Photosynthetic oxygen evolution: changes in magnetism of the water-oxidizing enzyme

Changes in magnetic susceptibility produced by single-turnover flashes of light have been measured for the first time for four of the oxidation states, so-called S states, produced during oxygen evolution in Photosystem II (PSII) complexes of spinach. The data reveal new insights into the structure and bonding of the manganese cluster responsible for catalysis of water oxidation. In samples that have been dark adapted for 15 min or longer to favor population of the resting S{sub 1} state, a train of six flashes increases the paramagnetism on flashes 1, 3, and 5, while no or small increases are observed on flashes 2, 4, and 6. Advancement to the S{sub 1} state does not restore the dark level of S{sub 1} magnetism. This is due to two effects: formation of net paramagnetism from O{sub 2} release on the S{sub 4} {yields} S{sub 0} reaction (scavengeable by glucose oxidase) and a large increase in magnetism for the S{sub 1}(resting) {yields} S{sub 2} reaction, which is not restored without dark readaptation. Comparison of these data with models proposed for the structure of the manganese site reveals that models in which oxidation of substrate water occurs prior to S{sub 4} or oxidation of magnetically isolated Mn ions cannot account for the susceptibility changes observed. The large increase of 17 {mu}{sub B}{sup 2}/PSII observed for the S{sub 1} (resting) {yields} S{sub 2} oxidation is opposite in sign to the decrease in paramagnetism reported for oxidation of synthetic Mn dimers containing the {mu}{sub 2}-oxo-di-{mu}{sub 2}-carboxylato and di-{mu}{sub 2}-oxo-{mu}{sub 2}-carboxylato bridges undergoing the oxidation Mn{sub 2}(III,III) {yields} Mn{sub 2}(III,IV).