Orientation of Ligand Field for Dangling Manganese in Photosynthetic Oxygen-Evolving Complex of Photosystem II.

The spin structure in the S2 state and the crystal structure of the manganese cluster of the oxygen-evolving complex of plant photosystem II were combined by the quantitative evaluation of the magnetic anisotropy of the g = 4 signal. The g-values of 3.93 and 4.13 were obtained for the g = 4 signal in the directions parallel and perpendicular to the membrane normal, respectively. The peak-to-peak separations were 270 and 420 G for the parallel and perpendicular orientations to the membrane, respectively. By comparison with the crystal structure, the z-axis of the zero-field splitting was ascribed to the direction of the dangling Mn connecting water oxygen, Mn4-O(W1), in the manganese cluster. The results give the first experimental evidence that the valence of the dangling Mn is Mn(III) in the S2 high-spin state. We showed that the strong exchange coupling of Mn4 to Mn3 was required for g = 4.1 spin state in the four-spin couplings, estimated as > ∼|-30 cm-1|, indicating that the present closed cubane model in quantum mechanics/molecular mechanics (QM/MM) calculation cannot explain the g = 4.1 spin structure. The onsite zero-field splitting of the dangling Mn was evaluated as -2.3 cm-1 under the strong antiferromagnetic couplings (-50 cm-1) with the dangling Mn to the cubane frame in the four coupled spin state. From the viewpoint of the arrangement of the Mn valences in the cluster, a closed cubane model is effective, but without a large structural deviation from the S1 state crystal structure.