Ta. Roelofs et al., OXIDATION-STATES OF THE MANGANESE CLUSTER DURING THE FLASH-INDUCED S-STATE CYCLE OF THE PHOTOSYNTHETIC OXYGEN-EVOLVING COMPLEX, Proceedings of the National Academy of Sciences of the United Statesof America, 93(8), 1996, pp. 3335-3340
The Mn K-edge x-ray absorption spectra for the pure S states of the te
tranuclear Mn cluster of the oxygen-evolving complex of photosystem II
during flash-induced S-state cycling have been determined. The relati
ve S-state populations in samples given 0, 1, 2, 3, 4, or 5 flashes we
re determined from fitting the flash-induced electron paramagnetic res
onance (EPR) multiline signal oscillation pattern to the Kok model. Th
e edge spectra of samples given 0, 1, 2, or 3 flashes were combined wi
th EPR information to calculate the pure S-state edge spectra. The edg
e positions (defined as the zero-crossing of the second derivatives) a
re 6550.1, 6551.7, 6553.5, and 6553.8 eV for S-0, S-1, S-2, and S-3, r
espectively. In addition to the shift in edge position, the S-0 --> S-
1 and S-1 --> S-2 transitions are accompanied by characteristic change
s in the shape of the edge, both indicative of Mn oxidation. The edge
position shifts very little (0.3 eV) for the S-2 --> S-3 transition, a
nd the edge shape shows only subtle changes. We conclude that probably
no direct Mn oxidation is involved in this transition. The proposed M
n oxidation state assignments are as follows: S-0 (II, III, IV, IV) or
(III, III, III, IV), SI (III, III, IV, IV), S-2 (III, IV, IV, IV), S-
3 (III, IV, IV, IV).