6-Thio-2′-deoxyinosine: Synthesis, incorporation, and evaluation as a postsynthetically modifiable base in oligonucleotides

The synthesis of 6-thio-2′-deoxyinosine (d S6 I) in a form suitably protected for solid-phase oligonucleotide synthesis is reported. This thionucleic acid was incorporated in high yield into short oligodeoxynucleotides, and the thiocarbonyl group could be modified by S-alkylation with complete chemoselectivity. The quantitation of incorporation and facile post-synthetic modification was demonstrated by enzymatic digestion and HPLC analysis, and the effect of covalent alkylation was determined by ΔT m measurements of the corresponding duplex oligonucleotides with dC as the complementary base.