Jp. Barnes et Wa. Bernhard, ONE-ELECTRON-REDUCED CYTOSINE IN ACIDIC GLASSES - CONFORMATIONAL STATES BEFORE AND AFTER PROTON-TRANSFER, Journal of physical chemistry, 98(3), 1994, pp. 887-893
Electron paramagnetic resonance spectra of cytosine, one-electron-redu
ced at 4 K in a 5 M NaH2PO4 aqueous glass, indicate the radical is tra
pped in an intermediate state between the neutral radical, protonated
at N3, and the radical cation, protonated at N3 and the amino group. I
n the intermediate state, the amino group has one N-H bond directed si
milar 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 t
he amino group's lone pair electrons. Upon annealing the NaH2PO4 glass
to similar to 100 K, the amino group protonates and assumes an eclips
ed conformation, accompanied by a substantial shift in the unpaired el
ectron distribution. This same transition between the intermediate sta
te and amino-protonated state occurs at similar to 30 K in the more ac
id glasses of 37% HCl and 85% H3PO4.