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

UV resonance Raman spectroscopy was used to detect and estimate the frequency of the unfavored imino tautomer of the transition mutagen 5-hydroxy-2′-deoxycytidine (HO⁵dCyt) in its anionic form. In DNA, this 2′-deoxycytidine analog arises from the oxidation of 2′-deoxycytidine and induces C → T transitions with 10² greater frequency than such spontaneous transitions. An imino tautomer marker carbonyl band (≈1650 cm^-1) is enhanced at ≈65°C against an otherwise stable spectrum of bands associated with the favored amino tautomer. This band is similarly present in the UV resonance Raman spectra of the imino cytidine analogs N³-methylcytidine at high pH and N⁴-methoxy-2′-deoxycytidine at pH 7 and displays features attributable to the imino form of C residues and their derivatives. The fact that the imino tautomer of HO⁵dCyt 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 mispair intermediates of replication leading to substitution mutations.