LAUSR

Abstract Guanine is the most susceptible base to oxidation damage induced by reactive oxygen species including singlet oxygen (1O2, 1Δg). We clarify whether the first step of guanine oxidation in B−DNA proceeds via either a zwitterionic or a diradical intermediate. The free energy profiles are calculated by means of a combined quantum mechanical and molecular mechanical (QM/MM) method coupled with the adaptive biasing force (ABF) method. To describe the open‐shell electronic structure of 1O2 correctly, the broken‐symmetry spin‐unrestricted density functional theory (BS−UDFT) with an approximate spin projection (AP) correction is applied to the QM region. We find that the effect of spin contamination on the activation and reaction free energies is up to ∼8 kcal mol−1, which is too large to be neglected. The QM(AP−ULC−BLYP)/MM‐based free energy calculations also reveal that the reaction proceeds through a diradical transition state, followed by a conversion to a zwitterionic intermediate. Our computed activation energy of 5.2 kcal mol−1 matches experimentally observed range (0∼6 kcal mol−1).