Synthesis and properties of DNA oligomers containing stereopure phosphorothioate linkages and C-5 modified deoxyuridine derivatives.

Phosphorothioate (PS) modification, where a non-bridging oxygen atom in a phosphodiester linkage is replaced by a sulfur atom, is widely used to improve the properties of nucleic acid drugs. Each PS linkage can be found in two stereoisomers, R p and S p. Since one non-bridging oxygen or sulfur atom in S p-PS or R p-PS, respectively, is located close to the C-5 substituent of uracil in a DNA/RNA hybrid duplex, the combination of the stereochemistry of the PS linkages and the type of the C-5 modification of uracil bases is expected to affect the properties of the hybrid duplexes. Herein, DNA oligomers containing both stereopure phosphorohioate linkages and C-5 modified deoxyuridine derivatives were synthesized. The thermodynamic stability of the DNA/RNA and DNA/DNA duplexes and RNase H activity of the DNA/RNA duplexes were evaluated. The combination of 5-propynyluracil and ( R p)-PS linkages in a DNA strand could significantly increase the thermal stability of a DNA/RNA hybrid duplex without reducing its RNase H activity.