The influence of oligodeoxyribonucleotide phosphorothioate pyrimidine strands on triplex formation.

Analogues of the homopyrimidine oligonucleotide dT15 that contained phosphorothioate bonds of a mixture of diastereoisomers or one of the two stereoisomers (either Rp or Sp) were synthesized. The analogues were mixed under conditions conducive to the formation of triple-stranded assemblies. The mixtures were characterized by their thermal stabilities (Tm values), CD spectra, and gel electrophoresis pattern. The 34-mer duplexes containing 15 central purines on one strand and 15 complementary pyrimidines on the other strand gave no detectable triple helix upon combination with dT15S14. On the other hand, 34-bp duplexes with dT15S1, having Rp or Sp, formed triple helixes. This suggests that a steric factor plays an important role in triple helix formation.