Electronic Structure Studies of the Spin Density Distribution of the QA Plastosemiquinone Free Radical of Photosystem II

Abstract

Density functional calculations are used to calculate the spin density distribution for the plastosemiquinone anion radical in the QA binding site of Photosystem II. A number of models are examined which explore the effect of iron depletion on the QA site semiquinone spin density distribution and resultant hyperfine couplings. For a model system with a divalent metal ion in the non-heme site the calculated spin density in the QA site model suggests that differential hydrogen-bonding strength to the O1 and O4 oxygen atoms of the radical results in an asymmetric spin density distribution in the semiquinone anion free radical form. The hydrogen bond to the proximal O1 atom is significantly stronger. This is similar to the situation shown to exist previously in the bacterial reaction centre of Rba sphaeroides. Various models of depleted non-heme site metal show the profound effect that the presence of a divalent ion in this site has on the spin density distribution of the QA site semiquinone. The variation in calculated spin density distribution of the QA site plastosemiquinone as a function of the occupancy of the non-heme site needs to be taken into account in the interpretation of experimental paramagnetic resonance data. For Type II reaction centres a major role for Fe2+ in the non-heme site may be the raising of the redox potential of the QA/QA − couple to ensure that electron transfer from the (bacterio)pheophytin anion free radical occurs at a sufficient rate to compete with wasteful back-reactions.KeywordsA DFT, density functional theoryB3LYP, Becke3 Lee Yang ParrQM, Quantum MechanicsMM Molecular MechanicsPQ, Plastoquinone, PSQ, PlastosemiquinonePS II, Photosystem II, ONIOM, Our Own N-layered Integrated Molecular Orbital and Molecular MechanicsEPR, electron paramagnetic resonanceENDOR, electron nuclear double resonanceESEEM, electron spin echo envelope modulation