Radical–Radical Interactions among Oxidized Guanine Bases Including Guanine Radical Cation and Dehydrogenated Guanine Radicals

We present here a theoretical investigation of the structural and electronic properties of di-ionized GG base pairs (G•⁺G•⁺,G(-H1)•G•⁺, and G(-H1)•G(-H1)•) consisting of the guanine cation radical (G•⁺) and/or dehydrogenated guanine radical (G(-H1)•) using density functional theory calculations. Different coupling modes (Watson–Crick/WC, Hoogsteen/Hoog, and minor groove/min hydrogen bonding, and π–π stacking modes) are considered. We infer that a series of G•⁺G•⁺ complexes can be formed by the high-energy radiation. On the basis of density functional theory and complete active space self-consistent (CASSCF) calculations, we reveal that in the H-bonded and N–N cross-linked modes, (G•⁺G•⁺)WC, (G(-H1)•G(-H1)•)WC, (G(-H1)•G(-H1)•)ₘᵢₙI, and (G(-H1)•G(-H1)•)ₘᵢₙIII have the triplet ground states; (G•⁺G•⁺)HₒₒgI, (G(-H1)•G•⁺)WC, (G(-H1)•G•⁺)HₒₒgI, (G(-H1)•G•⁺)ₘᵢₙI, (G(-H1)•G•⁺)ₘᵢₙII, and (G(-H1)•G(-H1)•)ₘᵢₙII possess open-shell broken-symmetry diradical-characterized singlet ground states; and (G•⁺G•⁺)HₒₒgII, (G•⁺G•⁺)ₘᵢₙI, (G•⁺G•⁺)ₘᵢₙII, (G•⁺G•⁺)ₘᵢₙIII, (G•⁺G•⁺)HₒHₒ, (G(-H1)•G•⁺)ₘᵢₙIII, (G(-H1)•G•⁺)HₒHₒ, and (G(-H1)•G(-H1)•)HₒHₒ are the closed-shell systems. For these H-bonded diradical complexes, the magnetic interactions are weak, especially in the diradical G•⁺G•⁺ series and G(-H1)•G(-H1)• series. The magnetic coupling interactions of the diradical systems are controlled by intermolecular interactions (H-bond, electrostatic repulsion, and radical coupling). The radical–radical interaction in the π–π stacked di-ionized GG base pairs ((G•⁺G•⁺)ππ, (G(-H1)•G•⁺)ππ, and (G(-H1)•G(-H1)•)ππ) are also considered, and the magnetic coupling interactions in these π–π stacked base pairs are large. This is the first theoretical prediction that some di-ionized GG base pairs possess diradical characters with variable degrees of ferromagnetic and antiferromagnetic characteristics, depending on the dehydrogenation characters of the bases and their interaction modes. Hopefully, this work provides some helpful information for the understanding of different structures and properties of the di-ionized GG base pairs.