Proton Transfer of Guanine Radical Cations Studied by Time-Resolved Resonance Raman Spectroscopy Combined with Pulse Radiolysis

The oxidation of guanine (G) is studied by using transient absorption and time-resolved resonance Raman spectroscopies combined with pulse radiolysis. The transient absorption spectral change demonstrates that the neutral radical of G (G^•(−H⁺)), generated by the deprotonation of G radical cation (G^•+), is rapidly converted to other G radical species. The formation of this species shows the pH dependence, suggesting that it is the G radical cation (G^•+)′ formed from the protonation at the N7 of G^•(−H⁺). On one hand, most Raman bands of (G^•+)′ are up-shifted relative to those of G, indicating the increase in the bonding order of pyrimidine (Pyr) and imidazole rings. The (G^•+)′ exhibits the characteristic CO stretching mode at ∼1266 cm^–1 corresponding to a C–O single bond, indicating that the unpaired electron in (G^•+)′ is localized on the oxygen of the Pyr ring