Transient and pulsed EPR study of17O-substituted methyl-naphthoquinone as radical anion in the A1 binding site of photosystem I and in frozen solution

Quinones have been studied in considerable detail as functional cofactors in membrane-bound protein-cofactor systems, in particular in reaction centers (RCs) of photosynthesis. For both types of RCs, they act primarily as one-electron gates during light-induced charge separation but at very different redox potential. Hydrogen bonding between the RC protein and the two, 1,4-quinone carbonyl groups constitutes a major protein-cofactor interaction in control of function. In contrast to symmetric H-bonding for quinones in isotropic solution, asymmetric H-bonding is a characteristic feature of the quinone binding sites in RC proteins. A simple valence bond model correlates the asymmetry of respective H-bond strength with the asymmetric spin density distribution derived from observable hyperfine couplings of the quinone anion state. Among all quinone-protein systems studied so far, the A1 acceptor site in photosystem (PS) I exhibits the highest asymmetry. Since the carbonyl groups carry most of the total unpaired electron spin density, isotopic labelling of the carbon (13C) and oxygen (17O) appears to be the proper way to characterize the H-bond asymmetry by hyperfine couplings. Indeed, recent13C hyperfine studies, together with data for protons in specific ring substituents, confirm the high asymmetry correlated with only one dominant H-bond in the A1 site of PS I, which is consistent with the structure model derived from X-ray structure (1JB0) for the ground state of the PS I protein complex.17O hyperfine tensors measured for the A1 site of PS I yield high hyperfine coupling constants but very small asymmetry for the two carbonyl groups. The asymmetry is even three times smaller than the already small one observed for the QA site of purple bacterial RCs. A small asymmetry is however consistent with previous studies on model systems which showed an insensitivity of the17O hyperfine coupling to H-bond-induced changes of the unpaired electron spin density. The large17O hyperfine coupling itself appears to depend on the electrostatics seen by the radical anion. It is slightly larger when A 1 − is part of the functional transient radical ion pair state as compared with the photoaccumulated stable radical anion. Possible explanations and consequences of these results are discussed.