formylcytidine (f^C), which is found at the first position of the anticodon of bovine mitochondrial tRNA^e^, were analyzed by ^H-NMR spectroscopy. f^C has a normal amino tautomeric form at position 4 of the base moiety. The results indicate the presence of an intramolecular hydrogen bond between the carbonyl of the 5-formyl group and the 4-amino function. f 5 C was found to exhibit the C3’-endo conformation exclusively and the enthalpy difference (AH) between the C2’-endo and C3’-endo forms was found to be 1.56 ± 0.13 kcal/mol, indicating f5C to be one of the most conformationally rigid nucleosides yet analyzed. The conformational rigidity of f C may contribute to regulation of codon recognition by tRNAMet. Conformational properties of a novel modified nucleoside, 5-.
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