LNA units present in [ R P -PS]-(DNA#LNA) chimeras enhance the thermal stability of parallel duplexes and triplexes formed with (2'-OMe)-RNA strands.

The results of CD measurements indicate that 2-4 LNA units distributed along 12 nt P-stereodefined phosphorothioate [ R P -PS]-(DNA#LNA) chimeras impose a C3'- endo conformation on the 2'-deoxyribonucleosides. Under neutral and slightly acidic conditions homopurine [ R p -PS]-(DNA#LNA) hybridizes with 9-12 nt Hoogsteen-paired (2'-OMe)-RNA strands to form parallel duplexes, which are thermally more stable than the reported earlier analogous complexes containing LNA-free [ R P -PS]-DNA oligomers (Δ T m = 7 °C per LNA unit at pH 5.4). Upon addition of the corresponding Watson-Crick-paired (2'-OMe)-RNA strands, parallel triplexes are formed with further increased thermal stability.