SITE OF PROTONATION OF ONE-ELECTRON-REDUCED CYTOSINE AND ITS DERIVATIVES IN AQUEOUS-METHANOL GLASSES

Electron paramagnetic resonance spectra of one-electron-reduced cytosine, cytidine, 2'-deoxycytidine, 5'-dCMP, and 3'-dCMP produced by Co-60 gamma-irradiation in aqueous lithium chloride glasses (LiCl/H2O) and aqueous methanol glasses (MeOH/H2O) at 77 K exhibited a triplet which has been interpreted in terms of the radical anion protonating on the exocyclic amino group of the base C(N4+H)(.) (II), The exchangeable nature of the extra proton coupling is demonstrated by the collapse of the triplet into a doublet in LiCl/D2O and CD3OD/D2O glasses. The EPR spectrum of cytidine-5,6-d(2) in either LiCl glasses or aqueous methanol led to a doublet in each case, consistent with the loss of the coupling to the hydrogen on C6, with the remaining coupling due to the protonation on the exocyclic amino group. Comparable experiments in LiCl/D2O or CD3OD/D2O glasses led directly to a singlet. The results in aqueous methanol glasses parallel closely those in aqueous LiCl glasses and provide clear evidence for protonation on the exocyclic amino group even in the absence of metal ion coordination to N3. Hence, N4 protonation of the cytosine radical anion is not a result of metal ion coordination to N3 as has been suggested by others. We suggest that N4 protonation is kinetically controlled, possibly brought about by solvation effects, whereas the N3 protonation would occur under thermodynamic control.