• cluster compounds;
  • density functional calculations;
  • EPR spectroscopy;
  • photosystem II;
  • structure elucidation


Metal and ligand hyperfine couplings of a previously suggested, energetically feasible Mn4Ca model cluster (SG2009−1) for the S2 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 MnIII(MnIV)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 55Mn hyperfine couplings (HFCs) for SG2009−1 gives excellent agreement with experiment. However, at the current level of spin projection, the 55Mn 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 MnIII site at the MnC center, which is coordinated by histidine (D1-His332). The computed histidine 14N HFC anisotropy for SG2009−1 gives much better agreement with ESEEM data than the other models, in which MnC is an MnIV site, thus supporting the validity of the model. The 13C HFCs of various carboxylates have been compared with 13C ENDOR data for PSII preparations with 13C-labelled alanine.