Secondary Stabilization Reactions and Proton-Coupled Electron Transport in Photosystem II Investigated by Electroluminescence and Fluorescence Spectroscopy

The oxidized primary electron donor in photosystem II, P680 +, is reduced in several phases, extending over 4 orders of magnitude in time. Especially the slower phases may reflect the back-pressure exerted by water oxidation and provide information on the reactions involved. The kinetics of secondary electron-transfer reactions in the microseconds time range after charge separation were investigated in oxygen-evolving thylakoids suspended in H2O or D2O. Flash-induced changes of chlorophyll fluorescence yield and electric field-induced recombination luminescence were decomposed into contributions from oxidation states S0, S1, S2, and S3 of the oxygen-evolving complex and interpreted in terms of stabilization kinetics of the initial charge-separated state S j YZP680 +QA -QB. In approximately 10% of the centers, only charge recombination took place. Otherwise, no static heterogeneity was involved in the microsecond reduction of P680 + by YZ (stabilization) or QA - (recombination). The recombination component in active centers occurs mainly upon charge separation in S3, and, in the presence of D2O, in S2 as well and is tentatively attributed to the presence of YZ oxS j - 1 in equilibrium with YZS j . A 20−30 μs stabilization occurs in all S-states, but to different extents. Possible mechanisms for this component are discussed. D2O was found to decrease: (i) the rate of the reaction YZ oxS1 to YZS2, (ii) the equilibrium constant between P680+YZS2 and P680YZ oxS2, (iii) the rate of the slow phase of P680 + reduction for the S3 → S0 transition, and (iv) the rate of electron transfer from QA - to QB /QB -. The increased ‘miss probability' in D2O is due to (iii).