Hybridization and Mismatch Discrimination Abilities of 2′,4′-Bridged Nucleic Acids Bearing 2-Thiothymine or 2-Selenothymine Nucleobase

Oligonucleotides modified with 2′-O,4′-C-spirocyclopropylene-bridged nucleic acid (scpBNA) exhibit excellent duplex-forming ability with their complementary single-stranded RNA (ssRNA). Here, we demonstrate that scpBNA bearing a 2-thiothymine (scpBNA-S2T) or 2-selenothymine (scpBNA-Se2T) nucleobase provides robust mismatch discrimination capabilities to oligonucleotides without compromising their high binding affinities toward the full complementary ssRNA. X-ray crystallographic analysis of a self-assembling oligonucleotide featuring 2′,4′-BNA/LNA-2-thiothymine (2′,4′-BNA/LNA-S2T, where 2′,4′-BNA and LNA stand for “2′-O,4′-C-methylene-bridged nucleic acid” and “locked nucleic acid”, respectively), a prototype of scpBNA-S2T, revealed that the 2-thiocarbonyl moiety plays a crucial role in the destabilization of thymine–guanine mismatched wobble base pairs.