The thermodynamic constraints for the operation of the water oxidizing Mn4/Ca cluster within Photosystem II (PS II) are discussed. These are then examined in the light of the known redox chemistry of hydrated Mn-oxo systems and relevant model compounds. It is shown that the latest high resolution crystal structure of cyanobacterial PS II suggests an organization of the mono Ca tetranuclear Mn cluster that naturally accommodates the stringent requirements for successive redox potential constancy, with increasing total oxidation state, which the enzyme function imposes. This involves one region of the Miu/Ca cluster being dominantly involved with substrate water binding, while a separate, single Mn is principally responsible for the redox accumulation function. Recent high level computational chemical investigations by the authors' strongly support this, with a computed pattern of Mn oxidation states throughout the catalytic cycle being completely consistent with this interpretation. Strategies to design synthetic, biomimetic constructs utilizing this approach for efficient electrolytic generation of hydrogen fuel within artificial photosynthesis are briefly discussed.
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