title: The photosynthetic oxygen evolving complex requires chloride for its redox state S2-->S3 and S3-->S0 transitions but not for S0-->S1 or S1-->S2 transitions.
abstract: 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 MnIII to MnIV on the S1-->S2 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 S3-state. A rapid method was developed to remove Cl- after producing the S3-state by two flashes. The lifetime of the S3-state was found to be unaffected by Cl(-)-depletion, in contrast to the 20-fold stabilization of the S2 lifetime by Cl- removal, and the Cl(-)-depleted S3-state did not proceed to S0 on flash illumination. However, when the same Cl(-)-depletion procedure was applied after producing the S0-state by three flashes, further advance to S2 by two additional flashes was not impaired by the absence of Cl-. The requirement for Cl- only on the S2-->S3 and S3-->S0 transitions can be rationalized by the hypothesis that Cl- is required for electron transfer between manganese ions within the oxygen-evolving complex.
authors.affiliation: Biophysics Department, State University, Leiden, The Netherlands.
authors.forename: H
authors.initials: H
authors.lastname: Wincencjusz
authors.forename: H J
authors.initials: HJ
authors.lastname: van Gorkom
authors.forename: C F
authors.initials: CF
authors.lastname: Yocum
mesh_terms: D002712:Chlorides / Q000378:metabolism*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D013054:Spectrophotometry, Atomic; D013056:Spectrophotometry, Ultraviolet
chemical_list: D002712:Chlorides; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010088:Oxidoreductases