Electronic and Magnetic Properties of Neutral and Charged Quinone and Plastoquinone Radicals

Semiquinones are of vital importance in a number of biological systems, where they act as mediators in electron transport. In the present work we have employed hybrid and gradient-corrected density functional methods to investigate theoretically the electronic and magnetic properties of 1,4-benzoquinone, its ethylated counterpart, and a model plastoquinone. The structures are optimized at the B3LYP/6-311G(d,p) level, followed by single-point B3LYP/6-311+G(2df,p) and PWP86/6-311G(2d,p) energy and hyperfine properties calculations. Hydrogen bonding to the quinone and plastoquinone oxygens are modeled. Based on comparisons with experimental ESR data, the results strongly support the presence of hydrogen-bonding moieties to both oxygens of the quinone radical anion Q A - in photosystem II. These hydrogen-bonding groups are shown to increase the electron affinity of the quinones by ca. 0.6 eV and are hence of crucial importance for the functionality of the entire photochemical process. As a final part of the paper, we outline briefly the energetics involved in the electron−proton/H-atom transport in the quinone pool of the thylakoid membranes, linking photosystem II to photosystem I.