THIOLATION OF URIDINE CARBON-2 RESTRICTS THE MOTIONAL DYNAMICS OF THE TRANSFER-RNA WOBBLE POSITION NUCLEOSIDE

Thiolation of transfer RNA wobble position uridines produces a preferred conformation of the nucleoside in solution at ambient temperature that is of biological significance to codon recognition [Sierzputowska-Gracz, H.; Sochacka, E.; Malkiewicz, A.; Kuo, K.; Gehrke, C.; Agris, P. F. J. Am. Chem. Soc. 1987,109, 7171-7177]. We investigated and compared, by proton nuclear magnetic resonance (NMR) spectroscopy, the thermodynamic stability of the conformations of 2-thiouridine and five biologically important 5-position derivatives and the six analogous uridines. Under physiological conditions, there were 4.8 times larger values of enthalpy and an average change of 1 kcal/mol, DELTA-G, for the C(2') to C(3') endo transitions of the 2-thiouridines, found to favor the C(3') endo conformation, than for the respective non-thiolated uridines, found preferentially in the C(2') endo conformation. The effect of an adjacent nucleoside on the structures and dynamics of 2-thiouridine and uridine was studied by analyzing the dinucleoside s2UpU. Within the dinucleoside the individual nucleosides neither differed in structure nor dynamics from their respective mononucleosides. Therefore, the 2-position thiolation, and not the 5-position modification, produced a significantly more stable, motionally more restricted, C(3') endo, gauche plus, anti conformer. This thermodynamically preferred structure may be best suited for anticodon base stacking and loop and stem stability. The result in tRNA is a modified-wobble selection of adenine as the only suitable third base of the codon.