Intermediates of the S3 state of the oxygen-evolving complex of photosystem II

The S-3 state of the water-oxidizing complex (WOC) of photosystem II (PSII) is the last state that can be trapped before oxygen evolution occurs at the transient S-4 state. A number of EPR-detectable intermediates are associated with this critical state. The preceding paper examined mainly the decay Of S-3 at cryogenic temperatures leading to the formation of a proton-deficient configuration of S-2 termed S-2'. This second paper examines all intermediates formed by the near-IR light (NIR) excitation of the S-3 state and compares these with the light-excitation products of the S-2' state. The rather complex set of observations is organized in a comprehensive flowchart, the central part of which is the S-3...Q(A)(-) state. This state can be converted to various intermediates via two main pathways: (A) Excitation Of S-3 by NIR light at temperatures below 77 K results presumably in the formation of an excited S-3 state, S-3*, which decays via either of two pathways. Slowly at liquid helium temperatures but much faster at 77 K, S-3* decays to an EPR-silent state, denoted S-3, which by raising the temperature to ca. 190 K converts to a spin configuration of the Mn cluster, characterized by g = 21, 3.7 in perpendicular and g = 23 in parallel mode EPR, denoted S-3'. Upon further warming to 220 K, S3' relaxes to the untreated S3 state. Below about 77 K and more favorably at liquid helium temperatures, an alternative pathway Of S-3* decay via the metallo-radical intermediate S-2'Z(.)...Q(A)(-) can be traced. This leads to the metastable state S-2'Z...Q(A) via charge recombination. S-2'Z(.) is characterized by a split-radical signal at g = 2, while all S-2' transients are characterized by the same g = 5/2.9 (S = 7/2) Configuration of the Mn cluster with small modifications, reflecting an influence of the tyr Z oxidation state on the crystal-field symmetry at the Mn cluster. (B) S-2'...Q(A) can be reached alternatively by the slow charge recombination Of S-3 and Q(A)(-) at 77 K. White-light illumination Of S-2'...Q(A) below about 20 K results in charge separation, reforming the intermediate S-2'Z(.)...Q(A)(-). Thermally activated branches to the main pathways are also described, e.g., at elevated temperatures tyr Z(.) reoxidizes S-2' to the S-3 state. The above observations are discussed in terms of a molecular model of the S-3 state of the OEC. Main aspects of the model are the following. Intermediates, isoelectronic to S-3, are attributed to the NIR-induced translocation of the positive hole to different Mn ligands, or to tyr Z. On the basis of a comparison of the electron-donating efficiency of tyr Z and tyr D at cryogenic temperatures, it is inferred that the Mn cluster acts as the main proton acceptor from tyr Z. Water associated with the Mn cluster is assumed to be in hydrogen-bonding equilibrium with tyr Z, and an array comprising this water and adjacent water (or OH or O) ligands to Mn followed by a sequence of proton acceptors is proposed to act as an efficient proton translocation pathway. Oxidation of the tyrosine by P-680(+) repels protons to and out from the Mn cluster. This proposed role of tyr Z in the water-splitting process is described as a proton repeller/electron abstractor.