Chloride is an essential cofactor for photosynthetic water oxidation. However, its location and functional roles in active photosystem II are still a matter of debate. We have investigated this issue by studying the effects of Cl- replacement by Br- in active PSII. In Br- substituted samples, Cl- is effectively replaced by Br- in the presence of 1.2M NaBr under room light with protection of anaerobic atmosphere followed by dialysis. The following results have been obtained. i) The oxygen-evolving activities of the Br--PSII samples are significantly lower than that of the Cl--PSII samples; ii) The same S2 multiline EPR signals are observed in both Br- and Cl--PSII samples; iii) The amplitudes of the visible light induced S1TyrZ• and S2TyrZ• EPR signals are significantly decreased after Br- substitution; the S1TyrZ• EPR signal is up-shifted about 8G, whereas the S2TyrZ• signal is down-shifted about 12G after Br- substitution. These results imply that the redox properties of TyrZ and spin interactions between TyrZ• and Mn-cluster could be significantly modified due to Br- substitution. It is suggested that Cl-/Br- probably coordinates to the Ca2+ ion of the Mn-cluster in active photosystem II.
- Bromide substitution
- Electron paramagnetic resonance
- Photosystem II
- Tyrosine Z