Simulation of the 'S0 state' EPR signal from the Mn cluster in photosystem II. Evidence for a piece of a thermally accessible O2 state

Abstract

The magnetic state of the Mn4-cluster in the S0-state of photosystem II (PSII) cycles was studied by simulation of a "g=2 multiline" EPR signal observed by K.A. ¿hring et al. [Biochemistry, 1998, 37, 8115-8120]. So far, no body has succeeded in simulating this S0-state EPR signal in terms of the proper spin Hamiltonian. Here, we present the first simulation result to suggest that this signal may be best described by a superposition of at least two EPR signals: the majority component(~92 %), which was suitably assigned to a "diamond-structure" tetranuclear Mn4(III,III,III; IV) cluster, could explain 26-27 main lines in the magnetic field 2300-4400 G, and the minority(~7.4 %), which could be exclusively assigned to an O2-binding Mn4(III,III,III; IV) cluster, was required to provide a Gaussian background spectrum with the peak-to-bottom width, ~270 G, to fit the central part of the spectrum. The symmetries of g- and A-tensors of three MnIII Jahn-Teller ions in the cluster are totally consistent with the diamond-structure model, and the spin projection factors in a S=1/2 ground state with no thermally accessible excited state are capable of explaining a much wider multiline than the S2-state, without relying upon a dimeric MnII-MnIII subunit to be included. A small population of the O2-Mn4(III,III,III; IV) clusters must be like that the Mn4 spin (=1/2) and an O2-spin (=1) are strong-antiferromagnetically coupled to each other, yielding S=1/2. This indicates that a thermally accessible S4-state, which is assumed to be binding an O2 molecule, may be the origin of this species.