ONE-ELECTRON-REDUCED CYTOSINE IN ACIDIC GLASSES - CONFORMATIONAL STATES BEFORE AND AFTER PROTON-TRANSFER

Electron paramagnetic resonance spectra of cytosine, one-electron-reduced at 4 K in a 5 M NaH2PO4 aqueous glass, indicate the radical is trapped in an intermediate state between the neutral radical, protonated at N3, and the radical cation, protonated at N3 and the amino group. In the intermediate state, the amino group has one N-H bond directed similar to 30 degrees below the ring plane. This is suggested to be due to the presence of a hydrogen bond between a matrix water proton and the amino group's lone pair electrons. Upon annealing the NaH2PO4 glass to similar to 100 K, the amino group protonates and assumes an eclipsed conformation, accompanied by a substantial shift in the unpaired electron distribution. This same transition between the intermediate state and amino-protonated state occurs at similar to 30 K in the more acid glasses of 37% HCl and 85% H3PO4.