Disulfide-linked oligonucleotide phosphorothioates: novel analogues of nucleic acids

The synthesis of phosphorothioate analogues of oligonucleotides by the oxidation of deoxyadenosine 3',5'-bisphosphorothioate (3) was attempted. Cyclization of 3 is much more efficient than oligomerization under all the conditions investigated. However, a preformed oligonucleotide carrying a 5'-terminal phosphorothioate group undergoes efficient chain-extension when oxidized in the presence of 3.