EPR AND ENDOR CHARACTERIZATION OF SEMIQUINONE ANION-RADICALS RELATED TO PHOTOSYNTHESIS

EPR and ENDOR experiments were performed on the radical anions of 1,4- benzoquinone, 2,3-dimethyl-1,4-benzoquinone, 2,3,5,6-tetramethyl-1,4-b enzoquinone, 2,3-dimethyl-5-ethyl-1,4-benzoquinone, plastoquinone-1, p lastoquinone-9 and ubiquinone-10. In liquid solutions the isotropic hy perfine coupling constants could be determined, In the frozen state hy perfine tensors for alpha-protons and the freely rotating methyl group s were measured whereas other beta-protons (CH2) were broadened beyond detection probably caused by several superimposed chain conformations , In the immobilized radicals proton hyperfine couplings of hydrogen b onds formed between the alcoholic solvent and the CO groups of the qui nones could be determined by experiments in protonated and deuterated solvents, Two different types of strong hydrogen bonds exist in these systems: in-plane with a(iso) approximate to 0 and out-of-plane with a (iso) > 0. The latter type is observed if substituents at both carbons adjacent to the CO groups force the hydrogen bond out of the molecula r plane. By use of the point-dipole model and oxygen spin densities ob tained from O-17 hyperfine couplings, hydrogen band lengths could be d etermined from the measured dipolar proton hyperfine tensors, These fi ndings are very important for the binding and function of quinones in biological systems such as the photosynthetic reaction centres of plan ts and bacteria.