Density Functional Calculations of Mn-55, N-14 and C-13 Electron Paramagnetic Resonance Parameters Support an Energetically Feasible Model System for the S-2 State of the Oxygen-Evolving Complex of Photosystem II

Metal and ligand hyperfine couplings of a previously suggested, energetically feasible Mn4Ca model cluster (SG2009(-1)) for the S, state of the oxygen-evolving complex (OEC) of photosystem II (PSII) have been studied by broken-symmetry density functional methods and compared with other suggested structural and spectroscopic models. This was carried out explicitly for different spin-coupling patterns of the S=(1)/(2) ground state of the Mn-III(Mn-IV)(3) cluster. By applying spin-projection techniques and a scaling of the manganese hyperfine couplings, computation of the hyperfine and nuclear quadrupole coupling parameters allows a direct evaluation of the proposed models in comparison with data obtained from the simulation of EPR. ENDOR, and ESEEM spectra. The computation of Mn-55 hyperfine couplings (HFCs) for SG2009(-1) gives excellent agreement with experiment. However, at the current level of spin projection, the Mn-55 HFCs do not appear sufficiently accurate to distinguish between different structural models. Yet, of all the models studied, SG2009(-1) is the only one with the Mn-III site at the Mn-C center, which is coordinated by histidine (D1-His332). The computed histidine N-14 HFC anisotropy for SG2009(-1) gives much better agreement with ESEEM data than the other models, in which Mn-C is an Mn-IV site, thus supporting the validity of the model. The C-13 HFCs of various carboxylates have been compared with C-13 ENDOR data for PSII preparations with C-13-labelled alanine.