ESR study of the guanine cation

It has been proposed that the primary direct radiation damage products in DNA are guanine cations and thymine anions. Experiments reported here characterize a guanine cation observed in a single crystal of guanine:HCl:H2O. ESR experiments were performed by x-irradiating and observing the crystals at 15 K. Spectral parameters for the cation include N3 and N10 hyperfine couplings, a C8–Hα hyperfine coupling, and two small exchangeable couplings presumably from the N10 protons. The computed spin densities of ρ(N3)=0.283, ρ(N10)=0.168, and ρ(C8)=0.182 agree nicely with those observed for the guanine cation in DNA. In the single crystal the native molecule is protonated at N7. It is proposed that once the native molecule is oxidized it rapidly deprotonates at N7 to form the cation observed.