Mechanism of Formation of Guanine-Guanine Crosslinks as Products of One-Electron Oxidation of 1-Methylguanosine in Acidic Medium

Reactive oxygen species (ROS) from normal metabolism and environmental exposure may cause oxidative DNA damage. Guanine (G) is the most oxidizable nucleobase. One-electron oxidation (OEO) of G in produces guanine radical cation (G•⁺), which deprotonates to form guanyl radical G(-H)•.This radical exists in two tautomeric forms: G(N1-H)• and G(N2-H)•. Dimerization of guanyl radical yields G-G cross-links (D1/D2); a tautomeric form for dimer formation remains elusive. This study presents the first characterization of G• dimerization products from the OEO of 1-MeGuo in acidic medium. Photolysis at 470 nm in the presence of S₂O₈²⁻ and Ru(II)(bpy)₃²⁺ generates two major stereoisomers (D1AW* and D1BW*), which are chemically and structurally analogous to D1; further oxidation of D1W* forms D2W*. HPLC, LC-MS, and ¹H-NMR analyses demonstrated that the exocyclic amino hydrogens (N2) but not the N1 hydrogens are required for D1/D2 analog formation, supporting the hypothesis that the G(N2-H)• tautomer predominates during dimerization.