▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

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Proton-Coupled Electron Transfer

Life on earth is almost entirely solar-powered. We can get some idea of the enormous quantity of energy received from the sun by noting that during daylight hours, the sun provides several thousand times more power to the surface of the U.S.A. than is produced by all of the nation’s electrical power stations. 1,2 Around 50% of the radiation that reaches the earth’s surface, roughly the visible region, is of a frequency useful to photosynthetic organisms. Oxygenic photosynthetic organisms convert this radiation into chemical energy, in the form of carbohydrate and dioxygen, at an optimal efficiency of something like 25%. 3 These products together sustain the rest of aerobic life, with carbohydrate acting as a source of high-energy electrons and dioxygen providing a lower-energy destination for these electrons. The overall equation of oxygenic photosynthesis is given in eq 1, where (CH2O) represents carbohydrate:

Water-splitting chemistry of photosystem II.

The manganese and calcium ions of photosynthetic oxygen evolution.

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn–Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25–90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn–Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn com...

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

In plants and algae, photosystem II uses light energy to oxidize water to oxygen at a metalloradical site that comprises a tetranuclear manganese cluster and a tyrosyl radical. A model is proposed whereby the tyrosyl radical functions by abstracting hydrogen atoms from substrate water bound as terminal ligands to two of the four manganese ions. Molecular oxygen is produced in the final step in which hydrogen atom transfer and oxygen-oxygen bond formation occur together in a concerted reaction. This mechanism establishes clear analogies between photosynthetic water oxidation and amino acid radical function in other enzymatic reactions.

https://science.sciencemag.org/content/sci/277/5334/1953.full.pdf

A metalloradical mechanism for the generation of oxygen from water in photosynthesis

Temperature dependence of S-state transition in a thermophilic cyanobacterium, Synechococcus vulcanus Copeland measured by absorption changes in the ultraviolet region

Interaction of 1,4-benzoquinones with Photosystem II in thylakoids and Photosystem II membrane fragments from spinach

Mechanistic aspects of photosynthetic water oxidation were analysed in dark-adapted PS II fragments from cyanobacteria (Synechococcus vulcanus Copeland) and higher plants (spinach) by detecting flash-induced absorption changes at 830 nm, at 350 nm and at 562 or 580 nm, reflecting the turnover of the photoactive chlorophyll a, P680, of the PS II reaction centre, of the univalent redox transitions Si → Si+1, at the catalytic site of water oxidation and of external indicator dye response due to protolytic reactions, respectively. The possible role of hydrogen bonds for the structural integrity of the oxygen-evolving complex was investigated by measurements of the average oxygen yield per flash in thylakoids after heat treatment of suspensions in H2O or D2O.

B. Hanssum

Papers

Temperature dependence of S-state transition in a thermophilic cyanobacterium, Synechococcus vulcanus Copeland measured by absorption changes in the ultraviolet region

Interaction of 1,4-benzoquinones with Photosystem II in thylakoids and Photosystem II membrane fragments from spinach

New Results on the Mechanism of Photosynthetic Water Oxidation