NMR analyses on the molecular mechanism of the conformational rigidity of 2‐thioribothymidine, a modified nucleoside in extreme thermophile tRNAs

1H-NMR analyses have been made on the conformations of 2-thioribothymidine (s2T), 2-thiodeoxyribothymidine (s2dT), as well as ribothymidine (T) and deoxyribothymidine (dT). s2T and s2dT exclusively take the anti form rather than the syn form. The C3'-endo-gg form of the sugar moiety is remarkably stabilized on modification of T to s2T, but not on modification of dT to s2dT. The steric effects of the 2-thiocarbonyl group and the 2'-hydroxyl group cause the rigidity of the C3'-endo-gg form of s2T. Such rigidity of s2T probably contributes to the thermostability of 2-thiopyrimidine polyribonucleotides and extreme thermophile tRNAs.