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STOICHIOMETRY OF THE REDOX CHANGES OF MANGANESE DURING THE PHOTOSYNTHETIC WATER OXIDATION CYCLE

Abstract— The UV absorbance change associated with the oxygen‐evolution cycle is investigated as to its correlation with the successive redox (S‐) states of the system. This change is probably due to a manganese oxidation within a multinuclear Mn‐cluster. The stoichiometry of its formation and destruction during the S‐cycle has been a matter of controversy: (0,1,0,‐1) according to velthuys, B. R. (1981) in Photosynthesis II, (Edited by G. Akoyunoglou), pp. 75–85. Balaban International Science Services, Philadelphia, or (1, 1, 1, ‐3) according to Dekker, J. P., Van Gorkom, H. J., Wensink, J. and Ouwehand, L. (1984) Biochim. Biophys. Acta 767, 1–9. We show that the data obtained in flash‐series experiments on dark‐adapted material may be fitted as satisfactorily by both models. More discriminating tests were devised, varying parameters such as the double‐hits probability or the initial distribution of the S‐states. These experiments clearly indicate that no (or little) absorbance change occurs both upon S0→S1 and S2→S3 transitions. This suggests that the correct stoichiometry is (0,l,0,‐1). The possibility of a non‐integer stoichiometry may also be considered.