The state of manganese in the photosynthetic apparatus: 5. The chloride effect in photosynthetic oxygen evolution. Is halide coordinated to the EPR-active manganese in the O 2-evolving complex? Studies of the substructure of the low-temperature multiline EPR signal

Abstract

The role of chloride in the manganese-containing oxygen-evolving complex of Photosystem II has been studied by observing the amplitude of the multiline EPR signal as a function of Cl − concentration or when Cl − is replaced by Br − or F −. The correlation of the multiline EPR signal intensity and O 2 activity with the concentration of Cl − shows that chloride is involved in oxygen evolution at the S 2 or earlier S states, and that it is necessary for the production of an EPR-detectable S 2 state. We have developed a new method for the preparation of subchloroplast PS II particles containing Br − and F −) and have used these particles for studying the EPR fine structure at high resolution. The fine structure shows a multiplet of 4–6 lines with 10–15 G spacing; at the resolution of our experiment there are no significant differences between the Cl −-and Br −-containing samples, suggesting that the halide is not a ligand of the EPR-active Mn. Various structural possibilities for the Mn complex, which would account for the observed fine structure of the multiline EPR spectrum are discussed.