The function of the chloride ion in photosynthetic oxygen evolution

The involvement of Cl- and several other monovalent anions in photosynthetic oxygen evolution was studied using photosystem 11 membranes depleted of Cl- by dialysis. The results of these studies differ significantly from results obtained using other depletion methods. Binding studies with glycerol as a cryoprotectant confirm our previous observations with sucrose of two interconvertible binding states of photosystem 11 with similar activities and with slow or fast exchange, respectively, of the bound ion. With glycerol, Cl- depletion decreased the oxygen evolution rate to 55% of that with Cl- present without decreasing the quantum efficiency of the reaction, supporting our previous conclusion that oxygen evolution can proceed at high rates in the absence of Cl-. Further, after Cl- depletion the S-2 state multiline signal displayed the same periodic appearance with the same signal yield after consecutive laser flashes as with Cl- present. Br-, I-, and NO3-, although with different capacities to reactivate oxygen evolution, also showed two binding modes. I- inhibited when bound in the low-affinity, fast-exchange mode but activated in the high-affinity mode. A comparison of the EPR properties of the S-2 state with these anions suggests that the nature of the ion or the binding mode only has a minor influence on the environment of the manganese. In contrast, F- completely inhibited oxygen evolution by preventing the S-2 to S-3 transition and shifted the equilibrium between the g = 4.1 and multiline S-2 forms toward the former, which suggests a considerable perturbation of the manganese cluster. To explain these and earlier observations, we propose that the role of chloride in the water-splitting mechanism is to participate together with charged amino acid side chains in a proton-relay network, which facilitates proton transfer from the manganese cluster to the medium. The structural requirements likely to be involved may explain the sensitivity of oxygen evolution to Cl- depletion or other perturbations.