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T1 - Water oxidation chemistry of photosystem II

Molecular evolution of photosynthetic water oxidation …

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Light-induced water oxidation in photosynthesis

Understanding biological water oxidation is central to achieving artificial photosynthesis and providing cheap and efficient hydrogen production. However, cracking the mystery of such a complex system has resulted in two competing oxidation state schemes, accompanied by controversy and debate over which is correct. Now, , of the Max Planck Institute for Chemical Energy Conversion in Germany, and his colleagues .

The mechanism of photosynthetic water oxidation …

Artificial photosynthesis relies on the creation of efficient and stable water-oxidising photocatalysts, preferably ones made of earth abundant materials. Biological catalytic cycles are complex, especially those with more than one metal ion. Part of nature’s catalyst for biological photosynthesis is a manganese cluster of four manganese ions that takes part in oxygen-evolving photosystem II, a catalytic cycle of five Si states (known as the , where i = 0–4). Controversy has arisen regarding the identity of the oxidation states of the manganese ions during the cycle, with the high-valent scheme (where, for example, S2 states are III, IV, IV, IV) and the low-valent scheme (where, for example, S2 states are III, III, III, IV) battling for supremacy.

that catalyzes oxidation of water during photosynthesis

Sodium periodate was characterized as a primary chemical oxidant for the catalytic evolution of oxygen at neutral pH using a variety of water-oxidation catalysts. Sodium periodate was found to function only for water-oxidation catalysts with low overpotentials. Studying oxygen-evolution catalysis by using sodium periodate as a primary oxidant may, therefore, provide preliminary evidence that a given catalyst has a low overpotential.

A detailed understanding of the structure and function of Photosystem II (PSII) which is responsible for the light-induced oxidation of water to O2, remains elusive despite the intense scrutiny to which this protein complex has been subjected. At the heart of this system is a metal active site which contains four Mn and one Ca atom. We are using computational chemistry to build up a structural model of the Mn4/Ca cluster in all its catalytically relevant oxidation states in order to reconcile the very different arrangements of the Mn atoms in the PSII core seen in the X-ray crystal structures, and also to rationalize data obtained from recent spectroscopic and water substrate binding studies. This model structure will have important implications as to the mechanism of water oxidation in PSII and will guide subsequent studies in the design of systems capable of artificial photosynthesis.

The Water Oxidation Bottleneck in Artificial Photosynthesis: ..

AB - The O2-evolving complex of photosystem II catalyses the light-driven four-electron oxidation of water to dioxygen in photosynthesis. In this article, the steps leading to photosynthetic O2 evolution are discussed. Emphasis is given to the proton-coupled electron-transfer steps involved in oxidation of the manganese cluster by oxidized tyrosine Z (YZ·), the function of Ca2+ and the mechanism by which water is activated for formation of an O-O bond. Based on a consideration of the biophysical studies of photosystem II and inorganic manganese model chemistry, a mechanism for photosynthetic O2 evolution is presented in which the O-O bond-forming step occurs via nucleophilic attack on an electron-deficient MnV=O species by a calcium-bound water molecule. The proposed mechanism includes specific roles for the tetranuclear manganese cluster, calcium, chloride, YZ and His190 of the D1 polypeptide. Recent studies of the ion selectivity of the calcium site in the O2-evolving complex and of a functional inorganic manganese model system that test key aspects of this mechanism are also discussed.

N2 - The O2-evolving complex of photosystem II catalyses the light-driven four-electron oxidation of water to dioxygen in photosynthesis. In this article, the steps leading to photosynthetic O2 evolution are discussed. Emphasis is given to the proton-coupled electron-transfer steps involved in oxidation of the manganese cluster by oxidized tyrosine Z (YZ·), the function of Ca2+ and the mechanism by which water is activated for formation of an O-O bond. Based on a consideration of the biophysical studies of photosystem II and inorganic manganese model chemistry, a mechanism for photosynthetic O2 evolution is presented in which the O-O bond-forming step occurs via nucleophilic attack on an electron-deficient MnV=O species by a calcium-bound water molecule. The proposed mechanism includes specific roles for the tetranuclear manganese cluster, calcium, chloride, YZ and His190 of the D1 polypeptide. Recent studies of the ion selectivity of the calcium site in the O2-evolving complex and of a functional inorganic manganese model system that test key aspects of this mechanism are also discussed.

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Chemistry for Biologists: Photosynthesis

The O2-evolving complex of photosystem II catalyses the light-driven four-electron oxidation of water to dioxygen in photosynthesis. In this article, the steps leading to photosynthetic O2 evolution are discussed. Emphasis is given to the proton-coupled electron-transfer steps involved in oxidation of the manganese cluster by oxidized tyrosine Z (YZ·), the function of Ca2+ and the mechanism by which water is activated for formation of an O-O bond. Based on a consideration of the biophysical studies of photosystem II and inorganic manganese model chemistry, a mechanism for photosynthetic O2 evolution is presented in which the O-O bond-forming step occurs via nucleophilic attack on an electron-deficient MnV=O species by a calcium-bound water molecule. The proposed mechanism includes specific roles for the tetranuclear manganese cluster, calcium, chloride, YZ and His190 of the D1 polypeptide. Recent studies of the ion selectivity of the calcium site in the O2-evolving complex and of a functional inorganic manganese model system that test key aspects of this mechanism are also discussed.

Oxidation-reduction reaction | chemical ..

"This has really upped the standard from the other known homogeneous WOCs," says Emory inorganic chemist Craig Hill, PhD. "It's like a home run compared to a base hit." For this process to become viable, say the researchers, the water oxidation catalyst must have the "triple 's' qualities" of selectivity, stability, and speed. Homogeneity is necessary since it boosts efficiency and makes the WOC easier to study and optimize.

The Process of Photosynthesis in Plants: An Overview

Pantazis acknowledges the potential controversy but feels the work offers ‘a definitive answer’ to the question of what the structures and oxidation states of the manganese ions are and how they change as the catalytic cycle progresses towards the final step of water oxidation.

Oxidation and Reduction - WWW Project Top Page

Recently, Emory chemists have developed the most effective homogeneous catalyst so far for water oxidation, a crucial component of generating hydrogen fuel from water using only sunlight. They aim to mimic the natural process of photosynthesis with a carbon-free, molecular water oxidation catalyst (WOC).

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