Patent Application
20060094026
The ability to determine the presence of an analyte in a sample is of significant benefit. For example, many metals and metal ions, such as lead, mercury, cadmium, chromium, and arsenic, pose significant health risks when present in drinking water supplies. To prevent the contamination of drinking and other water supplies, it is common to test industrial waste-streams before their release to the water treatment plant. Biological fluids, such as blood and those originating from body tissues, also may be tested for a variety of analytes to determine if the body has been exposed to harmful agents or if a disease state exists. For example, recently there has been the need to detect trace amounts of anthrax and other biologically harmful agents in a variety of samples.
Colorimetric methods are commonly used for the detection of metals and ions in soil, water, waste-streams, biological samples, body fluids, and the like. In relation to instrument based methods of analysis, such as atomic absorption spectroscopy, colorimetric methods tend to be rapid and require little in the way of equipment or user sophistication. For example, colorimetric test are available to aquarists that turn darker shades of pink when added to aqueous samples containing increasing concentrations of the nitrate (NO3−) ion. In this manner, calorimetric tests show that the analyte of interest, such as nitrate, is present in the sample and also may provide an indicator of the amount of analyte in the sample through the specific hue of color generated.
While colorimetric test are extremely useful, they only exist for a limited set of analytes, often cannot detect very small or trace amounts of the analyte, and depending on the nature of the sample, can generate unacceptable levels of false positive or negative results. False positives occur when the colorimetric reagents produce the color associated with the presence of an analyte when the analyte is not present, while false negatives occur when the analyte of interest is present in the sample, but the expected color is not produced. False positives are often the result of constituents in the sample that the colorimetric test cannot distinguish from the analyte of interest. False negatives often result from sample constituents that interfere with the chemical reaction that provides the color associated with the analyte.
As can be seen from the above description, there is an ongoing need for colorimetric tests that can identify trace amounts of a broader scope of analytes. Furthermore, colorimetric tests having a lower incidence of false positive and/or negative results also would provide significant benefit.
In one aspect of the invention, a sensor system is disclosed that includes a nucleic acid enzyme, a substrate for the nucleic acid enzyme, first particles, and invasive DNA. The substrate may include first polynucleotides and the first particles may include second polynucleotides that are coupled to the first particles. The invasive DNA may include fourth polynucleotides. The first polynucleotides may be at least partially complementary to the second and fourth polynucleotides. The sensor system also may include second particles that include third polynucleotides that are at least partially complementary to the first polynucleotides.
In another aspect of the invention, a method of detecting an analyte is disclosed that includes combining an aggregate, a sample, and invasive DNA to detect a color change responsive to the analyte. The aggregate may include a substrate and first particles. The aggregate also may include second particles and an endonuclease.
In another aspect of the invention, a kit for detecting an analyte is disclosed that includes a first container containing a system for forming aggregates that includes first polynucleotides and first particles and a second container containing invasive DNA.
In order to provide a clear and consistent understanding of the specification and claims, the following definitions are provided.
The term “sample” or “test sample” is defined as a composition that will be subjected to analysis that is suspected of containing the analyte of interest. Typically, a sample for analysis is in a liquid form, and preferably the sample is an aqueous mixture. A sample may be from any source, such as an industrial sample from a waste-stream or a biological sample, such as blood, urine, or saliva. A sample may be a derivative of an industrial or biological sample, such as an extract, a dilution, a filtrate, or a reconstituted precipitate.
The term “analyte” is defined as one or more substance potentially present in the sample. The analysis process determines the presence, quantity, or concentration of the analyte present in the sample.
The term “colorimetric” is defined as an analysis process where the reagent or reagents constituting the sensor system produce a color change in the presence or absence of an analyte.
The term “sensitivity” refers to the lower concentration limit at which a sensor system can detect an analyte. Thus, the more sensitive a sensor system is to an analyte, the better the system is at detecting lower concentrations of the analyte.
The term “selectivity” refers to the ability of the sensor system to detect the desired analyte in the presence of other species.
The term “hybridization” refers to the ability of a first polynucleotide to form at least one hydrogen bond with at least one second nucleotide under low stringency conditions.
The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale and are not intended to accurately represent molecules or their interactions, emphasis instead being placed upon illustrating the principles of the invention.
In a related application, U.S. Ser. No. 10/144,679, filed May 10, 2002, entitled “Simple catalytic DNA biosensors for ions based on color changes,” a calorimetric sensor was disclosed that in one aspect utilized heating to speed the analyte catalyzed disaggregation of an aggregate. In this prior sensor system, a sample was added to a DNAzyme/Substrate/particle aggregate. The mixture was then heated to bring about the disaggregation of the aggregate if the sample included the selected analyte.
The present invention makes use of the discovery that the addition of invasive DNA to a DNA-RNAzyme/Substrate/particle aggregate can speed disaggregation of the aggregate without heating. In this manner, a light-up calorimetric sensor is provided that undergoes the desired color change in response to a selected analyte at room temperature, thus overcoming a disadvantage of the sensor system disclosed in U.S. Ser. No. 10/144,679.
In one aspect, the analyte 105 may be any ion that can serve as a co-factor for a cleavage reaction, as discussed further below. Preferable monovalent metal ions having a +1 formal oxidation state (I) include Li(I), Tl(I), and Ag(I). Preferable divalent metal ions having a +2 formal oxidation state (II) include Mg(II), Ca(II), Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Cu(II), Pb(II), Hg(II), Pt(II), Ra(II), Sr(II), Ni(II), and Ba(II). Preferable trivalent and higher metal ions having +3 (III), +4 (IV), +5 (V), or +6 (VI) formal oxidation states include Co(II), Cr(III), Ce(IV), As(V), U(VI), Cr(VI), and lanthanide ions. More preferred analyte ions include Ag(I), Pb(II), Hg(II), U(VI), and Cr(VI) due to the toxicity of these ions to living organisms. At present, and especially preferred analyte ion is Pb(II).
Once the analyte 105 is selected in 110, in 120 directed evolution 122 may be performed to isolate nucleic acid enzymes, such as DNAzyme 124 or RNAzyme 126, which will catalyze substrate cleavage in the presence of the analyte. The directed evolution 122 is preferably a type of in vitro selection method that selects molecules on the basis of their ability to interact with another constituent. Thus, the procedure of the directed evolution 122 may be selected to provide the DNA-RNAzymes that demonstrate enhanced substrate cleavage in the presence of the selected analyte 105 (thereby providing sensor sensitivity). The procedure also may be selected to exclude DNA-RNAzymes that demonstrate cleavage in the presence of selected analytes, but additionally demonstrate cleavage in the presence of non-selected analytes and/or other species present in the sample 102 (thereby providing sensor selectivity).
The directed evolution 122 may be any selection routine that provides nucleic acid enzymes that will catalyze the cleavage of a substrate in the presence of the desired analyte with the desired sensitivity and selectivity. In one aspect, the directed evolution 122 may be initiated with a DNA library that includes a large collection of strands (e.g. 1016 sequence variants), each having a different variation of bases. Phosphoramidite chemistry may be utilized to generate the strands. The DNA library is then screened for strands that bind the analyte. These strands are isolated and amplified, such as by PCR. The amplified strands may then be mutated to reintroduce variation. These strands are then screened for strands that more effectively bind the analyte. By repeating the selection, amplification, and mutation sequence while increasing the amount of binding efficiency required for selection, strands that more effectively bind the analyte, thus providing greater sensitivity, may be generated.
In one aspect, a technique referred to as in vitro selection and evolution may be utilized to perform the directed evolution 122. Details regarding this technique may be found in Breaker, R. R., Joyce, G. F., “A DNA enzyme with Mg2+-dependent RNA phosphoesterase activity,” Chem. Biol. 1995, 2:655-660; and in Jing Li, et al., “In Vitro Selection and Characterization of a Highly Efficient Zn(II)-dependent RNA-cleaving Deoxyribozyme,” Nucleic Acids Res. 28, 481-488 (2000).
In another aspect, nucleic acid enzymes having greater selectivity to a specific analyte may be obtained by introducing a negative selection process into the directed evolution 122. After selecting the strands having high sensitivity to the analyte, a similar selection, amplification, and mutation sequence may be applied, but to be selected, the strand must not bind closely related analytes.
For example, a DNAzyme may be selected that specifically binds Pb(II), while not significantly binding Mg(II), Ca(II), Co(II), or other competing metal ions. In one aspect, this may be achieved by isolating DNAzymes that bind Pb(II), then removing any DNAzymes that bind Mg(II), Ca(II), or Co(II). In another aspect, DNAzymes that bind Mg(II), Ca(II), or Co(II) are first discarded and then those that bind Pb(II) are isolated. In this manner, the selectivity of the DNAzyme may be increased. Details regarding one method to increase DNAzyme selectivity may be found in Bruesehoff, P. J., et al., “Improving Metal Ion Specificity During In Vitro Selection of Catalytic DNA,”Combinatorial Chemistry and High Throughput Screening, 5, 327-355 (2002).
The DNA-RNAzymes 124, 126 are nucleic acid enzymes having the ability to catalyze chemical reactions, such as hydrolytic cleavage, in the presence of a co-factor. The DNAzyme 124 includes deoxyribonucleotides, while the RNAzyme 126 includes ribonucleotides. The nucleotides from which the DNA-RNAzyme 124, 126 are formed may be natural, unnatural, or modified nucleic acids. Peptide nucleic acids (PNAs), which include a polyamide backbone and nucleoside bases (available from Biosearch, Inc., Bedford, Mass., for example), also may be useful.
The table below lists specific analytes, the Figure in which the corresponding nucleic acid enzyme sequence that utilizes the analyte as a cleavage co-factor may be found, and the reference or references where each nucleic acid enzyme sequence is described.
While both DNAzymes and RNAzymes can form duplexes with a DNA-based substrate, such as substrate 134 discussed below, the RNAzyme/Substrate duplex may be less stable than the DNAzyme/Substrate duplex. Additionally, DNAzymes are easier to synthesize and more robust than their RNAzyme counterparts.
The deoxyribonulceotides of the DNAzyme 124 and the complementary substrate strand 134 may be substituted with their corresponding ribonucleotides, thus providing the RNAzyme 126. For example, one or more ribo-cytosines may be substituted for the cytosines, one or more ribo-guanines may be substituted for the guanines, one or more ribo-adenosines may be substituted for the adenosines, and one or more uracils may be substituted for the thymines. In this manner, nucleic acid enzymes including DNA bases, RNA bases, or both may independently hybridize with complementary substrate strands that include DNA bases, RNA bases, or both.
After selecting an appropriate nucleic acid enzyme or enzymes in 120, an aggregate 132 may be formed in 130. The aggregate 132 includes the nucleic acid enzymes; the substrate 134; and oligonucleotide functionalized particles 136. Considering the physical size of its components, the aggregate 132 may be quite large. In fact, transmission electron microscopy (TEM) studies suggest that individual aggregates may range from 100 nm to 1 micron, and may agglomerate to form larger structures.
The substrate 134 may be any oligonucleotide that may hybridize with and be cleaved by the nucleic acid enzyme in the presence of the analyte 105. The oligonucleotide may be modified with a cleavage species, which allows cleavage of the substrate into two fragments by the nucleic acid enzyme. In one aspect, the substrate 134 is a strand complementary to the nucleic acid enzyme and may be extended to form a 12-mer overhang on each end to hybridize with the oligonucleotide functionalized particles 136. For example, if an oligonucleotide functionalized particle had a base sequence of 5′-CACGAGTTGACA, an appropriate overhang sequence for the substrate could be 3′-GTGCTCAACTGT.
Because the particles 136 demonstrate distance-dependent optical properties, the particles are one color when closely held in the aggregate 132 and undergo a color change as the distance between the particles increases. For example, when the particles 136 are gold nanoparticles, the aggregate 132 displays a blue color in aqueous solution that turns red as disaggregation proceeds. Disaggregation occurs when the substrate 134 that holds the functionalized particles 136 together is cleaved, thereby allowing the particles to separate from the aggregate 132. Thus, as the particles 136 diffuse away from the aggregate 132, the solution changes from blue to red.
The particles 136 may be any species that demonstrate distance-dependent optical properties and are compatible with the operation of the sensor system. Suitable particles may include metals, such as gold, silver, copper, and platinum; semiconductors, such as CdSe, CdS, and CdS or CdSe coated with ZnS; and magnetic colloidal materials, such as those described in Josephson, Lee, et al., Angewandte Chemie, International Edition (2001), 40(17), 3204-3206. Specific useful particles may include ZnS, ZnO, TiO2, Agl, AgBr, Hgl2, PbS, PbSe, ZnTe, CdTe, In2S3, In2Se3, Cd3P2, Cd3As2, InAs, and GaAs.
In a preferred aspect, the particles are gold (Au) nanoparticles and have an average diameter from 5 to 70 nanometers (nm) or from 10 to 50 nm. In an aspect especially preferred at present, gold nanoparticles having an average diameter of from 10 to 15 nm are functionalized to the oligonucleotides.
For a more detailed treatment of how to prepare gold functionalized oligonucleotides, See U.S. Pat. No. 6,361,944; Mirkin, et al., Nature (London) 1996, 382, 607-609; Storhoff, et al., J. Am. Chem. Soc. 1998, 20, 1959-1064; and Storhoff, et al., Chem. Rev. (Washington, D.C.) 1999, 99, 1849-1862. While gold nanoparticles are presently preferred, other fluorophores, such as dyes, inorganic crystals, quantum dots, and the like that undergo a distance-dependent color change also may be attached to oligonucleotides and utilized.
In 140 the aggregate 132 from 130 may be combined with the sample 102 and invasive DNA 144. In 150 the sample 102 is monitored for a color change. If a color change does not occur, then the analyte 105 is not present in the sample 102. If a color change does occur in 160, the analyte 105 is present in the sample 102. Thus, the analytic method 100 provides a “light-up” sensor system because a color change occurs in the presence of the analyte 105.
The color change signifies that the analyte 105 is an appropriate co-factor to catalyze cleavage of the substrate 134, which is hybridized with the oligonucleotide functionalized particles 136. This cleavage is believed to cause the substrate 134 to split into two fragments, thus allowing the particles 136 to diffuse away from the aggregate 132 and into the solution of the sample 102. While this cleavage of the substrate 134 is believed to proceed at room temperature, it is thought that a significant portion of the 9 base pairs forming each cleaved portion of the substrate remain hybridized with the nucleic acid enzyme. Thus, for disaggregation to occur, it is preferable to disrupt this hybridization.
The invasive DNA 144 is believed to “invade” the aggregate 132 and assist in releasing of the cleaved substrate fragments. While not wishing to be bound by any particular theory, it is believed that equilibrium forces causes a competition for the sites on the cleaved portions of the substrate 134 to occur between the nucleic acid enzyme and the invasive DNA 144. Because this equilibrium favors hybridization of the substrate 134 with the invasive DNA 144, the cleaved portions of the substrate 134 are pulled away from the nucleic acid enzyme, thus speeding disaggregation. As the cleaved portions of the substrate 134 hybridize with the invasive DNA 144, the attached particles 136 diffuse away from the aggregate 132 and provide the desired color change. Although the term “invasive DNA” is used throughout this specification and appended claims for consistency, if the substrate 134 includes ribonucleotides, the invasive DNA 144 also may include ribonucleotides.
While the invasive DNA 144 may be any oligonucleotide that is at least partially complementary to the cleaved fragments of the substrate 134, preferably, the invasive DNA 144 includes relatively short pieces of DNA. In one aspect, the invasive DNA 144 includes at least two types of DNA strands, each being at least partially complementary to one of the two cleaved substrate fragments. In another aspect, at least one terminal base of each of the cleaved substrate fragments is complementary to at least one terminal base of each of the invasive DNA strands. In yet another aspect, the invasive DNA 144 includes at least two types of DNA strands, each being fully complementary to one of the two cleaved substrate fragments. In yet another aspect, the invasive DNA 144 has from 2 to 10 or from 4 to 8, which includes 2, 4, 6, or 8 fewer bases capable of hybridizing with the corresponding cleaved substrate fragment. At present, especially preferred invasive DNA strands have 6 fewer complementary bases than the corresponding cleaved substrate fragment.
The degree the color changes in response to the analyte 105 may be quantified by colorimetric quantification methods known to those of ordinary skill in the art in 170. Various color comparator wheels, such as those available from Hach Co., Loveland, Colo. or LaMotte Co., Chestertown, Md. may be adapted for use with the present invention. Standard samples containing known amounts of the selected analyte may be analyzed in addition to the test sample to increase the accuracy of the comparison. If higher precision is desired, various types of spectrophotometers may be used to plot a Beer's curve in the desired concentration range. The color of the test sample may then be compared with the curve and the concentration of the analyte present in the test sample determined. Suitable spectrophotometers include the Hewlett-Packard 8453 and the Bausch & Lomb Spec-20.
In yet another aspect, the method 100 may be modified to determine the sensitivity and selectivity of an endonuclease, such as a nucleic acid enzyme, for detecting the analyte 105. In this aspect, an aggregate is formed from the substrate 134 and the particles 136, but without the DNA-RNAzymes 124, 126 in 130. This aggregate is combined with the analyte of interest and the invasive DNA in 140. The endonuclease, such as one created by the directed evolution 122, is then added. If the endonuclease can cleave the substrate 134 with the desired sensitivity and selectivity in the presence of the analyte 105, the endonuclease may be used to analyze for the analyte 105 in a calorimetric sensor system. In this aspect, the endonuclease or nucleic acid enzyme also may be considered an analyte. In this manner, multiple endonucleases generated from the directed evolution 122 may be tested for use in a calorimetric sensor system.
The base pairing regions of the DNAzyme 224 and the complementary substrate strand 234 may be extended or truncated, as long as sufficient bases exist to maintain the desired cleavage of the substrate. While many modifications to the enzyme and substrate are possible, modifications made to the catalytic core region of the enzyme can have significant effects on the catalytic efficiency or analyte specificity of the enzyme. A more detailed discussion of such modifications and the resulting effects on catalytic activity may be found in Brown, A., et al., “A Lead-dependent DNAzyme with a Two-Step Mechanism,” Biochemistry, 42, 7152-7161 (2003).
The ribo-adenosine (rA) 235 provides a cleavage site 237, where the DNAzyme 224 is believed to hydrolytically cleave the substrate 234 in the presence of the co-factor, as depicted in
The DNAzyme 224 and the complementary substrate strand 234 may be separate strands, as depicted in
The invasive DNA 344 includes a 3′ strand 387 and a 5′ strand 386. In the presence of the analyte 305 and the invasive strands 386 and 387, the blue aggregate 332 begins to disaggregate to form partial aggregate 390. This partial disaggregation adds red color to the blue solution as the particles diffuse away from the aggregate 332, thus giving a purple solution. If enough of the analyte 305 is present in the sample, the reaction will continue until the aggregate 332 is completely disaggregated, to give 395. This results in a red solution due to the greater distance between the nanoparticles.
The alignment of the particles (tail-to-tail or head-to-tail) with respect to each other may influence how tightly the moieties that form the aggregate bind together.
At present, the tail-to-tail hybridization arrangement of
The ionic strength of the sample may influence how tightly the moieties that form the aggregate bind together. Higher salt concentrations favor aggregation, thus slowing sensor response, while lower salt concentrations may lack the ionic strength necessary to maintain the aggregates. In one aspect, the sample may include or be modified with a reagent to include a monovalent metal ion concentration of 30 mM and greater. The ionic strength of the sample may be modified with Na+ ions, for example. In a preferred aspect, the monovalent metal ion concentration of the sample, which contains the aggregate, is from 28 to 40 mM. At present, an especially preferred monovalent metal ion concentration is about 30 mM. pH also may influence the aggregate binding, possibly attributable to the protonation of the polynucleotide base pairs at lower pH. In one aspect, an approximately neutral pH is preferred.
Thus, the performance of the sensor may be improved by adjusting the ionic strength and pH of the sample prior to combining it with the aggregate. Depending on the sample, it may be preferable to add the sample or analyte to a solution containing the aggregate (where the ionic strength and pH may be controlled) or the reverse.
The sensor system, including the substrate, oligonucleotide functionalized particles, and invasive DNA may be provided in the form of a kit. In one aspect, the kit includes the desired analyte specific endonuclease or nucleic acid enzyme that is at least partially complementary to the substrate. In yet another aspect, the kit excludes the endonuclease/nucleic acid enzyme, which is then provided by the user or provided separately. In this aspect, the kit also may be used to determine the specificity and/or selectivity of various endonucleases to a selected analyte. Thus, the kit may be used to select an appropriate endonuclease in addition to detecting the analyte. In yet another aspect, the kit includes an exterior package that encloses a DNAzyme, a complementary substrate, oligonucleotide functionalized particles, and invasive DNA.
One or more of these components may be separated into individual containers, or they may be provided in their aggregated state. If separated, the aggregate may be formed before introducing the sample. The invasive DNA may be held in a separate container so it may be added to the sample prior to combination with the aggregate. Additional buffers and/or pH modifiers may be provided in the kit to adjust the ionic strength and/or pH of the sample.
The containers may take the form of bottles, tubs, sachets, envelopes, tubes, ampoules, and the like, which may be formed in part or in whole from plastic, glass, paper, foil, MYLAR®, wax, and the like. The containers may be equipped with fully or partially detachable lids that may initially be part of the containers or may be affixed to the containers by mechanical, adhesive, or other means. The containers also may be equipped with stoppers, allowing access to the contents by syringe needle. In one aspect, the exterior package may be made of paper or plastic, while the containers are glass ampoules.
The exterior package may include instructions regarding the use of the components. Color comparators; standard analyte solutions, such as a 10 μm solution of the analyte; and visualization aids, such as thin layer chromatography (TLC) plates, test tubes, and cuvettes, also may be included. Containers having two or more compartments separated by a membrane that may be removed to allow mixing may be included. The exterior package also may include filters and dilution reagents that allow preparation of the sample for analysis.
In another aspect, in addition to the sensor system of the present invention, the kit also may include multiple sensor systems to further increase the reliability of analyte determination and reduce the probability of user error. In one aspect, multiple light-up sensor systems in accord with the present invention may be included. In another aspect, a “light-down” sensor system may be included with the light-up sensor system of the present invention.
The presently claimed sensor system may be considered a light-up sensor because a color change occurs (blue to red) in the presence of the analyte. Conversely, in a light-down sensor system, a color change is not observed in the presence of the analyte. Thus, a light-up system may give a false result by lighting up when the analyte is absent, while a light-down sensor system may not undergo a color change when the analyte is present. Combining a sensor system using light-down chemistry with the presently claimed light-up sensor may reduce the probability of an inaccurate analyte determination.
Suitable light-down sensors for inclusion in the presently claimed kit may rely on DNAzyme/Substrate/particle aggregates that are not formed in the presence of the selected analyte. Thus, for these sensors, a color change from aggregate formation is observed when the selected analyte is not present in the sample. In one aspect, these light-down sensors may rely on a tail-to-tail particle arrangement coupled with nanoparticles having average diameters of about 43 nm to provide aggregation at room temperature in the absence of the analyte. A more detailed description of suitable light-down sensor systems for inclusion in the presently claimed kit may be found, for example, in U.S. patent application Ser. No. 10/756,825, filed Jan. 13, 2004, entitled “Biosensors Based on Directed Assembly of Particles,” which is hereby incorporated by reference.
The preceding description is not intended to limit the scope of the invention to the preferred embodiments described, but rather to enable a person of ordinary skill in the art to make and use the invention. Similarly, the examples below are not to be construed as limiting the scope of the appended claims or their equivalents, and are provided solely for illustration. It is to be understood that numerous variations can be made to the procedures below, which lie within the scope of the appended claims and their equivalents.
All DNA samples were purchased from Integrated DNA Technology Inc., Coralville, Iowa. The substrates and enzyme portions of the DNAzyme were purified by HPLC prior to use. Gold nanoparticles having an average diameter of 13 nm were prepared and functionalized with 12-mer thiol-modified DNA following literature procedures, such as those disclosed in Storhoff, J., et al., “One-pot calorimetric differentiation of polynucleotides with single base imperfections using gold particle probes,” JACS 120: 1959-1964 (1998), for example. The average diameter of the gold nanoparticles was verified by transmission electronic microscope (JEOL 2010).
The enzyme (17E, 400 nM), substrate (35SubAu, 100 nM), 3′DNAAu (6 nM), and 5′DNAAu (6 nM) were mixed with a 25 mM Tris acetate buffer, pH 8.2, 300 mM NaCl. The mixture (usually in 1 mL volume) was heated at 65° C. for 3 minutes and allowed to cool slowly to room temperature for approximately 4 hours. Blue-colored nanoparticle aggregates formed and precipitated. Optionally, the aggregates were further precipitated with a bench-top centrifuge and the supernatant was removed. The precipitated aggregates were washed three times with a buffer containing 100 mM NaCl and 25 mM tris acetate (pH 8.2) and re-dispersed in 200 μL of fresh 25 mM Tris acetate buffer, but with 100 mM NaCl.
The concentration of the aggregates in this undiluted mixture was standardized by adding 10 μL of the aggregate containing mixture to 80 μL of deionized water to disperse the aggregates. The extinction of this 9× diluted mixture was then measured at 522 nm. From this measurement, the amount of buffer solution required to provide an extinction value of 1 at 522 nm was calculated for the undiluted mixture. The appropriate amount of buffer containing 100 mM of NaCl was then added to the undiluted mixture. In this manner, the aggregate concentration in the buffer solution was adjusted so that when disaggregated; the mixture would provide an extinction of ˜9 or of ˜1 after 9 dilutions at 522 nm.
The sequences of the 17E DNAzyme (SEQ ID NO: 1) and the 17DS substrate (SEQ ID NO: 2; r denotes a single ribonucleotide) extended on each end with 12 bases 35SubAu (SEQ ID NO: 3; r denotes a single ribonucleotide) to hybridize to the 5′DNAAu (SEQ ID NO: 4) and 3′DNAAu (SEQ ID NO: 5) oligonucleotide functionalized gold nanoparticles are given in Table 1 below.
All sequences are listed from 5′ to 3′.
rA denotes the cleavage site of the substrate (35SubAu), with r being a single nucleotide.
For 5′DNAAU and 3′DNAAU, a gold nanoparticle was attached via a 5′- and 3′-end thiol-linkage, respectively.
An 80 μL solution including 21 mM NaCl, 25 mM tris acetate (pH 8.2), 2.25 μM invasive DNA, and a Pb(OAc)2 concentration 12.5% higher than desired in the test sample was combined with 10 μL of the 100 mM NaCl solution containing the aggregates from Example 1. The resultant test sample had concentrations of 30 mM NaCl, 2 μM invasive DNA, and the desired Pb(II) concentration. The color change of the solution was determined after about 5 minutes at ˜22° C.
The color change of the sample from Example 2 was monitored by UV-vis extinction spectroscopy.
These experiments demonstrated the utility of the sensor system to detect an analyte, but suggested that selection of the appropriate invasiveness of the invasive DNA could provide a sensor with decreased background color change and a decreased propensity to give false positives.
To find a less invasive DNA, a series of invasive DNA strands having a reduced number of base-pairings with the cleaved fragments of the DNA substrate were tested as follows. A quarts UV-vis spectrophotometer cell (Hellma, Germany) was prepared as a blank by combining 60.3 μL of 25 mM tris acetate (pH 8.2), 17 μL of 100 mM NaCl-25 mM tris acetate (pH 8.2), 1.8 μL of 0.1 mM invasive DNA, and 1 μL of 1 mM Pb(OAc)2. After a baseline measurement, 10 μL of the aggregate mixture from Example 1 was added to the cell. This addition gave a final NaCl concentration of 30 mM and a final invasive DNA concentration of 2 μM for each DNA strand. The final Pb(II) concentration was 10 μM. Samples without the Pb(II) analyte were similarly prepared, except 61.3 μL instead of 60.3 μL of the 25 mM tris acetate (pH 8.2) buffer was added to make up the sample volume.
The preferred reduced base-pairing invasive DNA strands that were prepared as outlined above are listed as Inva-2 (SEQ ID NO: 8 and SEQ ID NO: 9, left to right, respectively), Inva-4 (SEQ ID NO: 10 and SEQ ID NO: 11), Inva-6 (SEQ ID NO: 12 and SEQ ID NO: 13), and Inva-8 (SEQ ID NO: 14 and SEQ ID NO: 15) in Table 2 below. Inva refers to the 22-mer invasive DNA strands (SEQ ID NO: 6 and SEQ ID NO: 7) utilized to generate the data for
From “Inva” to “Inva-8”, each invasive DNA contains two DNA strands, each being at least partially complementary to one of the two cleaved portions of the substrate, respectively. Inva-6 (highlighted) was selected for the sensor design due to the rapid color response and low background increase.
Additional reduced base-pairing invasive strands that were tested are listed as Inva-2A (SEQ ID NO: 18 and SEQ ID NO: 19, left to right, respectively), Inva-4A (SEQ ID NO: 20 and SEQ ID NO: 21), Inva-6A (SEQ ID NO: 22 and SEQ ID NO: 23), and Inva-8A (SEQ ID NO: 24 and SEQ ID NO: 25) in Table 3 below. Inva-A refers to the 23 and 21-mer invasive DNA strands (SEQ ID NO: 16 and SEQ ID NO: 17) utilized in Example 3 and to generate the data for
From “Inva-A” to “Inva-8A”, each invasive DNA contains two DNA strands, each being at least partially complementary to one of the two cleaved portions of the substrate, respectively. Of these additional sequences, Inva-4A (highlighted) had the best combination of color response and low background increase.
The preferred Inva DNA sequences are fully complementary to each of the two fragments of the cleaved substrate, while the additional Inva-A DNA sequences are partially complementary, being mismatched by one base. While reducing the number of base-pairings for either the fully complementary Inva or the mismatched Inva-A strands decreased the overall invasiveness of the DNA and provided a desirable reduction in the level of background color change without the analyte, the reduced base Inva-6 strands maintained a rapid rate of disaggregation in the presence of the analyte.
Thus, the Inva-6 strands, having 6 fewer bases than the cleaved portions of the substrate, were chosen as the best compromise between the rate of color change in response to the analyte and the level of background color change attributable to invasive DNA only disaggregation. For these reasons, the Inva-6 strands were used to test the sensitivity and selectivity of the sensor.
While not wishing to be bound by any particular theory it is believed that the number of complementary base-pairings has a greater effect on invasiveness (thermodynamic control), while the rate of disaggregation is more strongly dependant on the ability of the ends of the cleaved substrate fragments to initially hybridize with the invasive DNA strands (kinetic control). By altering these parameters of the invasive DNA, the background levels and rate of the color change may be optimized for a specific DNA-RNAzyme and/or analyte.
In addition to reducing complementarity by reducing the base number of the invasive DNA strands in relation to the cleaved substrate fragments, other methods of reducing complementarity also may be used. For example, the invasive DNA strands may include bases that do not effectively hybridize with the bases of the cleaved substrate fragments. In another aspect, the bases from which the substrate and the invasive DNA are assembled may be selected to more weakly hybridize in relation to other base-pairs. Other methods of reducing the strength of hybridization between the cleaved substrate fragments and the invasive DNA strands as known to one of ordinary skill in the art also may be used.
In a quarts UV-vis spectrophotometer cell (Hellma, Germany), 60.3 μL of 25 mM tris acetate (pH 8.2), 17 μL of 100 mM NaCl-25 mM tris acetate (pH 8.2), 1.8 μL of 0.1 mM Inva-6 invasive DNA, and 1 μL of a 0.5 mM solution containing a metal salt were combined. Samples were prepared that included the following metal salts: Pb(OAc)2, CoCl2, ZnCl2, CdCl2, NiCl2, CuCl2, MgCl2, and CaCl2. After a baseline measurement, 10 μL of the aggregate mixture from Example 1 was added to each cell. This addition gave a final NaCl concentration of 30 mM, a final Inva-6 invasive DNA concentration of 2 μM for each DNA strand, and a final metal ion concentration of 5 μM for each metal tested. After complete dispersion, the extinction at 522 nm was ˜1.
The dispersion kinetics for each metal ion was monitored as a function of time using a Hewlett-Packard 8453 spectrophotometer.
In addition to the instrumental method of
To facilitate the rapid dispersion of the aggregates from Example 1, the aggregates were suspended in a buffer containing NaCl to determine the lowest NaCl concentration capable of stabilizing the aggregates.
As any person of ordinary skill in the art will recognize from the provided description, figures, and examples, that modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of the invention defined by the following claims and their equivalents.
This subject matter of this application may have been funded in part under the following research grants and contracts: DOE Grant No. DEFG02-01-ER63179, NSF CTS-0120978, and NSF DMR-0117792. The U.S. Government may have rights in this invention.
