Optical characterization of intermediates in the water-splitting enzyme system of photosynthesis — possible states and configurations of manganese and water

Abstract

Absorption changes coupled with the individual transitions S 0–S 3 and redox reactions in the water-splitting enzyme system S of photosynthesis have been measured. The principal difficulties of measuring the very small absorption changes in the ultraviolet coupled with those reactions have been reduced drastically through the use of a highly purified Photosystem II complex isolated from the Cyanobacterium synechococcus. The general problem caused by the mixing of the S states during a train of flashes and the falsification through the overlap with absorption changes of Q B (binary oscillations) have been treated as follows. (1) The binary oscillations were bypassed through the use of silicomolybdate and high concentrations of DCBQ, respectively, as external electron acceptor. (2) Stable absorption changes of the mixed S-state transitions have been deconvoluted through fitting procedures to get the changes of the individual transitions of S 1 → S 2 → S 3 → S 0 → S 1. (3) Kinetically resolved absorption changes of the S-states in the 100-μs range gave independent information on the individual transitions. (4) Stable absorption changes of the S 0 → S 1 transitions in the forefront were induced after shifting the S states through low concentrations of NH 2OH two units backwards. Analysis of the resulting sequence S x → S 0 → S 1 → S 2 → S 3 → S 0, beginning with an NH 2OH depending pre-state, S x , and followed by an S 0 → S 1 transition not mixed with the opposite S 3 → S 0 transition, increased the conclusiveness considerably. It results that the ultraviolet spectrum of the S 0 → S 1 transition is different from the spectra of the S 1 → S 2 and S 2 → S 3 transition. Possible states of manganese, water and surplus charges responsible for these spectra are presented.