Identification by UV resonance Raman spectroscopy of an imino tautomer of 5-hydroxy-2 '-deoxycytidine, a powerful base analog transition mutagen witha much higher unfavored tautomer frequency than that of the natural residue 2 '-deoxycytidine

UV resonance Raman spectroscopy was used to detect and estimate the frequen cy of the unfavored imino tautomer of the transition mutagen 5-hydroxy-2' - deoxycytidine (HO(5)dCyt) in its anionic form. In DNA, this 2'-deoxycytidin e analog arises from the oxidation of 2'-deoxycytidine and induces C --> T transitions with 10(2) greater frequency than such spontaneous transitions. An imino tautomer marker carbonyl band (approximate to 1650 cm(-1)) is enh anced at approximate to 65 degrees C against an otherwise stable spectrum o f bands associated with the favored amino tautomer. This band is similarly present in the UV resonance Raman spectra of the imino cytidine analogs N-3 -methylcytidine at high pH and N-4-methoxy-2'-deoxycytidine at pH 7 and dis plays features attributable to the imino form of C residues and their deriv atives. The fact that the imino tautomer of HO(5)dCyt occurs at a frequency consistent with its high mutagenic enhancement lends strong support to the hypothesis that unfavored base tautomers play important roles in the mispa ir intermediates of replication leading to substitution mutations.