyzed first. For example 4,5-cleavage produces 4-nitro-2,4,-diaza-butanal (V) whereas 3,4 cleavage produces methylenedinitramine (VI) (
In conclusion,
Roman numeral reference indicia used in the Example pertain to
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX, I), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), are known to hydrolyze at pH>10 to produce end products including NO2−, HCHO, HCOOH, NH3, and N2O, but little information is available on their precursors apart from the tentatively identified pentahydro-3,5-dinitro-1,3,5-triaza-1-ene (II). In the present study, we hydrolyzed RDX, MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine) and HMX in aqueous solution (pH 10-12.3) and found that the removal of the nitramine was accompanied by the formation of one molar equivalent of nitrite and the accumulation of the key ring cleavage product 4-nitro-2,4-diaza-butanal (4-NDAB, O2NNHCH2NHCHO). Most of the remaining C and N content of RDX, MNX and HMX was found in NO2−, HCHO, N2O, HCOOH, and NH3. Consequently, we selected RDX as a model cyclic nitramine compound and hydrolyzed it in aqueous acetonitrile solutions (pH 10-12.3) in the presence and absence of hydroxypropyl-β-cyclodextrin (HP-β-CD) to explore other early intermediates in more detail. We observed a transient LC-MS peak with a [M−H] at 192 Da that was tentatively identified as 4,6-dinitro-2,4,6-triaza-hexanal (O2NNHCH2NNO2CH2NHCHO, III) considered as the hydrolyzed product of II. In addition we detected another novel intermediate with a [M−H] at 148 Da that was tentatively identified as a hydrolyzed product of III, namely, 5-hydroxy-4-nitro-2,4-diaza-pentanal (HOCH2NNO2CH2NHCHO, IV). Both III and IV can act as precursors to 4-NDAB. The results provide strong evidence that initial denitration of cyclic nitramines in water is sufficient to cause ring cleavage followed by spontaneous decomposition to form the key product 4-NDAB. Finally, in the case of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), denitration (two NO2−) also led to the formation of HCOOH, NH3, and N2O, but neither HCHO nor 4-NDAB were detected.
Introduction. The cyclic nitramines hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are highly energetic compounds used in various propellants and conventional munitions (
Numerous investigations have established that the typical decomposition or degradative routes (photochemical, thermal, microbiological, or hydrolytic) of RDX produces end products including nitrite (NO2−), nitrous oxide (N2O), N2, ammonia (NH3), formaldehyde (HCHO), formic acid (HCOOH) and CO2. Hoffsommer et al., (1997) reported that RDX degradation in alkaline medium was initiated by a single denitration step to first form pentahydro-3,5-dinitro-1,3,5-triazacyclohex-1-ene (II), which then hydrolyzed further leading to rapid ring cleavage and decomposition (
Both Rhodococcus sp. strain DN22 and light (350 nm) can successfully decompose RDX in water via initial denitration to produce 4-nitro-2,4-diazabutanal (4-NDAB, O2NNHCH2NHCHO) as a major ring cleavage product in both cases. In addition the end products found in photochemical and enzymatic degradative routes are basically similar and include NO2−, N2O, NH3, HCHO, CO2, and H2NCHO.
While many of the end products of RDX hydrolysis are known, intermediates and decomposition pathways are not well understood. Heilmann et al. (1996) suggested that RDX and HMX contaminated waters could be economically treated using granular activated carbon (GAC) to first adsorb the explosive, followed by off-line regeneration of the laden GAC through alkaline hydrolysis. However, for this technology to be optimized, knowledge of intermediates and the degradation pathways of these two monocyclic nitramines is critical both to predict their behavior and enhance their removal from the environment.
The end products of RDX decomposition during enzymatic biodegradation with strain DN22 were similar to those obtained via both alkaline hydrolysis and photolysis. This Example investigates whether following initial denitration of the cyclic nitramine, ring cleavage and subsequent decomposition are largely dictated by the aqueous chemistry of the resulting intermediates. Therefore, if other cyclic nitramines such as MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine), HMX, and CL-20 produce end products similar to those of RDX, then their decomposition mechanisms should also be similar. Hence, this Example provides additional insight into the degradation pathways of these cyclic nitramines by investigating the initial steps and intermediates that are involved in their decomposition under alkaline conditions. RDX was selected as a model cyclic nitramine compound and hydrolyzed it in aqueous acetonitrile solutions (pH 10-12.3) in the presence and absence of hydroxypropyl-β-cyclodextrin (HP-β-CD) in an attempt to explore other early intermediates in more detail.
Chemicals. Commercial grade RDX (purity >99%), ring-labelled [15N]-RDX (purity ca. 98%), HMX (purity>99%), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX, 99% pure) were provided by the Defense Research and Development Canada (DRDC), Valcartier, Canada. 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (ε-CL-20) was provided by A.T.K. Thiokol Propulsion (Brigham City, Utah). Hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX, 99% pure) was provided by R. Spanggord (SRI, Menlo Park, Calif.). 4-nitro-2,4-diazabutanal was obtained as described by Fournier et al. (2002), and methylene dinitramine was obtained from the rare chemical department, Aldrich, Canada. All other chemicals used in this work were of reagent grade.
Hydrolyses of RDX, MNX, HMX and CL-20 in Aqueous Solution at pH 10. To a series of dry 20 mL vials (each wrapped in Aluminum foil) was added an aliquot of cyclic nitramine stock solution prepared in acetone. The acetone was evaporated in a fume hood, followed by the addition of 10 mL of a NaOH solution (pH 10) to give the following concentrations 215, 180, 110 and 100 ppm for RDX, MNX, HMX and CL-20, respectively. The initial concentrations were kept well in excess of the maximum water solubility of each nitramine (ca. 50 ppm for RDX, 5 ppm for HMX, and 3.6 ppm for CL-20) in an attempt to generate sufficient amounts of intermediates to allow detection. The vials were sealed with Teflon coated serum caps and placed in a thermostated benchtop shaker at 30° C. and 200 rpm. Some vials were sparged with Argon for 2 hours to allow analysis for N2.
We used a gas-tight syringe to measure gaseous products (N2O and N2) in the head space but for the analysis of the liquid medium we first quenched the reaction by adding HCl taken in a MeCN solution (10 mL) (pH 4.5). MeCN was used to solubilize unreacted starting material for subsequent analysis. The quenched vials were then stored in a refrigerator at 4° C. until analyzed.
Hydrolysis of RDX in 70% MeCN:30 H2O (v/v) Mixture. A predetermined weight of RDX (0.1 g) was placed into a vial to which a 10 mL solution of NaOH (pH 12.3) in 70:30 (v/v) acetonitrile: water was added, giving an initial RDX concentration of 10,000 ppm. MeCN was used to solubilize the high RDX concentration used, while a high pH was used to accelerate the reaction. The vial was sealed with a Teflon coated serum cap and placed in a thermostated benchtop shaker at 30° C. and 200 rpm. After 22 h, the reaction was quenched by the addition of HCl (1 M) and the volume of the quenched mixture was then reduced under reduced pressure (18 mm Hg). The remaining mixture was passed through a column containing 3-aminopropyl-functionalized silica gel (Aldrich) using a 70% acetonitrile:30% water (v/v) mixture to remove chloride and nitrite anions, preventing interference during LC-MS (ES−) analysis (discussed below). This experiment was then repeated using [15N]-RDX.
Hydrolysis of RDX in the Presence of Hydroxypropyl-β-Cyclodextrin (HP-β-CD). HP-β-CD was used in an effort to stabilize putative early intermediates such as pentahydro-3,5-dinitro-1,3,5-triazacyclohex-1-ene (II) and its ring cleavage products (O2NNHCH2NNO2CH2NHCHO (III)) and HOCH2NNO2CH2NHCHO (IV) by forming inclusion complexes. RDX (0.1 g) or [15N]-RDX (0.1 g) was placed in a vial, followed by the addition of 10 mL NaOH (pH 12.3) in 65:35 (v/v) acetonitrile:water containing 3% (wt/v) HP-β-CD. Hydrolysis and sample preparation for subsequent analysis was conducted as discussed above.
Analytical Procedures. The concentration of RDX, MNX, HMX and CL-20 was determined using a reversed-phase HPLC connected to a photodiode array (PDA) detector. Formaldehyde (HCHO), 15N14NO (45 Da) and 14N14NO (44 Da) analyses were carried out as described by Sheremata et al. (2000). Analyses of nitrite (NO2−), nitrate (NO3−), fornate (HCOO−) and ammonium (NH4+) were performed by capillary electrophoresis on a Hewlett-Packard 3D-CE equipped with a Model 1600 photodiode array detector and a HP capillary part number 1600-61232. The total capillary length was 64.5 cm, with an effective length of 56 cm and an internal diameter of 50 μm. For nitrite, nitrate and formate, analyses were performed using sodium borate (25 mM) and hexamethonium bromide (25 mM) electrolytic solution at pH 9.2. The ammonium cation was analyzed using a formic acid (5 mM), imidazole (10 mM) and 18-crown-6 (50 mM) electrolytic solution at pH 5. In all cases, UV detection was performed at 215 nm.
A Micromass™ bench-top single quadropole mass detector attached to a Hewlett Packard 1100 Series HPLC system equipped with a DAD detector was used to analyze for RDX ring cleavage intermediates. The samples were injected into a 4 μm-pore size Supelcosil CN column (4.6 mm ID by 25 cm; Phenomenex, Torrance, Calif.) at 35° C. The solvent system was composed of 20% acetonitrile: 80% water (v/v): For mass analysis, ionization was performed in a negative electrospray ionization mode, ES(−), producing mainly the deprotonated molecular mass ions [M−H]. The mass range was scanned from 30 to 400 Da with a cycle time of 1.6 s, and the resolution was set to 1 Da (width at half height).
Results and Discussion
Alkaline Hydrolysis of Cyclic Nitramines: Product Distribution.
In addition, it was found that MNX hydrolysis proceeds via initial denitration (i.e., elimination of HNO2) followed by ring cleavage to produce NO2−, N2O (trace quantities), HCHO, and 4-NDAB (Table 4 and
We were unable to observe any of the RDX nitroso products (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX)), which is also consistent with other reports on the alkaline hydrolysis of RDX. Indeed we found that hydrolysis on MNX under the same conditions failed to produce the dinitroso (DNX) or the trinitroso (TNX) compounds. Likewise, when we hydrolyzed HMX and CL-20 at pH 10, none of their corresponding nitroso products were observed. These results provide strong evidence that initial denitration is a key step responsible for the decomposition of cyclic nitramines in water.
Table 4 illustrates normalized molar yields for the products obtained upon alkaline hydrolysis (pH 10) of the cyclic nitramines RDX, MNX, HMX, and CL-20. Stoichiometries are calculated based on the number of moles of product observed for each mole of reactant consumed.
aValues are the average of data from three replicate measurements.
bValues are the average of date from duplicate measurements.
cDue to its slow reaction rate, monitoring of HMX hydrolysis was stopped after 21 days. It is possible that NH3 and HCOO− formed, but were at levels less than the detection limit of the capillary electrophoresis method.
dValues for HCOO− are as follows for RDXa, MNXa, HMXb and CL-20b respectively: 0.070 ± 0.005; 0.152 ± 0.008;
n.d.c; and 0.493 ± 0.007.
Table 4 summarizes the normalized molar yields for the products obtained in the hydrolysis of RDX, MNX, HMX and CL-20 at pH 10. From Table 4, it is apparent that, except for CL-20, these compounds lose one equivalent of NO2− and form HNNO2CH2NHCHO (4-NDAB) as a major product. For CL-20, we found that hydrolysis (pH 10) was accompanied by the release of two molar equivalents of nitrite ion rather than one and the formation of HCOOH rather than HCHO. The observation of NO2− emphasized once again the importance of initial denitration in the subsequent decomposition of CL-20. However, we were unable to observe 4-NDAB as was the case with the other nitramines examined. This is not surprising in view of the structural differences between the rigid caged polycyclic CL-20 which is characterized by the presence of C—C bonds (elongated) that are absent in the case of the mono cyclic ones (
For RDX, it was found that C mass balance of 97% distributed as follows: HCHO (31.3%), HCOOH (2.3%), and HNNO2CH2NHCHO (63.4%) and a N mass balance of 90.7% distributed as follows: NO2− (16.2%), N2O (25.6%), NH3 measured as NH4+ (1.3%), and HNNO2CH2NHCHO (47.6%). For MNX, it was found that C mass balance of 87.8% in: HCHO (26.6%), HCOOH (5.05%), and HNNO2CH2NHCHO (56.1%) and a N mass balance of 60.45% distributed as follows: NO2− (15.6%), N2O (0.14%), NH3 measured as NH4+ (2.35%), and HNNO2CH2NHCHO (42.1%). Whereas for HMX, it was found that C mass balance of 90% distributed as follows: HCHO (46.2%), HNNO2CH2NHCHO (43.8%) and a N mass balance of 86% distributed as follows: NO2− (14.7%), N2O (38.5%), HNNO2CH2NHCHO (32.8%). Due to the slow reaction rate of HMX at pH 10 the presence of NH3 and HCOOH, could not be unequivocally confirmned. However, when the pH was increased to 12.3 a product distribution (NO2−, N2O, NH3 and HCOOH) was obtained, but 4-NDAB was found to decompose at this pH thereby explaining why this important intermediate was previously found in hydrolysis studies.
Stoichiometry. Table 5 summarizes the percentage of N-containing and C-containing products produced by RDX degradation at pH 10. The stoichiometries of nitrogen and carbon during the alkaline hydrolysis of RDX is shown, calculated on the basis of percentage of reacted RDX using total theoretical number of N and C atoms. We found a C mass balance of 97% distributed as follows: HCHO (31.3%), HCHOO (2.3%), and V (63.4%) and a N mass balance of 90.7% distributed as follows: NO2− (16.2%), N2O (25.6%), NH3 measured as NH4+ (1.3%), and V (47.6%).
aPercentages are the average of data from three replicate measurements.
bTotal includes the amount of N2O calculated to be in the aqueous phase, and also the amount calculated as being in the form of nitramide. Subtracting these theoretical values gives a total nitrogen recovery of 90.7%.
In addition, taking the concentrations of N2O measured in these experiments as an equilibrium quantity, the aqueous phase concentration of N2O can be calculated using Henry's Law. For N2O, the Henry's Law co-efficient (kH) is 0.02522 M atm−1 (Sander et al.). Furthermore, if it is assumed that the measured N2O arose from the hydrolysis of nitramide (NH2NO2), the concentration of nitramide in solution can be calculated based on the rate (or half-life) of N2O formation. With a half-life of 91 h, the reaction had proceeded to ca. 94% at the time the stoichiometry was quantified (t=316 h). Since the N2O that was present accounted for 91% of the total nitrous oxide expected to form, it was assumed that the remainder was in the form of NH2NO2. Hence, 0.3% of the nitrogen is calculated as being in the form of aqueous N2O, while 2.5% of the nitrogen is thought to be still in nitramide. Thus, the combination of observed and calculated nitrogen mass balances yields a total recovered nitrogen of 93.5%.
Identification of Early Intermediates in RDX Hydrolysis. In none of the LC/MS traces shown (
Meanwhile, the peak labeled IV exhibited a [M−H] at 148 Da, matching a molecular mass formula of C3H7N3O4. IV showed three other mass ion fragments at 46, 61 and 118 Da, representing —NO2, —NHNO2, and the [M−H] of O2NNHCH2NHCHO, respectively (
When RDX (1% w/v) was hydrolyzed in the presence of 3% HP-β-CD in a 65:35 (v/v) acetonitrile: water mixture at pH 12.3, we only observed III and 4-NDAB (
Decomposition Pathways. Based on product distribution and mass balance studies (Table 4 and Table 5) and the critical observation that denitration was a key step responsible for degradation of cyclic nitramines we proposed a degradation pathway that is consistent with previous reports (
Intermediates III and IV have different C—N bonds that are subject to hydrolytic cleavage. A plausible route for the formation of the novel intermediate observed with a [M−H] at 148 Da (IV) could be via nucleophilic attack by OH at the C atom of the NH—Cl bond III, accompanied by the loss of NH2NO2 (
Clearly, nucleophilic attack at the C1 position in III is favoured because C1, being surrounded on both sides by stronger electron-withdrawing nitramine groups, is expected to be more electrophilic than C2. Consequently, C1 is more susceptible to nucleophilic attack by OH− than C2, explaining why the route depicted in
Both MNX and HMX also hydrolyze via initial denitration to produce NO2−, followed by a ring cleavage and spontaneous decomposition to N2O, HCHO, and 4-NDAB. These findings indicate that the mechanism postulated for the alkaline hydrolysis of RDX applies more broadly to other mono cyclic nitramine explosives. For MNX, the analogous intermediate to III is ONNHC1H2NNO2C2H2NHCHO (4-nitro-6-nitroso-2,4,6-triaza-hexanal). Since the C-2 position of this analog is expected to be more electrophilic than the C-2 position in III itself, HNCHO (evolved as formamide) becomes more amenable for nucleophilic substitution by OH−. As a result, the minor pathway producing NH3 and HCOO− (
In contrast to the single denitration event seen during either alkaline hydrolysis in this Example or photodenitration, two enzymatic denitration events occur en route to producing 4-NDAB. However, under extreme conditions ([NaOH]>19 M), two denitration events may also occur.
Although the polycyclic nitramine CL-20 is structurally different from the monocyclic RDX, MNX and HMX, its decomposition was also found to proceed via initial denitration (two molar equivalents of NO2−) to give end products including N2O, NH3, and HCOO− (Table 4).
In conclusion, the present Example confirms that a successful initial (primary) attack, denitration in this case, occurs in the degradation of monocyclic and polycyclic nitramines. Once denitration occurs, ring cleavage becomes spontaneous and is dictated by the chemistry of the ensuing intermediates in water. Similarities are observed between the decomposition patterns of cyclic nitramines under various chemical, biochemical and biological conditions. The rapid denitration process may be used as a probe to detect early stage decomposition events.
The above-described embodiments of the invention are intended to be examples of the present invention. Alterations, modifications and variations may be effected the particular embodiments by those of skill in the art, without departing from the scope of the invention which is defined solely by the claims appended hereto.
This application claims priority from U.S. Provisional Patent Application No. 60/404,147 (filed on Aug. 19, 2002) and from U.S. Provisional Patent Application No. 60/395,316 (filed Jul. 12, 2002), both of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA02/01799 | 11/27/2002 | WO | 9/9/2005 |
| Number | Date | Country | |
|---|---|---|---|
| 60395316 | Jul 2002 | US |
