Spin-exchange interactions in the S2-state manganese tetramer in photosynthetic oxygen-evolving complex deduced from g=2 multiline EPR signal

Possible spin-exchange structures of the Mn(III,IV,IV,IV) cluster in an S-2 state of plant photosystem II were computer-searched, within the range compatible with X-ray absorption data, by diagonalizing each Heisenberg spin-exchange Hamiltonian and then by checking whether it can take the S = 1/2 ground state capable of explaining the effective Mn-55 hyperfine constants determined from oriented multiline spectra and the first excited state with excitation energy around 20-50 cm(-1), or not. The possible spin-exchange structures were found to be distributed in those that contain only one strong-antiferromagnetic coupling and at most two intermediate coupling(s). The most probable structures are discussed in detail. (C) 1999 Elsevier Science B.V. All rights reserved.