How protonation and deprotonation of 9-methylguanine alter its singlet O2 addition path: about the initial stage of guanine nucleoside oxidation

Abstract

Mutagenicity of singlet O₂ is due to its oxidatively generated damage to the guanine nucleobases of DNA. Oxidation of neutral guanosine has been assumed to be initiated by the formation of a transient 4,8-endoperoxide via a Diels–Alder cycloaddition of singlet O₂. Protonation and deprotonation of guanosine represent another factor related to DNA damage and repair. Herein, 9-methylguanine was utilized as a model substrate to mimic the correlation between singlet O₂ oxidation of the nucleoside and its ionization states, both in the absence and in the presence of water ligands. We used guided-ion-beam scattering tandem mass spectrometry to detect and quantify transient intermediates at room temperature. To provide a reliable description of reaction potential surfaces, different levels of theory including restricted and unrestricted density functional theory, CCSD(T), MP2, and multi-reference CASSCF and CASMP2 were applied. By means of molecular potential, kinetic and direct dynamics simulations, two reaction pathways were identified and neither follows the mechanism for neutral guanosine. Singlet O₂ oxidation of protonated 9-methylguanine begins by a concerted cycloaddition; but it is mediated by a 5,8-endoperoxide. By contrast, a concerted cycloaddition does not occur for deprotonated 9-methylguanine. The latter involves a stepwise addition starting with the formation of an 8-peroxide, which subsequently evolves to a 4,8-endoperoxide. This dichotomy implies that acidic and basic media may lead to different chemistries for guanosine oxidation in aqueous solutions, starting from initial stage. The comparison with oxidation of protonated/deprotonated guanine illustrates the different mechanisms and products and particularly the suppressed oxidizability of 9-methylguanine vs. free guanine.