Water Oxidation in Photosystem II

The next illustration of how DFT can be appHed to bioinorganic systems is the oxidation of water in photosystem II. Two different tyrosyl radicals have been detected with one of them, Tyrz, near the water oxidising complex (woe). The woe involves a Mn4 cluster which mediates the evolution of one molecule of O2 from a molecule of water for every four photons absorbed. 

The mononuclear model shown in Fig. 6 was developed but the activation free energy for the HAT process was too high. One might suppose that the chemical model was at fault, being too simplistic a representation of the true 

There were some problems with the separate enthalpic and entropic contributions which could be attributable to tunnelling effects and the nature of the model system. However, the advantage of these calculations is that not only do they suggest the FT pathway as most likely, they also provide a good 

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GT Babcock, BA Barry, RJ Debus, CW Floganson, M Atamian, L McIntosh, I Sithole, and CF Yocum (1989) Water oxidation in photosystem II From radical chemistry to multielectron chemistry. Biochemistry 28 9557-9565... 

Siegbahn P (2011) Recent theoretical studies of water oxidation in photosystem II. J Photochem Photobiol B Biol 104 94... 

INTERACTION OF CPa-I WITH COMPONENTS INVOLVED WITH WATER OXIDATION IN PHOTOSYSTEM II MAPPING OF NHS-BIOTINYLATION SITES AND THE EPITOPE OF THE MONOCLONAL ANTIBODY FAC2 TO THE LARGE EXTRINSIC LOOP REGION OF CPa-l ... 

Interaction of CPa-1 with Components Involved with Water Oxidation in Photosystem II Mapping of NHS-Biotinylation Sites and the Epitope of the Monoclonal Antibody FAC2 to the Large Extrinsic Loop Region of CPa-1 639... 

Tentatively, it seems that DCCD-binding to a LHCII-poljrpeptide is responsible for the redirection of protons from water oxidation across photosystem II to the bound quinone. Consequently, LHCII-pol3rpeptides must be involved in normal proton conduction from the Mn-centre into the lumen. 

Huang, R Kurz, R Styring, S. 2007. EPR investigations of synthetic manganese complexes as bio-mimics of the water oxidation complex in photosystem II. Appl. Magn. Reson. 31 301-320. 

The present discussion is only concerned with the structure/redox capacity of the site responsible for the oxidation of water. The starting point is the evidence that the photosynthetic pathway is triggered by photooxidation of the chlorophylls in photosystem II. The need for chlorophylls to recover the electrons lost in photooxidation (in order to regenerate their ability to absorb light) induces water to undergo oxidation, according to ... 

Mn was first shown to play an important role in photosynthetic 0 evolution by nutritional studies of algae (7). The stoichiometry of Mn in photosystem II was determined by quantitating Mn released from thylakoid membranes by various treatments (8). These experiments established that Mn is specifically required for water oxidation and that four Mn ions per photosystem II are required for optimal rates of 0 evolution (9). More recently, photosystem II preparations with high rates of Oj evolution have been isolated from a variety of sources (for a review see 10). The isolation of an O2-evolving photosystem II has proved to be a major step forward in both the biochemical and spectroscopic characterization of the O2-evolving system. These preparations contain four Mn ions per photosystem II (11), thus confirming that four Mn ions are functionally associated with each O2-evolving center. 

I have presented an overview of the current state-of-the-art in studies of the Mn complex in photosystem II. There are many unresolved questions and a clear picture of the structure and function of Mn in photosynthetic water oxidation is still not available. One useful approach to help determine the structure of the Mn complex in photosystem II involves the synthesis and characterization of Mn model complexes for comparison with the properties of the Mn complex in photosystem II. Recently, several tetrameric high-valent Mn-oxo complexes have been reported (see the chapter in this volume by G. Christou). Further characterization of existing and new high-valent tetrameric Mn-oxo model complexes, especially EPR and EXAFS measurements, will no doubt help clarify the present uncertain picture of the structure of the Mn complex in photosystem II. 

In photosystem II an intermediate tyrosyl radical is formed which then repetitively oxidizes an adjacent manganese cluster leading to a four-electron oxidation of two water molecules to dioxygen. In broad detail, the model compounds" described above were demonstrated to undergo similar reactions on photochemical excitation of the respective ruthenium centers. 

Photosystems I and II operate in concert. Their interaction is described in the Z scheme (shown in outline in Figure 18). In photosystem II, the primary oxidant is able to remove electrons from water. These electrons are transported to photosystem I via plastoquinone and plastocyanin to replace PSI electrons that have been used in the reduction of iron-sulfur proteins and transferred via NADP to 0O2. Electron flow between PSII and PSI is accompanied by the synthesis of Atp 367 These oxidizing and reducing aspects of photosynthesis can be separated and other substrates incorporated. 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

Dasgupta J, Ananyev G, Dismukes GC. Photoassembly of the water-oxidizing complex in photosystem II. Coord Chem Rev 2008 252 347-60. 

The electrons that are provided by photosystem I are finally used to reduce CO2 to carbohydrates, while in photosystem II, water is oxidized to oxygen. Intense research over many decades has partially revealed the extremely complicated mechanism of natural photosynthesis. It follows that it is obviously rather difficult to imitate this in an artificial photosynthesis that is intended to convert and store solar energy in simple but energy-rich chemicals. Different approaches have been developed to solve this problem (i). It has been suggested to facilitate artificial photosynthesis by the assistance of redoxactive metal complexes in homogeneous systems. Generally, photoredox reactions of metal... 

At pH 5, which is a reasonable estimate of the thylakoid internal volume pH, the water/oxygen couple has an oxidation-reduction potential of -t-0.93 V. The energy required to drive this reaction is supplied by photon absorption in photosystem II which produces the oxidized reaction center chlorophyll, P-680 ... 

This work > is of importance in relevant to the water oxidation reaction in the photosynthetic membrane. The oxidation of two water molecules to evolve dioxygen in photosystem II is understood to be realized by an enzymatic organization of a special Mn cluster in a protein architecture This oxygen... 

Figure 5 Transition state structure for 0-0 bond formation in a simple model of the water-oxidizing cluster in photosystem II.

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