Nitrosation mechanisms, kinetics, and dynamics of the guanine and 9-methylguanine radical cations by nitric oxide—Radical–radical combination at different electron configurations.

This article investigates the nitrosation reactions between nitric oxide (•NO) and guanine radical cations (G•+), specifically focusing on unsubstituted guanine radical cations (9HG•+) and 9-methylguanine radical cations (9MG•+), a nucleoside-mimicking model. Using guided-ion beam tandem mass spectrometry combined with advanced computational methods—including density functional theory, coupled cluster theory, and complete active space perturbation theories—the study elucidates the reaction mechanisms, spin dynamics, and energetics of multiple spin states (open-shell singlet, closed-shell singlet, and triplet) involved in these radical–radical interactions. The major exothermic product identified is the closed-shell singlet 1,CS[7-NO-9MG]+ adduct, while a minor endothermic triplet product 3[8-NO-9MG]+ forms at higher collision energies. The findings highlight that nitrosation yields are limited by spin selectivity and rapid decay of singlet products, providing insight into the role of •NO in enhancing DNA radiosensitivity through reactions with guanine radical cations under ionizing radiation.