Oxidative photosynthetic water splitting: energetics, kinetics and mechanism

Abstract

This minireview is an attempt to summarize our current knowledge on oxidative water splitting in photosynthesis. Based on the extended Kok model (Kok, Forbush, McGloin (1970) Photochem Photobiol 11:457-476) as a framework, the energetics and kinetics of two different types of reactions comprising the overall process are discussed: (i) P680+* reduction by the redox active tyrosine YZ of polypeptide D1 and (ii) Yz (ox) induced oxidation of the four step sequence in the water oxidizing complex (WOC) leading to the formation of molecular oxygen. The mode of coupling between electron transport (ET) and proton transfer (PT) is of key mechanistic relevance for the redox turnover of YZ and the reactions within the WOC. The peculiar energetics of the oxidation steps in the WOC assure that redox state S1 is thermodynamically most stable. This is a general feature in all oxygen evolving photosynthetic organisms and assumed to be of physiological relevance. The reaction coordinate of oxidative water splitting is discussed on the basis of the available information about the Gibbs energy differences between the individual redox states Si+1 and Si and the data reported for the activation energies of the individual oxidation steps in the WOC. Finally, an attempt is made to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and on the active role of the protein in tuning the local proton activity that depends on time and redox state Si. The O-O linkage is assumed to take place at the level of a complexed peroxide.

Mediator-assisted water oxidation by the ruthenium “blue dimer”cis,cis-[(bpy)2(H2O)RuORu(OH2)(bpy)2]4+

“…They also add to the limited insight available for water oxidation and make a possible connection with water oxidation in PSII, in which a peroxido intermediate has also been proposed (6,7). Preliminary electrochemical experiments in 0.1 M HNO 3 with a glassy carbon working electrode demonstrate that mediator-assisted electrocatalytic water oxidation (Scheme 2) is attainable with a turnover number of 19, which has already been achieved.…”

“…catalysis ͉ redox mediator ͉ electron transfer W ater oxidation is a key reaction in photosynthesis, the basis for most of life as we know it (1)(2)(3)(4)(5)(6)(7)(8). It is also a central reaction in artificial photosynthesis, an example being solardriven splitting of water into hydrogen and oxygen, 2 H 2 O 3 O 2 ϩ 2 H 2 (9)(10)(11).…”

Structural Basis of Cyanobacterial Photosystem II Inhibition by the Herbicide Terbutryn

“…Therefore, illumination prior to herbicide treatment will cause at least one turnover of the Mn 4 Ca cluster. Nevertheless, the subsequent dark adaptation period probably allows for a relaxation in the S 1 state (about 40 min), even in the presence of terbutryn (52). This excludes the possibility that the observed shift of the chloride positions is caused by redox state changes of the Mn 4 Ca cluster.…”

Solar water-splitting into H2 and O2: design principles of photosystem II and hydrogenases

“…This uniqueness underpins the high level of complexity that is required to perform this strongly oxidizing reaction within a protein matrix. Although not all facets are yet known, several important features of the water-splitting reaction in PSII have been unraveled due to intense research efforts by many groups in the field (for recent reviews see [26][27][28][29][30][31][32][33][34] ). These key features will be briefly outlined below, and their possible significance for constructing artificial catalysts for the watersplitting half-reaction (1) was not yet identified in crystal structures.…”