S-STATE DEPENDENCE OF CHLORIDE BINDING AFFINITIES AND EXCHANGE DYNAMICS IN THE INTACT AND POLYPEPTIDE-DEPLETED O-2 EVOLVING COMPLEX OF PHOTOSYSTEM-II

The Cl- binding properties in the successive oxidation states of the O -2 evolving complex of photosystem II were investigated by measurement s of UV absorbance changes, induced by a series of saturating flashes, that monitor manganese oxidation state transitions. In dark-adapted, intact photosystem II, Cl- can be replaced by NO3- in minutes, in an e xchange reaction that depends on the NO3- concentration and that is no t rate-limited by dissociation of Cl- from its binding site. Preillumi nation of dark-adapted photosystem II by one or two flashes accelerate d the NO3- substitution reaction by an order of magnitude. A quantitat ive analysis of the Cl- concentration dependence of UV absorbance chan ges, measured in photosystem II preparations depleted of extrinsic 17 and 23 kDa polypeptides, shows that the Cl- binding properties of phot osystem II change with the oxidation state of the oxygen evolving comp lex. Although the affinity for the individual S-states could not be de termined with precision, it is shown that the affinity is an order of magnitude lower in the S-2 state than in the S-1 state. Comparison of the results obtained using intact photosystem II and preparations depl eted of the 17 and 23 kDa extrinsic polypeptides suggests that these p roteins constitute a diffusion barrier, which prevents fast equilibrat ion of the Cl- binding site with the medium, but does not change the C l- affinity of the binding site.