Primary Processes of Photosynthesis, Part 2: Principles and Apparatus
Chapter 17: Photosynthetic Water Splitting
Published:29 Nov 2007
J. Messinger and G. Renger, in Primary Processes of Photosynthesis, Part 2: Principles and Apparatus, ed. G. Renger and G. Renger, The Royal Society of Chemistry, 2007, vol. 9, ch. 17, pp. 291-349.
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This chapter reviews our current state of knowledge on the structure and functional pattern of the water oxidizing complex (WOC) in photosynthesis. The reactions leading to oxidative water splitting into molecular oxygen and four protons take place at a multimeric metal center, the Mn4OxCa cluster (x symbolizes the number of oxo-bridges). The overall process consists of a reaction sequence of four oxidation steps (Kok cycle), which is energetically driven by the strongly oxidizing cation radical P680+˙ with tyrosine YZ acting as intermediary redox carrier. The energetics and kinetics of these reactions are described. Based on the Kok-scheme as a fingerprint for the interpretation of spectroscopic data, information is obtained on the electronic configuration and nuclear geometry of the different redox states Si, in particular on the valence states and the distances between the metal centers. Furthermore, the exchange kinetics of substrate water in the Si states are presented. The transitions S0 → S1 and S1 → S2 are shown to be metal-centered redox steps, while S2 → S3 is favored to be a ligand-centered reaction. The redox states S3 and/or S3YZOX could comprise redox isomerism and tautomerism equilibria. Finally, we attempt to cast our current knowledge about photosynthetic water splitting into a mechanism.