2',5'-linked oligo-3'-deoxyribonucleoside phosphorothioate chimeras: thermal stability and antisense inhibition of gene expression.

2',5'-Linked oligo-3'-deoxyribonucleotides bind selectively to complementary RNA but not to DNA. These oligonucleotides (ODNs) do not recognize double-stranded DNA by Hoogsteen triplex formation and the complexes formed by these ODNs with RNA are not substrates for Escherichia coli RNase H. Substitution of the 2',5'-phosphodiester backbone by phosphorothioate linkages gives 2',5'-linked oligo-3'-deoxynucleoside phosphorothioate ODNs that exhibit significantly less non-specific binding to cellular proteins or thrombin. Incorporation of a stretch of seven contiguous 3',5'-linked oligo-2'-deoxynucleoside phosphorothioate linkages in the center of 2',5'-linked ODNs (as a putative RNase H recognition site) afford chimeric antisense ODNs that retain the ability to inhibit steroid 5alpha-reductase (5alphaR) expression in cell culture.