Thiophosphoryl modifications of the sugar-phosphate backbone of oligonucleotide provide important properties for DNA medical application. However, the thio-analogs have a systemic toxicity and the modifications can result in function-significant distortions of the oligomers tertiary structure (1). It is especially significant for aptamers, which biological activity is frequently directly conditioned by their conformation (2). In this connection, the developing of partially thiophosphorylated analogs of biologically significant oligonucleotides was carried out (3-4).

A series of antithrombin binding aptamer d(GGTTGGTGTGGTTGG) analogs carrying thiophosphoryl modifications at 14 definite localizations has been produced by original automatic parallel synthesis (5). Here, we studied conformations and thermodynamic stability of the 11 d(GGTTGGTGTGGTTGG) analogs. All studied tio-modification did not noticeably affect the oligonucleotide ability to form antiparallel quadruplex structure as revealed with circular dichroism (CD) spectra at 15 °C. The thermodynamic parameters of the aptamers formation were determined with analysis of thermal denaturation curves. Effects of specific thio-modifications on the aptamers stability were found out: (I) the substitutions placed between planes of G-quartets lead to a drop in free formation energy, the stability decreases linearly on a number of these modifications. (II) In contrast, the modifications made in the loops connecting the G4-quartets even slightly increase the quadruplex thermostability. Thus, thiophosphoryl modifications of the sugar-phosphate backbone placed in the aptamer quadruplex loops allows to construct aptamer analogs with an intact stable quadruplex conformation, which can be valuable in medical applications.

References and Footnotes

1. Besschetnova I. A. et al. J Biomolec Struct and Dynamics 22, 859-886 (2005).
2. Smirnov I., Shafer R. H. Biochem 39, 1462-1468 (2000).
3. Dean F. B. et al. Genome Res 11, 1095-1099 (2001).
4. Di Giusto D., King G. C. Nucl Acids Res 31, e7 (2003).
5. Lukyanova T. A. et al. Bioorg Khim 33in press (2007).