Photosynthetic Water Splitting: Apparatus and Mechanism

This chapter reviews our current knowledge on the reactions leading to light-induced water splitting into molecular oxygen and hydrogen, bound in the form of plastoquinol. This process takes place in the multimeric protein complex of Photosystem II (PS II). After a brief description of the basic principles of biological solar energy exploitation through photosynthesis, general features of the kinetics, energetics and the structural array and nature of the cofactors of PS II are presented. The overall reaction pattern of the water:plastoquinone oxidoreductase comprises three types of sequences: (a) light-induced charge separation leading to the “stabilized” radical pair \( \text{P68}{0}^{+•}{\text{Q}}_{\text{A}}^{-•}\); (b) oxidative water splitting into molecular oxygen and four protons released into the lumen, driven by P680+• as oxidant and with tyrosine \( {\text{Y}}_{\text{Z}}\) involved as intermediate, and (c) two-step reduction of plastoquinone to quinol under uptake of two protons from the cytoplasm/stroma by the use of \( {\text{Q}}_{\text{A}}^{-•}\) as reductant. Evidence is presented that within the (Chl a)4 \( Phe{o}_{x}\) (x = 0, 1 or 2) unit, which constitutes the photoactive pigment P680, the lowest singlet exciton state predominantly located on Chl a D1 acts as electron donor for the primary charge separation and that the exceptionally high reduction potential of P680 mainly originates from a specific hydrophobic protein microenvironment. The key step of oxidative water splitting is the O-O bond formation which is shown to occur most likely at the level of a binuclearly complexed peroxide. It is emphasized that the protein matrix plays a key role in tuning the energetics and kinetics. Examples are presented for effects of protein relaxation and flexibility on the reaction pattern of PS II.