G-rich isoDNA sequences forming G-quadruplex structure, their function as aptamers and a process for the preparation thereof

Abstract

The present invention relates to G-quadruplex forming isoDNA aptamers and a process for the preparation thereof. The present invention further relates to a stable, non-genetic, guanine rich 2-5 linked isoDNA selected from 3 deoxy 2-5 isoDNA and 3 deoxy 2-5 isoDNA-isoRNA hybrid. The instant 2-5 linked isoDNA such as the thrombin binding aptamer (isoTBA) can be used in deep vein thrombosis, where prolonged anticoagulant activity is required.

Claims

1. G-quadruplex forming isoDNA oligomers comprising a stable, non-genetic, guanine rich 2-5 linked isoDNA sequence selected from a 3 deoxy 2-5 isoDNA sequence and a 3 deoxy 2-5 isoDNA-isoRNA hybrid sequence, wherein the G-quadruplex forming isoDNA oligomers are configured to bind thrombin, and wherein the 3 deoxy 2-5 isoDNA sequence is selected from the group consisting of TABLE-US-00005 SeqIDNo:3 5-GGTTGGUGUGGTTGG-2, SeqIDNo:4 5-GGITGGTTGGTTGG-2, SeqIDNo:5 5-GGTGGTGTGGTGG-2, and SeqIDNo:6 5-GGTGGTGGTGG-2, or the 3 deoxy 2-5 isoDNA-isoRNA hybrid sequence is selected from the group consisting of TABLE-US-00006 SeqIDNo:7 5-GGTTGG.sup.XU.sup.FGTGGTTGG-2, SeqIDNo:8 5-GGTTGG.sup.XU.sup.FG.sup.XU.sup.FGGTTGG-2, SeqIDNo:9 5-GGTTGG.sup.rU.sup.FGTGGTTGG-2, and SeqIDNo:10 5-GGTTGG.sup.rU.sup.FG.sup.rU.sup.FGGTTGG-2.

2. The G-quadruplex forming isoDNA oligomers of claim 1, wherein the 2-5 linked isoDNA sequence forms unimolecular antiparallel G quadruplexes.

3. The G-quadruplex forming isoDNA oligomers of claim 1, wherein the 2-5 linked isoDNA sequence forms hybrids with isoRNA.

4. The G-quadruplex forming isoDNA oligomers of claim 1, wherein the 2-5 linked isoDNA sequence is Seq ID NO: 2 or Seq ID NO: 3.

5. A medicament for the treatment of deep vein thrombosis comprising the G-quadruplex forming isoDNA oligomers of claim 1.

6. A composition comprising the G-quadruplex forming isoDNA oligomers of claim 1 and one or more pharmaceutically acceptable excipients.

Description

BRIEF DESCRIPTION OF DRAWING

(1) FIG. 1 depicts genetic 3-5 DNA/RNA, non-genetic 2-5-iso DNA/RNA and 3-2TNA.

(2) FIG. 2 depicts the schematic representation of the Guanine tetrad, antiparallel unimolecular G-quadruplex and multimolecular parallel G-quadruplex.

(3) FIG. 3 A depicts the Circular Dichroism (CD) spectra of oligomers dTBA-1, isod-TBA-2 and isod-TBA-3 in K+ buffer and FIG. 3B depicts Temperature dependent changes in CD amplitude at 295 nm plotted against temperature.

(4) FIG. 4 depicts the quadruplex imino proton spectral region for d-TBA-1 and isod-TBA-2 in the presence of K.sup.+ from 4 C. to 50 C. Inset shows the imino proton spectral region in the absence of K.sup.+ at 4 C.

(5) FIG. 5 depicts the changes in CD signal at 295 nm upon addition of thrombin FIG. 5a) d-TBA-1 5b) isod-TBA-2 and 5c) isod-TBA-3.

(6) FIG. 6 depicts anti-thrombin activity measured by UV transmittance in the presence of d-TBA-1, isod-TBA-2 and isod-TBA-3. FIG. 6b depicts induction time which is indicated as a coagulation parameter (t.sub.i).

(7) FIG. 7 depicts the CD amplitude at 295 nm versus temperature during heating to get T.sub.m of d-TBA-1 and isod-TBA-2 in presence of Na.sup.+ (5 M strand in 10 mM Na-phosphate buffer pH 7.5 containing 100 mM NaCl).

(8) FIG. 8 depicts the CD hysteresis changes is CD amplitude at 295 nm with temperature during heating and cooling experiments for d-TBA1, isod-TBA-2, isod-TBA-3

(9) FIG. 9 depicts UV hysteresis-changes is absorbance at 295 nm during heating and cooling experiments for d-TBA1, isod-TBA-2

(10) FIG. 10. A. depicts CD of d-TBA-1; B. depicts isod-TBA-2 with incremental additions of Bovine serum albumin (BSA) at 4 C.

(11) FIG. 11 depicts stability of the aptamers d-TBA-1, isod-TBA-2 towards snake venom phosphodiesterase (SVPD) enzyme.

(12) FIG. 12 depicts non-denaturing PAGE analysis showing the relative mobilities of TBA-1 and isoTBA-2. (a) visualized by UV shadowing and (b) visualized by ethidium bromide staining.

(13) FIG. 13 depicts non-denaturing PAGE analysis showing the quadruplex formation by TBA-1, isoTBA-2 and isoTBA-3, visualized by UV-shadowing.

DETAILED DESCRIPTION OF THE INVENTION

(14) The invention will now be described in detail in connection with preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated. As envisaged above, the backbone element is the crucial governing entity for maintaining the functional structural quadruplex topology of any given sequence. In the present invention, isoDNA as an alternative backbone for the possible quadruplex formation and as a plausible fore-runner of finding most suitable carriers of genetic information is considered for the following reasons. (1) The 2-5 linkages maintain an extended backbone geometry due to the anomeric effect and the O4-C1-C2-O2 gauche effect on the substituted sugar leading to the N-type geometry sugar conformations. (2) The guanine base orientation could be either syn or anti. (3) The 2-5-linkages are known to form stable loop structures in the hairpin DNA/RNA motifs, and (4) the isoDNA oligomers are relatively stable to exonuclease degradation. In a preferred embodiment, the present invention provides an aptamer/oligomer comprising a stable, non-genetic, guanine rich 2-5 linked isoDNA. These stable, non-genetic, guanine rich 2-5 linked isoDNA oligomers form unimolecular antiparallel G quadruplexes as studied by CD spectroscopy that maintain biological molecular recognition and is useful in therapeutics and diagnostics (Nagatoishi, S., Tanaka, Y., Tsumoto K. Circular dichroism spectra demonstrate formation of the thrombin-binding DNA aptamer G-quadruplex under stabilizing-cation-deficient conditions. Biochem. Biophys. Res. Commun. 352, 812-817, 2007). In an optional embodiment, the 2-5 linked iso oligonucleotide is selected from isoDNA. Accordingly, the 3 deoxy 2-5 isoDNA oligomers is selected from the group consisting of 5-GGTTGGTTGGTTGG-2 (Seq ID No. 4); 5-GGTGGTGTGGTGG-2 (Seq ID No. 5); and 5-GGTGGTGGTGG-2 (Seq ID No 6). In another embodiment, the present invention discloses hybrids involving 2-5-phosphodiester linkages as part of the G-quadruplex-forming sequence/oligomer. They could be in combination with 3-5-phosphodiester linkages in DNA/RNA, or other linkages such as in TNA, LNA, UNA, 3-5-linked modified backbones such as 2-OMe, etc. The 3 deoxy 2-5 isoDNA-isoRNA hybrid is selected from the group consisting of sequences 5% GGTTGG.sup.XU.sup.FGTGGTTGG-2 Id No 7; 5-GGTTGG.sup.XU.sup.FG.sup.XU.sup.FGGTTGG-2 Id No 8; 5-GGTTGG.sup.rU.sup.FGTGGTTGG-2 Id No 9; 5-GGTTGG.sup.rU.sup.FG.sup.rU.sup.FGGTTGG-2 Id No 10. Where .sup.XU.sup.F U is 3deoxy, 3-xylofluoro-uridine, .sup.rU.sup.F is 3deoxy, 3-ribofluoro-uridine. In another preferred embodiment, the present invention provides that the guanine-rich 2-5-isoDNA sequence forming 3D antiparallel quadruplex of the instant invention is a thrombin-binding oligomer or a thrombin binding aptamer (TBA). The terms oligomer and aptamer with reference to the thrombin-binding are used interchangeably, hence the person skilled in the art will appreciate the same as such. The thrombin binding oligomer or aptamer (TBA) binds and inhibits thrombin, leading to inhibition of clotting. SELEX (Systematic Evolution of Ligands by Exponential Enrichment) generated G-quadruplex-forming thrombin binding aptamer (TBA), 5-GGTTGGTGTGGTTGG-3 (Seq ID No:1) is used as a reference to study the structural topology of the regioisomeric 2-5 backbone (isoTBA). Accordingly, the present invention provides guanine-rich 2-5-isoDNA sequence having Seq ID No. 2 comprising 5-(3-deoxy-GGTTGGTGTGGTTGG)-2 isod-TBA-2 synthetic oligonucleotide. (Table 1). Further, by replacing 3-deoxythymidine in the TGT loop at T7 and T9 position in Seq ID No. 2 by uridine affords UGU loop-modified sequence isod-TBA-3 (i.e. Seq ID No.3) 5-(3-deoxy-GGTTGGUGUGGTTGG)-2. In yet another preferred embodiment, the present invention provides the process for synthesizing 2-5 linked iso oligonucleotides comprising: a. Synthesizing 2-5-linked oligonucleotides (ON Id No 2, 4-6)using 3-deoxy-2-ribothymidine phosphoramidites and 2-deoxy-3-guanosine phosphoramidites, by -cyanoethyl phosphoramidite chemistry; Sequences Id No 3 and Id No 7-10 were synthesized using 3-deoxy-2-ribothymidine phosphoramidites, 3-deoxy-2-guano sine phosphoramidite and uridine (U)-2-phosphoramidite, 3-ribofluoro-3-deoxy-uridine (.sup.rU.sup.F)-2-phosphoramidite, 3-xylo fluoro-3-deoxy-uridine (.sup.xU.sup.F)-2-phosphoramidite at appropriate positions in the sequences by -cyanoethyl phosphoramidite chemistry. a. Desalting the oligonucleotides after post-synthetic treatment by passing through Pharmacia NAP-5 columns followed by purification by RP-HPLC using an increasing gradient of acetonitrile in 0.1M triethylammonium acetate (pH 7.0). b. Replacing the 3-5-phosphodiester linkages in control Seq ID no. 1 (dTBA-1) by 2-5-phosphodiester linkages to give 3-deoxy-2-5 oligomer 5-GGTTGGTGTGGTTGG-2Seq ID No 2 (isod-TBA-2); c. Replacing 3-deoxythymidine in the TGT loop (T7 and T9 in isod-TBA-2) by uridine affords UGU loop-modified sequence isod-TBA-3 (Seq ID no: 3); and d. Dissolving HPLC purified and lyophilised TBA-1, iso-TBA-2 and isod-TBA-3 in phosphate buffer pH7.5 containing KCl followed by lyophilization and further dilution. e. The process also used CD spectroscopy techniques to show the emergence of a positive band at near 295 nm characteristic of G-quadruplex by 2-5isoTBA sequences (sequence Id No 2 and 3) in the presence of cations similar to 3-5TBA (sequence Id No 1). The process also used CD spectroscopy techniques to show the emergence of a positive band at near 295 nm characteristic of G-quadruplex by 2-5isoTBA sequences (sequence Id No 2 and 3) in presence of thrombin similar to 3-5TBA (sequence Id No 1).

(15) Accordingly, the phosphoramidites are selected from the group consisting of Uridine 3-O-TBDMS 2-phosphoramidite (used for modified isod-TBA-3 sequence) and 3-deoxy-2-phosphoramidites.

(16) Further, 3-5-phosphodiester linkages in control sequence dTBA-1 (Seq ID No. 1) is replaced by 2-5-phosphodiester linkages to give 3-deoxy-2-5 oligomer 5-GGTTGGTGTGGTTGG-2 isod-TBA-2 (Seq ID No. 2). Replacement of 3-deoxythymidine in the TGT loop (T7 and T9 in isod-TBA-2) by uridine affords UGU loop-modified sequence isod-TBA-3 (Seq ID No.3). The 3-OH group shifts the equilibrium to S-type sugar conformation and imparts rigidity and stability to the loop similar to that imparted by the N-type sugars in DNA. In another embodiment, the present invention provides synthesis of the hybrid with 3-5-linked DNA. In yet another embodiment, the formation of G-quadruplex and stability of the oligomers, 2-5-linked isod-TBA-2 and isod-TBA-3 and control sequence d-TBA-1 of the instant invention is determined in the presence of monovalent cations and thrombin. The 2-5-linked isod-TBA-2 and isod-TBA-3 and control sequence d-TBA-1 are observed to exhibit intense maxima at 295 nm in CD spectra in presence of K.sup.+ ions (FIG. 3A) which correspond to the previously-described group-III antiparallel G-quadruplex topology for 3-5-DNA. This indicates the formation of antiparallel folded G-quadruplex in the presence of monovalent cations for isoDNA backbone. The stability of the G-quadruplexes is followed by change in the amplitude of the CD signal at 295 nm with temperature (FIG. 3B). The replacement of the TGT by the UGU in the loop region (isod-TBA-3) is observed to improve the stability of the quadruplex structure as seen by a positive change in T.sub.m in presence of K.sup.+ (Table 1) showing the influence of loop geometry on quadruplex stability as found earlier for DNA-TBA. Further, the hysteresis between the heating and cooling curves is found to be negligible in all the cases indicating unimolecular folding into G-quadruplex form (FIG. 8). The results are further supplemented by UV-T.sub.m and hysteresis measurements (FIG. 9) and also by T.sub.m experiments at differing concentrations, viz., 5 M and 20 M. In a further embodiment, the stability of 2-5-linked isod-TBA-2 and isod-TBA-3 is determined in the presence of thrombin without adding monovalent cation. The CD maximum at 295 nm which emerges for isod-TBA-2 and isod-TBA-3 and the isoelliptic points observed at 280 nm and 255 nm clearly indicate the two-state nature of the structural transition. Further, the temperature-dependent stability of the quadruplex structure determined by recording the CD amplitude at 295 nm with increasing temperature indicate the specific role of thrombin in inducing quadruplex structures and clearly point out the similarity in structural topology of isoTBA with TBA, necessary for interaction with thrombin. The characterisation and melting temperatures of G-quadruplexes of the instant invention are summarized in Table 1. In another embodiment, the stable, non-genetic, guanine rich 2-5 linked isoDNA/RNA capable of forming G-quadruplexes are further characterised by mass spectroscopy, NMR UV-Thermal denaturation, fluorescence spectroscopy and polyacrylamide gel electrophoresis.

(17) In another embodiment, the present invention provides use of guanine rich 2-5 linked isoDNA/RNA for medical applications like anti-thrombin, anti-HIV, anti-proliferative, anti-cancer, Telomerase inhibition and the like. The instant of guanine rich 2-5 linked isoDNA such as TBA is used in deep vein thrombosis, where prolonged anticoagulant activity is required. In an optional embodiment the guanine rich 2-5 linked isoDNA/RNA of the instant invention may include pharmaceutical composition prepared by the method known in the art. In another embodiment, the present invention provides a method for the treatment of cardiovascular surgery in which anticoagulation is desired, in the treatment of HIV, cancer, as inhibitors of thrombin, telomerase comprising administering the said composition to a subject in need thereof orally, parenterally, intraperitoneally or the like. Thus, the present invention provides that the homogeneous 2-5 linked isoDNA/RNA is capable of forming hitherto unknown unimolecular, antiparallel G-quadruplex structures that can retain the functional ability. The guanine-rich 2-5-isoDNA sequence can form 3D antiparallel quadruplex, having similar structural topology as the DNA quadruplexes. The G-rich isoTBA sequence not only forms stable G-quadruplex which is structurally similar to TBA but also exhibits unprecedented acceptance for a 100% isoDNA backbone while binding to a large protein, thrombin, originally evolved to function with a DNA backbone.

EXAMPLES

(18) The following examples are given by way of illustration and should not be construed to limit the scope of the invention:

Example 1: Process for Preparation of Guanine-Rich 2-5-isoDNA

(19) 3-5- and 2-5-linked oligonucleotides were synthesized in-house on a Bioautomation Mermade-4 DNA synthesizer employing -cyanoethyl phosphoramidite chemistry. The 2-deoxy-3-phosphoramidites, Uridine 3-OTBDMS 2-phosphoramidite (used for modified isod-TBA-3 sequence) were obtained from ChemGenes and 3deoxy-2-phosphoramidites were obtained from Glen Research. Universal columns procured from Bioautomation were used for 2-5 oligomer synthesis. Oligonucleotides after post-synthetic treatment were desalted by passing through Pharmacia NAP-5 columns, then purified by RP-HPLC on a C18 column using a Waters system (Waters Delta 600e quaternary solvent delivery system and 2998 photodiode array detector and Empower2 chromatography software). An increasing gradient of acetonitrile in 0.1M triethylammonium acetate (pH 7.0) was used. For NMR studies, HPLC purified and lyophilised TBA-1 and iso-TBA-2 were dissolved in 150 l of 10 mM K-phosphate buffer pH 7.5 containing 100 mM KCl and lyophilized, then diluted in 9:1 v/v H.sub.2O:D.sub.2O (150 l) to get a concentration of 100 M of each oligomer.

(20) Characterization:

(21) 1. Mass Spectrometry

(22) Mass of the oligomers was obtained by MALDI-ToF mass spectrometry. The MALDI-ToF spectra were recorded on Voyager-De-STR (Applied Biosystems). The matrix used for analysis was THAP (2,4,6-trihydroxyacetophenone). The result is given in Table 1 below.

(23) 2. CD Thermal Denaturation Studies of the TBA Sequences with and without Thrombin and BSA.

(24) CD spectra were recorded on Jasco J-815 CD Spectrometer equipped with a Jasco PTC-424S/15 peltier system. 2 mm path-length quartz cuvettes were used for a sample volume 500 l and strand concentration of 5 M in 10 mM Na/K-phosphate buffer (pH 7.5) containing 100 mM NaCl/KCl respectively. Oligomers prepared in buffer were annealed by heating at 95 C. for 5 minutes, then slowly cooled to room temperature, followed by refrigeration for 5-6 hours before use. Spectral scans were collected over a wavelength range 200-320 nm at a scanning rate of 100 nm min.sup.1. The spectra were collected as an average of three scans for each sample.

(25) CD experiments with d-TBA-1, isod-TBA-2 and isod-TBA-3 were carried out in the presence of increasing concentrations of thrombin at low temperature (FIG. 5 a,b,c) Nagatoishi, S., Tanaka, Y., Tsumoto K. Circular dichroism spectra demonstrate formation of the thrombin-binding DNA aptamer G-quadruplex under stabilizing-cation-deficient conditions. Biochem. Biophys. Res. Commun. 352, 812-817 (2007). In the case of control d-TBA-1, a CD maximum at 295 nm was observed even in the absence of either thrombin or K.sup.+, revealing the preference of G-quadruplex structure as reported earlier.sup.10 and also as seen by .sup.1H NMR in the present studies. An increase in the CD signal amplitude upon incremental addition of thrombin is observed even at relatively low concentrations of thrombin. In case of isod-TBA-2 and isod-TBA-3, CD band at 295 nm and G-quadruplex structure was not evident in the absence of either thrombin or K.sup.+, which is also supported by .sup.1H NMR. Upon incremental addition of thrombin, the CD maximum at 295 nm emerges for isod-TBA-2 and isod-TBA-3. Isoelliptic points were observed at 280 nm and 255 nm in all the cases. Further, the temperature-dependent stability of the quadruplex structure was followed by recording the CD amplitude at 295 nm with increasing temperature. The strength of d-TBA-1 G-quadruplex was the highest with T.sub.m=22 C., followed by iso-d-TBA-3 (T.sub.m 13 C.) and iso-d-TBA-2 (T.sub.m<10 C.). The CD signal at 295 nm was restored upon addition of K.sup.+ and the quadruplexes were as stable as with K.sup.+ alone at the thrombin concentrations used in the present study (Table 1).

(26) TABLE-US-00003 TABLE1 OligomerssynthesizedwithMALDI-ToF massspectrometriccharacterizationand meltingtemperaturesofG-quadruplexes MALDI-ToF Mass T.sub.m C. TBA Calcu- Ob- Throm- No sequences lated served Na.sup.+ K.sup.+ bin.sup.a 1 5-(2-deoxy- 4726 4728 22.2 52.0 22 GGTTGGTGTGG (53) TTGG)-3 d-TBA-1 (SeqID No1) 2 5-(3-deoxy- 4726 4731 24.9 37.1 <10 GGTTGGTGTGG (38) TTGG)-2 isod-TBA-2 (SeqID No2) 3 5-(3-deoxy- 4730 4732 nd 45.0 13.2 GGTTGGUGUG (45) GTTGG)-2 isod-TBA-3 (SeqID No3) .sup.aValues in parenthesis are in presence of K.sup.+

(27) Changes in CD spectral amplitudes were not observed when serum albumin was used instead of thrombin in these experiments confirming the specific role of thrombin in inducing quadruplex structures. (FIG. 10B).

(28) TABLE-US-00004 TABLE2 Listofsequences IdNo1 5-GGTTGGTGTGGTTGG-3 IdNo2 5-GGTTGGTGTGGTTGG-2 IdNo3 5-GGTTGGUGUGGTTGG-2 IdNo4 5-GGTTGGTTGGTTGG-2 IdNo5 5-GGTGGTGTGGTGG-2 IdNo6 5-GGTGGTGGTGG-2 IdNo7 5-GGTTGG.sup.XU.sup.FGTGGTTGG-2 IdNo8 5-GGTTGG.sup.XU.sup.FG.sup.XU.sup.FGGTTGG-2 IdNo9 5-GGTTGG.sup.rU.sup.FGTGGTTGG-2 IdNo10 5-GGTTGG.sup.rU.sup.FG.sup.rU.sup.FGGTTGG-2
3. Imino Proton NMR Spectra:

(29) The NMR measurements were performed on a Bruker AV 500 NMR spectrometer operating at 500.13 MHz for .sup.1H using a 5 mm BBFO probe. Samples (150 uL of 100 uM solution) were prepared in a standard 3 mm NMR tube in 10% D.sub.2O and 90% H.sub.2O. Water suppression was achieved by using a standard Bruker watergate W5 pulse sequence with gradients. 2000 transients were collected with an acquisition time of 3.54 sec and a pulse delay of 1 sec. The raw data were processed with a Gaussian function for improvement of signal to noise ratio. Temperature during the measurements was controlled by means of a Bruker BVT 3000 unit.

(30) The characteristic chemical shifts of imino proton signals of the eight H-bonds formed between the guanines of each G-quartet of a quadruplex structure were observed between 11.5 and 12.5 ppm range in the .sup.1H NMR spectrum. The imino proton chemical shifts for isod-TBA-2 were comparable with the control d-TBA-1 indicating the hydrogen-bonded quadruplex formation (FIG. 4). The temperature-dependent changes in the spectra were then recorded. As seen for the CD at 295 nm, the .sup.1H NMR signals between 11.5 and 12.5 ppm slowly disappeared with increasing temperature due to the loss of quadruplex structure near T.sub.m. In the case of d-TBA-1 and isod-TBA-2, the NMR signals were observed up to 50 C., and 40 C., respectively (FIG. 4).

(31) The broad peaks between 10.5-11.1 ppm in the isod-TBA-2 and d-TBA-1 spectra disappeared much below the melting temperature (20-25 C.) in each case, suggesting that these may not be involved in the H-bonded quadruplex structure. The appearance of characteristic H-bonded imino protons supports the fact that isod-TBA-2 does form a G-quadruplex structure. although less stable (Table 1, T.sub.m 37 C.) compared to the 3-5-linked d-TBA-1 (Table 1, T.sub.m 52 C.) quadruplex structure. It is evident from the .sup.1H NMR signals at 4 C. between 11.5 and 12.5 ppm that d-TBA-1 has significant quadruplex structure even in the absence of K.sup.+ whereas isod-TBA-2 is not structured without monovalent ions such as K.sup.+ (FIG. 4, Inset).

(32) 4. UV-Thermal Denaturation Studies of the TBA Oligomers:

(33) UV-Thermal denaturation studies of the TBA oligomers were performed using a 10 mm quartz cell in a Varian Cary 300 Bio UV-Visible Spectrophotometer. The TBA oligomers (5 M) were annealed in a 10 mM potassium phosphate buffer, pH 7.5, 100 mM KCl. The concentration was calculated on the basis of absorbance from molar extinction coefficients of the corresponding nucleobases of DNA/isoDNA. Absorbance versus temperature profiles were obtained by monitoring the absorbance at 295 nm from 10-85 C. at a ramp rate of 0.5 C. per minute. Both melting and re-annealing profiles were obtained to check reversibility of the process. A stream of dry nitrogen was gently applied through the sample compartment to prevent condensation of water on the cuvette walls at low temperatures.

(34) 5. Anti-Thrombin Activity Measurements:

(35) Anti-thrombin activity measurements were done by using Varian Cary 300 Bio UV-Visible spectrophotometer to measure % transmittance change over time. 0.1 NIH unit of thrombin (50 NIH/ml, bovine thrombin, Fibroscreen reagent, Tulip Diagnostics (P) LTD.) was added to the TBA-1, iso-TBA-2 aptamers dissolved in water to a concentration of 3.710.sup.8M and incubated for 15 minutes at 25 C. This was then added to 1 ml fibrinogen solution (Sigma product No F 3879, 310.sup.6 M) in saline and the transmittance was measured at 2 minute intervals for 90 minutes. The inhibitory activity of the aptamers on thrombin-catalyzed conversion of fibrinogen to fibrin (clotting) was determined by measuring the percent transmittance with time in the absence and presence of potassium ions. d-TBA-1 slowed down the coagulation with an increased induction time (t.sub.i as coagulation parameter), confirming its reported inhibitory activity (FIG. 6). The induction time for the isoDNA oligomers isod-TBA-2 and isod-TBA-3 were considerably less than for d-TBA-1, but more than that in the absence of any aptamer, with or without K.sup.+. Surprisingly, the higher thermal stability of isod-TBA-3 was not effectively reflected in its anti-thrombin activity. The experiment provides conclusive evidence that the isod-TBA oligomers are indeed capable of not only forming G-quadruplex structures but also hold similarity in structural topology capable of taking active part in the assigned function of the TBA, though less efficiently.

(36) 6. Enzymatic Stability of Aptamers to SVPD:

(37) Enzymatic hydrolysis of the aptamers d-TBA-1, isod-TBA-2 (7.5 M) was carried out at 37 C. in 100 l buffer (100 mM Tris-HCl (pH 8.5), 15 mM MgCl.sub.2, 100 mM NaCl) and SVPD (Snake Venom phosphordiesterase) (2 g, 1.210.sup.4 U) Aliquots were taken at several time intervals. Each aliquot was heated at 90 C. for 2 min to inactivate the nuclease enzyme. The intact oligomer at each time interval was monitored by RP-HPLC. Percentage of intact oligomer was plotted against time to show the degradation of oligomers with respect to time.

(38) To test the enzymatic stability of the isoDNA quadruplex-forming backbone as compared to the 3-5-DNA quadruplexes, the aptamers of the instant invention were treated to SVPD digestion (FIG. 11). The 2-5-linked isoDNA oligomer isod-TBA-2 was found to be digested much slower compared to the control d-TBA-1. The half-life of isoTBA-2 was found to be 120 min (FIG. 11) while that of TBA-1 was found to be 20 min at 37 C. The observed higher stability of the isoDNA oligomer offers obvious advantages for applications in biological systems, when the control unmodified oligomer has a relatively low half-life.

(39) 7. Non-Denaturing Gel Electrophoresis Study:

(40) The quadruplex-forming ability of the synthesized oligomers was assessed by non-denaturing polyacrylamide gel electrophoresis (PAGE). As reference oligonucleotides, two oligomers of differing lengths, viz., 5-dAACCGATTTCAG-3 (12-mer) and 5-dCACCATTGTCACACTCCA-3 (18-mer) were used. The TBA and isoTBA oligomers were annealed in buffer prior to loading on the gel. PAGE analysis indicated similar complex formation in the case of TBA-1 and isoTBA-2, evident from their similar mobility in gel. The gels were visualized by UV-shadowing and ethidium bromide staining (FIGS. 12 A and B). Thus, quadruplex formation was also confirmed by PAGE analysis for the oligomers of the current study (FIG. 13).

ADVANTAGE OF THE INVENTION

(41) 1. Stable to enzymatic degradation, non-genetic 2-5 linked isoDNA oligomers capable of forming unimolecular antiparallel G quadruplexes which maintain biological molecular recognition and is useful in therapeutics and diagnostics.