NMR-STUDIES OF THE EFFECTS OF THE 5'-PHOSPHATE GROUP ON CONFORMATIONAL PROPERTIES OF 5-METHYLAMINOMETHYLURIDINE FOUND IN THE FIRST POSITIONOF THE ANTICODON OF ESCHERICHIA-COLI TRNA(4)(ARG)

5-Methylaminomethyluridine (mnm(5)U) exists in the first position of t he anticodon (position 34) of Escherichia coli tRNA(4)(Arg) for codons AGA/AGG. In the present study, the temperature dependence of the ribo se-puckering equilibrium of pmnm(5)U was analyzed by proton NMR spectr oscopy. Thus, the enthalpy difference (Delta H) between the C2'-endo a nd C3'-endo forms was obtained as 0.65 kcal . mol(-1). By comparison o f the Delta H values of pU and pmnm(5)U, the 5-substitution was found to increase the relative stability of the C3'-endo form over the C2'-e ndo form significantly (by 0.56 kcal mol-l). Furthermore, this conform ational ''rigidity'' was concluded to depend on the 5'-phosphate group , because nucleoside U exhibits only a negligible change in the ribose -puckering equilibrium upon the 5-methylaminomethyl substitution. Furt her NMR analyses and molecular dynamics calculations revealed that int eractions between the 5-methylaminomethyl and 5'-phosphate groups of p mnm(5)U restrict the conformation about the glycosidic bond to a low a nti form, enhancing steric repulsion between the 2-carbonyl and 2'-hyd roxyl groups in the C2'-endo form. This intrinsic conformational rigid ity of the mnm(5)U residue in position 34 may contribute to the correc t codon recognition.