The photosynthetic oxygen evolving complex requires chloride for its redox state S-2->S-3 and S-3->S-0 transitions but not for S-0->S-1 or S-1->S-2 transitions

The Cl- requirement in the redox cycle of the oxygen-evolving complex (OEC) was determined by measurements of flash-induced UV absorbance changes in Cl--depleted and Cl--reconstituted photosystem II membranes, On the first flash after dark adaptation the spectrum and amplitude of those changes, known to reflect the oxidation of Mn-III to Mn-IV on the S-1-->S-2 transition, were the same in the presence or absence of Cl-. On the second and later flashes, however, absorbance changes in Cl--depleted samples revealed only electron transfer from tyrosine to quinone which reversed slowly in the dark by charge recombination and did not produce the S-3-state. A rapid method was developed to remove Cl- after producing the S-3-state by two flashes. The lifetime of the S-3-state was found to be unaffected by Cl--depletion, in contrast to the 20-fold stabilization of the S-2 lifetime by Cl- removal, and the Cl--depleted S-3-state did not proceed to S-0 on flash illumination, However, when the same Cl--depletion procedure was applied after producing the S-0-state by three flashes, further advance to S-2 by two additional flashes was not impaired by the absence of Cl-. The requirement for Cl- only on the S-2-->S-3 and S-3-->S-0 transitions can be rationalized by the hypothesis that Cl- is required for electron transfer between manganese ions within the oxygen-evolving complex.