SIMULATION OF THE S-2 STATE MULTILINE ELECTRON-PARAMAGNETIC-RESONANCESIGNAL OF PHOTOSYSTEM-II - A MULTIFREQUENCY APPROACH

The S-2 state electron paramagnetic resonance (EPR) multiline signal o f Photosystem II has been simulated at Q-band (35 Ghz), X-band (9 GHz) and S-band (4 GHz) frequencies. The model used for the simulation ass umes that the signal arises from an essentially magnetically isolated Mn-III-Mn-IV dimer, with a ground state electronic spin S-T = 1/2. The spectra are generated from exact numerical solution of a general spin Hamiltonian containing anisotropic hyperfine and quadrupolar interact ions at both Mn nuclei. The features that distinguish the multiline fr om the EPR spectra of model manganese dimer complexes (additional widt h of the spectrum (195 mT), additional peaks (22), internal ''superhyp erfine'' structure) are plausibly explained assuming an unusual ligand geometry at both Mn nuclei, giving rise to normally forbidden transit ions from quadrupole interactions as well as hyperfine anisotropy. The fitted parameters indicate that the hyperfine and quadrupole interact ions arise from Mn ions in low symmetry environments, corresponding ap proximately to the removal of one ligand from an octahedral geometry i n both cases. For a quadrupole interaction of the magnitude indicated here to be present, the Mn-III ion must be 5-coordinate and the Mn-IV 5-coordinate or possibly have a sixth, weakly bound ligand. The hyperf ine parameters indicate a quasi-axial anisotropy at Mn-III, which whil e consistent with Jahn-Teller distortion as expected for a d(4) ion, c orresponds here to the unpaired spin being in the ligand deficient, z direction of the molecular reference axis. The fitted parameters for M n-IV are very unusual, showing a high degree of anisotropy not expecte d in a d(3) ion. This degree of anisotropy could be qualitatively acco unted for by a histidine ligand providing pi backbonding into the meta l d(xy) orbital, together with a weakly bound or absent ligand in the x direction.