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‐benzoquinone, 2,3‐dimethyl‐5‐ethyl‐1,4‐benzoquinone, plastoquinone‐1, plastoquinone‐9 and ubiquinone‐10. In liquid solutions the isotropic hyperfine coupling constants could be determined. In the frozen state hyperfine tensors for α‐protons and the freely rotating methyl groups were measured whereas other β‐protons (CH 2 ) were broadened beyond detection probably caused by several superimposed chain conformations. In the immobilized radicals proton hyperfine couplings of hydrogen bonds formed between the alcoholic solvent and the CO groups of the quinones 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 > 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 molecular plane. By use of the point‐dipole model and oxygen spin densities obtained from 17 O hyperfine couplings, hydrogen bond lengths could be determined from the measured dipolar proton hyperfine tensors. These findings are very important for the binding and function of quinones in biological systems such as the photosynthetic reaction centres of plants and bacteria.