been uncovered. The roles of Cl− and the membrane proteins surrounding the metal cluster are not fully understood. Much of what is known has been deduced from flash experiments, EPR, and x-ray spectroscopy. The Mn4O5Ca cluster is the most striking feature of the OEC. It is crucial because it stores the three oxidizing equivalents in going from S0 to S3, and it binds the two substrate water molecules in such proximity that the oxygen-oxygen bond can be formed and in such a way that the protons of the water molecules can be extracted. Because of its obvious importance, some authors use the term oxygen-evolving complex to refer to the Mn4O5Ca cluster. The question of which manganese oxidation states are present in the cluster has been the subject of many investigations. This matter may have been decided by experiments that made use of the photoactivation process. Starting from Mn2+ ions, molecular oxygen was produced starting after nine flashes of light, which implies that in the S3 state all four manganese atoms have attained the Mn(IV) oxidation state. Working backward, S0 is Mn(III)3Mn(IV) and so on for the other S states. Binding the substrate water molecules is a second function of the Mn4O5Ca cluster. In S1, the best characterized state of the cluster, it has five oxo bridges, and two water molecules are bound to each of Mn4 and the calcium atom. Oxygen isotope exchange experiments showed that both substrate oxygen atoms are exchangeable and that one exchanges more readily than the other. It has been suggested that the more slowly exchanging oxygen atom is O5, which is a bridging ligand to Mn2, Mn3 and Ca. It binds already in S0, possibly as a hydroxo ligand. The second substrate water molecule is bound in S2, likely to Mn4, which in S2 is five coordinate Mn(III). During the S2 to S3 transition, Mn4 becomes a six-coordinate Mn(IV) with a new water-derived ligand as the substrate oxygen atom moves to a position much nearer to O5. These motions are described as a carousel mechanism for delivery of the substrate water molecule into the position where it can be incorporated into the product. The crystal structure of S3 suggests that the two substrate oxygen atom nuclei are within 2 Å of each other. Close proximity of the two substrate oxygen atoms is essential for forming the O-O bond in the S3 to S4 to S0