ELECTRONIC AND MAGNETIC-PROPERTIES OF NEUTRAL AND CHARGED QUINONE ANDPLASTOQUINONE RADICALS

Semiquinones are of viral importance ill a number of biological system s, where they act as mediators in electron transport. In the present w ork we have employed hybrid and gradient-corrected density functional methods to investigate theoretically the electronic and magnetic prope rties of 1,4-benzoquinone, its ethylated counterpart, and a model plas toquinone, The structures are optimized at the B3LYP/6-311G(d,p) level , followed by single-point B3LYP/6-311+G(2f,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 wi th experimental ESR data, the results strongly support the presence of hydrogen-bonding moieties to both oxygens of the quinone radical anio n Q(A)(-) in photosystem IT, These hydrogen-bonding groups are shown t o increase the electron affinity of the quinones by ca, 0.6 eV and are hence of crucial importance for the functionality of the entire photo chemical process, As a final part of the paper, we outline briefly the energetics involved in the electron-proton/H-atom transport in the qu inone pool of the thylakoid membranes, linking photosystem II to photo system I.