Structure of the oxygen-evolving complex of photosystem II: information on the S-2 state through quantum chemical calculation of its magnetic properties

Twelve structural models for the S-2 state of the oxygen-evolving complex (OEC) of photosystem II are evaluated in terms of their magnetic properties. The set includes ten models based on the 'fused twist' core topology derived by polarized EXAFS spectra and two related models proposed in recent mechanistic investigations. Optimized geometries and spin population analyses suggest that Mn(III), which is most often identified with the manganese ion at site D, is always associated with a penta- coordinate environment, unless a chloride is directly ligated to the metal. Exchange coupling constants were determined by broken- symmetry density functional theory calculations and the complete spectrum of magnetic sublevels was obtained by direct diagonalization of the Heisenberg Hamiltonian. Seven models display a doublet ground state and are considered spectroscopic models for the ground state corresponding to the multiline signal (MLS) of the S-2 state of the OEC, whereas the remaining five models display a sextet ground state and could be related to the g = 4.1 signal of the S-2 state. It is found that the sign of the exchange coupling constant between the Mn centres at positions A and B of the cluster is directly related to the ground state multiplicity, implying that interconversion between the doublet and sextet can be induced by only small structural perturbations. The recently proposed quantum chemical method for the calculation of Mn-55 hyperfine coupling constants is subsequently applied to the S-2 MLS state models and the quantities that enter into the individual steps of the procedure (site-spin expectation values, intrinsic site isotropic hyperfine parameters and projected Mn-55 isotropic hyperfine constants) are analyzed and discussed in detail with respect to the structural and electronic features of each model. The current approach performs promisingly. It reacts sensitively to structural distortions and hence may be able to distinguish between different structural proposals. Thus it emerges as a useful contributor to the ongoing efforts that aim at establishing correlations between the body of spectroscopic data available for the various Si states of the OEC and their actual geometric features.