Patent Application
20200268889
The invention relates generally to (R)-12-hydroxystearic acid hydrazides as gelators and the use thereof to generate self-standing molecular gels.
Molecular gels typically consist of a low molecular weight gelator (LMWG) and a liquid. To date, relatively few LMWGs have been found which have crystalline, self-assembled fibrillar networks (SAFINs) and show a high degree of thixotropy (i.e., they are able to substantially reform a large part of their viscoelasticity after the cessation of destructive strain).
Of particular interest also are super-gelators, LMWGs capable of gelating liquids at concentrations below 1.0 wt %, and those forming free-standing, shape-persistent gels. Apart from polymer gels (most of which are hydrogels), known LMWGs capable of forming self-standing molecular gels are normally metal complexes with strong metal-ligand interactions, derivatives of crown ethers that undergo host-guest interactions, or amides, peptides and sugars with strong hydrogen-bonding units. Such super-gelators also include highly luminescent oxadiazole-based stilbene molecules without hydrogen-bonding motifs. The SAFINs of these self-standing molecular gels in low-polarity liquids derive principally from intermolecular Π-Π interactions. Designing gelators with these desirable properties remains a huge challenge, and most have been found serendipitously.
A problem with many of these gelators, the solvents in which they produce gels, and their resulting gels themselves is that they lack biocompatibility; that is, they are unsuitable for contact or use with living tissue, such as, for example, contact with human skin or mucosa. There exists a need to develop highly efficient gelators capable of producing molecular gels, including self-standing gels, in, for example, the medical and pharmaceutical fields for potential drug delivery applications. A key challenge in these fields that must be addressed is that the gelators need to be capable of causing gelation in biocompatible solvents such that resulting molecular gels are safe for medical use.
The invention provides (R)-12-hydroxystearic acid hydrazides and gels derived therefrom. The (R)-12-hydroxystearic acid hydrazides are versatile and can produce gels in various solvents.
In some embodiments, a series of (R)-12-hydroxystearic acid hydrazides produce gels that are self-standing, self-healing, thixotropic, load-bearing and have moldable properties.
The gels may have self-assembled fibrillar networks (SAFINs) that are crystalline in structure. In some embodiments, the gels have demonstrated good potential for use in drug release.
The present invention generally relates to a series of hydrazides (“n-HSAH”) derived from (R)-12-hydroxystearic acid (
The integer n may be zero, or it may be an odd or even number. For each gel employing an n-HSAH according to the invention, the (S) isomer of the n-HSAH may also be present in an amount less than or equal to the amount of (R) isomer. Therefore, the (R) isomer constitutes at least 50% of the total of (R) and (S) isomers of any given hydrazide present in the gel. It may constitute at least 60%, or at least 70%, 80%, 90%, 95%, 98%, or 99% of the total. Gelators other than n-HSAH compounds may optionally be included in the gel, or they may be excluded.
Non-limiting examples of liquids that may be gelled with n-HSAH compounds according to the invention include water, organic liquids, and combinations thereof, for example solutions of organic liquids in water or vice versa. Exemplary organic liquids include C1-C10 aliphatic hydrocarbons (for example hexane and decane), silicone oils, toluene, each of the xylene isomers, CHCl3, chlorobenzene, ethyl acetate, THF, 1-butanol, ethanol, methanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, 1-octanol, nitrobenzene, DMF, acetonitrile, ethylene glycol, propylene glycol, glycerol, DMSO, propylene carbonate, and combinations thereof.
The n-HSAH compounds disclosed herein may form gels in a variety of liquids by self-assembly mechanisms that appear to depend on the specific value of n and the choice of liquid. This comportment is similar in some respects to those of the analogous (R)—N-alkyl-12-hydroxystearamides, but the specific properties of the hydrazides are very different in some important respects. Thus, in several of the hydrazides, the properties of the gels can be correlated with their mechanisms of formation. Notably, the parent hydrazide, (R)-12-hydroxystearic acid hydrazide (0-HSAH), was found to be a super-gelator in ethylene glycol, propylene glycol, and glycerol, and its gels exhibited self-standing, self-healing, moldable and load-bearing properties that are not found in the corresponding amide. Although other molecular gels have been shown to possess one or more of the properties reported here, none possesses all of them in one gel with such simply-structured gelators.
Certain derivatives of (R)-12-hydroxystearic acid hydrazides were specifically studied, including n-HSAH derivatives in which n=0, 2, 6, or 10.
For synthesizing (R)-12-hydroxystearic acid hydrazide (0-HSAH), the parent hydrazide, a mixture of methyl (R)-12-hydroxystearate (10.0 g, 3.2 mmol) and hydrazine hydrate (10.0 g, 16.9 mmol) was heated in ethanol (90 mL). After refluxing overnight, the hot solution was poured into 500 mL water with stirring. The precipitate was washed with 3×200 mL water and dried in vacuum to give 8.9 g (89%) of a pale yellow powder. 5.0 g of this material was refluxed twice in 100 g aliquots of hexane for 30 min. The solution was filtered hot. The filter cake was washed with a small amount of hexane and then recrystallized from ethyl acetate (200 g) and methanol (100 g) mixtures twice to give 2.2 g (44%) 0-HSAH as a white powder.
For synthesizing (R)—N′-ethyl-12-hydroxyoctadecane hydrazide (2-HSAH), 0-HSAH (2.0 g, 6.4 mmol) and acetaldehyde (1.7 g, 38.6 mmol) were mixed in methanol (7.0 g) in a thick glass tube at 0° C. Glacial acetic acid (0.8 g) was added as a catalyst. The tube was sealed with a screw cap and the mixture was stirred for 5 hours at 70° C. After cooling, the product was filtered and recrystallized from 30 g methanol to give 1.3 g (60%) of two geometric isomers of (R)—N′-ethylidene-12-hydroxy octadecane hydrazide. (R)—N′-ethylidene-12-hydroxy octadecane hydrazide (0.43 g, 1.26 mmol) was dissolved in 5 mL methanol at room temperature and stirred for 30 min. Sodium cyanoborohydride (156 mg, 2.48 mmol) and glacial acetic acid (156 mg, 2.60 mmol) were dissolved in 1 mL methanol and added drop-wise to the methanol solution over 10 minutes under a N2 atmosphere and then stirred for 5 hours at 0° C. and for 45 minutes at room temperature. Thereafter, the mixture was filtered. The filter cake was washed with water and recrystallized from ethyl acetate. The crude product was extracted with CHCl3, dried under vacuum and then recrystallized from a mixture of ethyl acetate (200 g) and methanol (100 g) twice to give 60 mg (14%) 2-HSAH.
For synthesizing (R)—N′-hexyl-12-hydroxyoctadecane hydrazide (6-HSAH), a mixture of 0-HSAH (3.1 g, 9.8 mmol) and hexanal (1.5 g, 15.0 mmol) was heated in methanol (30 mL). Glacial acetic acid (1.0 g) was added as a catalyst to the system. The mixture was refluxed under N2 atmosphere for 2 hours. After cooling, the product was filtered and recrystallized from methanol to give 2.3 g (60%) of two geometric isomers of (R)—N′-hexylidene-12-hydroxyoctadecanehydrazide. (R)—N′-Hexylidene-12-hydroxyoctadecane hydrazide (2.30 g, 5.80 mmol) was dissolved in 50 mL methanol at room temperature and stirred for 30 min. Sodium cyanoborohydride (0.74 g, 11.78 mmol) and glacial acetic acid (0.74 g, 12.32 mmol) were dissolved in 20 mL methanol and added drop-wise to the methanol solution over 30 minutes under a N2 atmosphere and then stirred for 1 hour at 0° C. and for 2 hours at room temperature. Thereafter, the mixture was filtered. The filter cake was washed with methanol and recrystallized from methanol. The crude product was extracted with CHCl3, dried under vacuum and then recrystallized from methanol to give 480 mg (20%) 6-HSAH.
For synthesizing (R)—N′-decyl-12-hydroxyoctadecane hydrazide (10-HSAH), a mixture of 0-HSAH (1.0 g, 3.2 mmol) and decanal (0.7 g, 4.5 mmol) was heated in methanol (10 mL). Glacial acetic acid (0.3 g) was added as a catalyst to the system. The mixture was refluxed under N2 atmosphere for 2 hours. After cooling, the product was filtered and recrystallized from methanol to give 1.2 g (83%) of two geometric isomers of (R)—N′-decylidene-I2-hydroxyoctadecane hydrazide. (R)—N′-Decylidene-12-hydroxyoctadecane hydrazide (0.20 g, 0.44 mmol) was dissolved in 20 mL methanol at room temperature and stirred for 30 minutes. Sodium cyanoborohydride (0.06 g, 0.95 mmol) and glacial acetic acid (0.06 g, 0.10 mmol) were dissolved in 10 mL methanol and added drop-wise into the methanol solution over 30 minutes under a N2 atmosphere and then stirred for 5 hours at room temperature. Thereafter, the mixture was filtered. The filter cake was washed with methanol and recrystallized from methanol. The crude product was extracted with CHCl3, dried under vacuum and then recrystallized from methanol to give 0.10 g (50%) 10-HSAH.
Known amounts of an (R)-12-hydroxystearic acid hydrazide gelator and a liquid were heated in a flame-sealed 5 mm (inside diameter) tube to 120° C. (i.e., slightly higher than the melting points of the gelators). Then, the hot tube was taken from the bath and left undisturbed at room temperature overnight. Samples that formed two layers at 120° C. are labeled “insoluble” (I); samples that dissolved after heating, but precipitated or formed viscous liquids when cooled, are classified as “precipitates” (P) or “viscous” (V), respectively. Those that appeared to be clear or only microscopically heterogeneous and did not flow when inverted are considered (by preliminary classification) to be gels (G). The critical gelator concentrations (CGCs) were determined by adding sequentially a small amount of liquid into the glass tubes that appeared to contain gels. After each addition, the tubes were resealed, reheated and recooled as described above. The CGCs were the lowest concentrations at which the inverted samples did not flow when inverted.
Gel-to-sol transition temperatures (Tgeis) at 5 wt % gelator were also determined by the invert tube method. The heating rate for the Tgeis test was 2° C./min in an oil bath. The temperature range was recorded from when the first drop of liquid fell to when the entire gel collapsed. For preparation of the self-standing gel blocks, hot solutions/sols of 2 wt % or 5 wt % 0-HSAH in ethylene glycol were poured into a two channel syringe; the gels were formed within 5 minutes. After 30 minutes, the gel was pushed out with the syringe plunger and the gel block was cut into two pieces. One was submerged into a 500 mg/L methylene blue-ethylene glycol solution for 10 min. The excessive solution on the surface of the gel block was absorbed with a tissue. Then, the two pieces were put together again and allowed to stand overnight to assess the self-healing properties. If they remained one piece when suspended horizontally, they were considered joined.
The gelation behaviors of 5 wt % n-HSAH (n=0, 2, 6, 10) in different liquids are summarized in Table 1. 0-HSAH shows much better gelation properties than the other gelators of the series. It can gelate a wide variety of liquids, including alkanes, alcohols, aromatic liquids, and even DMSO. By contrast, the other n-HSAH were unable to form gels in alkane liquids like hexane and decane and most of the alcohol liquids (except ethylene glycol). However, they were able to gelate aromatic liquids, DMSO and propylene carbonate. In addition, the CGCs of the gels formed by 0-HSAH are generally lower than those of the other n-HSAH, especially in ethylene glycol, in which the CGC of 0-HSAH was astonishingly low, 0.06 wt %. By contrast and demonstrating the importance of the hydroxyl group at C12, stearic acid hydrazide (SAH), the gelator analogous to 0-HSAH, but lacking a hydroxyl group at C12, required a minimum of 3 wt % to gelate ethylene glycol.
The 0-HSAH in ethylene glycol gels also showed self-standing, self-healing, thixotropic, load-bearing and moldable properties (
A self-standing cylindrical gel block was made with 2 or 5 wt % 0-HSAH in ethylene glycol, but could not be made with 0.5 or 1 wt % concentrations of the gelator. To demonstrate the self-healing properties more clearly, one 5 wt % 0-HSAH in ethylene glycol gel block was cut into two pieces and one of the pieces was submerged into a solution of methylene blue in ethylene glycol for 10 min. Thereafter, the two pieces were placed in contact as shown in
All of the n-HSAH derivatives were able to gelate silicone oil. As such and because it has a high boiling point and low volatility, silicone oil was selected as the liquid to study further the gelation properties of the series of hydrazides.
aOG = opaque gel, CG = clear gel, TG = translucent gel, P = precipitate, I = insoluble, V = viscous liquids;
dgels became precipitates within 22 h;
The mechanical properties of the silicone oil gels were investigated at different conditions within the linear viscoelastic regions (
The mechanical properties of the ethylene glycol gels were also investigated under different conditions. As shown in
In order to test further the stiffness of the 0-HSAH in ethylene glycol gel blocks, compression rheology experiments were performed on a 0.9 mm high and 25 mm diameter cylinder gel block. As the gap between the rheometer plates decreased, the normal force increased. Also, during further compression, the ethylene glycol began to be squeezed out. The normal force at this point, the load-bearing force (Fb), represents the ability of the network to support a static load. It is the critical force below which the network is able to resist compression elastically. Above this load bearing-force, the network underwent irreversible deformation. With an additional decrease of the gap, the normal force increased further until it reached the instrument limit of 50N. As the concentration of 0-HSAH in the gel was increased from 2 wt % to 5 wt %, Fb also increased from 10N to 47N, and the rate of increase of the 5 wt % gel, 1.95 N/μm block, was also greater than that of the 2 wt % gel, 1.23 N/μm (
aAll experiments were performed twice and the data were reproducible.
bEG = ethylene glycol; DMF = N,N-dimethylformamide.
Microstructure and Molecular Packing. Polarized optical micrographs (POMs) provided information about the microstructure of the SAFINs within the gels. Hot solutions/sols were poured into 0.4 mm path-length, flattened Pyrex capillaries and flamed-sealed. The samples were reheated in an oil bath before micrographs were recorded to ensure homogeneity. For fast-cooled samples, the hot capillaries were placed in the air at room temperature overnight. For slow-cooled samples, the capillaries were kept in the oil bath at 120° C. while allowing it to decrease to room temperature slowly. The morphologies of the gels consisting of 5 wt % 0-HSAH in ethylene glycol and n-HSAH (n=0, 2, 6, 10) in silicone oil of both fast-cooled and slow-cooled samples were observed. As shown in
In order to obtain additional insights into the nanostructures of these gels, XRD diffractograms of the organogels and neat solids of n-HSAH (n=0, 2, 6, 10) were also compared (
Attempts to index the powder diffraction peaks of 0-HSAH in order to assign a general cell packing were unsuccessful due to the presence of only 8 peaks. However, it was possible to make a preliminary assignment from analyses of FTIR data. For 2-HSAH (13 peaks), orthorhombic packing (a=8.64 Å, b=17.77 Å, c=30.94 Å); For 6-HSAH (14 peaks), orthorhombic packing (a=8.42 Å, b=24.11 Å, c=35.12 Å); For 10-HSAH (16 peaks), orthorhombic packing (a=15.53 Å, b=6.87 Å, c=41.57 Å); The low angle peaks in the XRD diffractograms of 10 wt % n-HSAH (n=0, 2, 6, 10) in silicone oil gels were very close to those of the neat powders. This similarity strongly suggests that the organizations of molecules in the SAFINs of the silicone oil gels and neat powders are very similar.
aExtended length calculated by MM2 method with Chem 3D Ultra 12.0 software (Cambridge Soft Corporation) and adding the van der Waals radii of the terminal atoms.55
bIndexed with JADE 9 software (Materials Data Inc.) .
cFrom infrared spectroscopic data; vide infra.
The nearly identical nature of the FT-IR spectra of n-HSAH (n=0, 2, 6, 10) in the silicone oil SAFINs and neat powders (
Based on the XRD and FT-IR data, two potential molecular stacking models for 0-HSAH (
For the other n-HSAH, the packing must be quite different. For example, in 10-HSAH, the frequencies of the —OH and NH—NH bands at 3278, 3241 and 3150 are similar to those of (R)-18-(pentylamino)octadecan-7-ol (HSN-5) (
[a]Extensive peak overlaps preclude unambiguous peak assignments above 3200 cm−1.
In order to explore the possible application of these gels as drug release agents, the diffusion coefficients for an anionic dye, methylene blue, and a cationic dye, erythrosine B, from 2 wt % 0-HSAH/ethylene glycol gel blocks into ethylene glycol liquids were determined at 25° C. using an early time approximation to Fick's second law (Eq 2). See
In this equation, Mt=the total amount of dye released during the measurement time t (summed for all ethylene glycol aliquots), which can be calculated based on the calibration curve of UV-vis absorbance (
A plot of Mt2 as a function of t provides a slope, 16DM∞2/Πλ2, from which the diffusion coefficient D can be calculated. From the slopes measured in
Placing a hydroxyl group along the alkyl chain of stearic acid and transformation of the acid into a hydrazide results in an excellent gelator, (R)-12-hydroxystearic acid hydrazide (0-HSAH) whose gelation properties differ substantially from those of the amide analogue, (R)-12-hydroxystearamide (0-HSAA). 0-HSAH forms gels in ethylene glycol that are self-standing, self-healing, partially thixotropic, moldable and high load-bearing. 0-HSAH also forms self-standing gels in propylene glycol and glycerol. The gelators obtained upon addition of an n-alkyl group to the terminal nitrogen atom of the hydrazide, n-HSAH (n=2, 6, 10), have reduced gelation efficiencies in certain respects, as indicated by the range of liquids gelated and the poorer thermal and mechanical properties of the corresponding gels. However, they are still capable of gelating a variety of liquids at relatively low concentrations, as has been found for the corresponding amides (n-HSAA). Thus, small alterations in the n-HSAH gelator structures cause large changes in the gelation properties.
Without wishing to be bound by any particular explanation, the inventors believe that the presence and length of the N-alkyl groups affect the molecular packing arrangements of the gelator molecules in their SAFINs. Whereas the 0-HSAH molecules are arranged in bilayers with separate H-bonding networks from neighboring hydrazide groups and from the secondary hydroxyl groups along the alkyl chains, the N-alkylated n-HSAH are arranged in orthorhombic subcells with lamellae of one molecular thickness. The subtle structure-property relationships among these (R)-12-hydroxystearic acid hydrazides (as well as between the hydrazides and their amide analogues) indicates how LMWG design can be fine-tuned to produce other new, efficient gelators that form self-standing gels with at least some thixotropic properties.
Notably, the diffusion coefficients of small cationic and anionic dyes in 0-HSAH/ethylene glycol gel blocks are much slower than the self-diffusion coefficient of ethylene glycol. For that reason, gels of 0-HSAH or other n-HSAH compounds are suitable for applications in several areas, including controlled release of drugs incorporated in the gels.
This application is related to and claims the benefit of U.S. Provisional Application No. 62/271,746, entitled “(R)-12-HYDROXYSTEARIC ACID HYDRAZIDES AS VERY EFFICIENT GELATORS AND THEIR SELF-STANDING GELS” filed 28 Dec. 2015, the contents of which are incorporated herein by reference in their entirety for all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2016/068862 | 12/28/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62271746 | Dec 2015 | US |
