Water oxidation center

Toward a Model of the Photosynthetic Water Oxidation Center... 

Manganese is an element that is essential for life. It is present at the active site of many en2ymes [4, 5]. Those en2ymes in which the metal center is involved in a redox process are manganese catalase [101], peroxidase [102], and SOD [103]. In addition, a cluster containing four Mn and one Ca atoms in the water-oxidizing center (WOC) of PSII is the site at which dioxygen is produced photosynthetically on Earth [3,104]. 

Water Oxidation Center Model Using Trinuclear Ruthenium... 

Fig. 1. Electron flow from water oxidation center in photosynthesis...

Fig. 16. A model water oxidation center composed of Nation membrane coated n-CdS semiconductor electrode and metal complex catalyst...

Recent years have witnessed great activity in studies of tetranuclear Mn compounds, largely because of their possible relevance to the putative Mn4 cluster at the water oxidation center (WOO of photosystem II (9, 21,22, 96-99) and because of their use as high-nuclearity cluster building blocks, the latter forming the basis of possible molecular magnetic materials (11, 12, 24, 27). 

How to determine protonation modes in reaction centers of enzymes is a very important issue in biochemistry [1-8], The protonation is obviously related to the catalytic activities of active side chains of amino residues the protonation and deprotonation to side chains of charged acids such as Glu, Asp, Arg, Lys, and His yield Brpnsted-Lowry acids and bases, catalyzing various chemical reactions. Also in metalloenzymes and these model systems, protonations are often critical parts of the reaction mechanisms. For instance, in (1) the water-oxidizing center (WOC) in photosystem II [9-14], (2) the Mn dimeric center in catalase [15-21], and (3) the Mn center in Mn superoxide dismutases (MnSODs)[22,23], the following reactions proceed, respectively ... 

That is, Mg2+ and Ba2+ may be more effective in preventing the rebinding of Mn " " than they are in causing Mn2+ removal. We do not know whether the Mn + is bound to the L-amino acid oxidase. If the enzyme was responsible for binding both ions, Mn2+ would probably be bound at a different site than Ca t. we have previously shown that some transition metals inhibit the L-arginine oxidation in a lower concentration range than the alkali earth metals (Pistorius, Voss, 1980). A dual requirement of Mn2+ and Ca2+ for the activation of the latent water-oxidation center has also been shown for intact chloroplasts isolated from wheat leaves grown under intermittent flash illumination (Ono,... 

Lightox A photochemical process for destroying organic materials in aqueous solution by oxidation with chlorine, activated by ultraviolet radiation. Developed by the Taft Water Research Center, United States in the 1960s. 

Cu(NH3)2BTC2/3 and finally copper hydroxide in the presence of water. The formation of the BTC salts was supported by the collapse of the structure after interaction of ammonia with unsaturated copper centers. The release of BTC and copper oxide centers provides sites for reactive adsorption of ammonia during the course of the breakthrough experiments. Interestingly, even though the structure collapses, some evidence of the structural breathing of the resulting materials caused by reactions with ammonia was found, based on the ammonia adsorption at equilibrium and the analysis of the heat of interactions [51]. 

R. Nakamura and H. Frei, Visible fight-driven water oxidation by Ir oxide clusters coupled to single Cr centers in mesoporous silica, J. Am. Chem. Soc., 128, 10668-10669 (2006). 

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. 

One view is that three distinct Mn centers function in the water oxidation process two mononuclear Mn centers and a blnuclear Mn center (26). It was proposed (26) that the blnuclear center gives rise to the multiline EPR signal, whereas, one of the... 

The oxidation of water to dioxygen occurs as the consequence of Photosystem 11-dependent generation of a very strong oxidant. Protons liberated by the water-oxidation reaction then contribute to the thylakoid transmembrane electrochemical gradient that drives ATP synthesis. Brudvig et aL describe how flash-induced proton-release measurements have resolved key steps that provide insights on how the 02-evolving center of PSll mediates this four-electron oxidation of water. 

PHOTOSYNTHETIC REACTION CENTER PHOTOSYNTHETIC WATER OXIDATION PHOTOSYSTEM I pH profile for inactivation,... 

Rieke proteins, 47 337, 347-355 superoxide dismutases and, 45 129 Photosynthetic bacteria, 2[4Fe-4S] and [4Fe-4S] [3Fe-4S] ferredoxins, 38 255-257 Photosystem 1, 38 303-304 Pa/Fb proteins, 38 262-263 reaction center X proteins, single [4Fe-4S] ferredoxins cluster bridging two subunits, 38 251-252 Photosystem II, 46 328 interatomic separations, 33 228 mechanisms for water oxidation, 33 244-247... 

Although catalytic water oxidation (dark reaction) is the first and important reaction of the electron flow in the photosynthesis represented by Fig. 19.1 whereby water is used as the source of electrons provided to the whole system, its catalyst and reaction mechanism are not yet established.10-13) In the photosynthesis Mn-protein complex works as a catalyst for the difficult four-electron oxidation of two molecules of water to liberate one 02 molecule (Eq. (19.2)). It is inferred that at least four Mn ions are involved in the active center, but its structure is not yet completely elucidated. 

Practical interest in high-molecular-weight poly (propylene oxide) centers in its potential use as an elastomer (19). Copolymerization of propylene oxide with allyl glycidyl ether gives a copolymer with double bonds suitable for sulfur vulcanization. Table IV shows the properties of elastomers made with a copolymer prepared with a zinc hexacyano-ferrate-acetone-zinc chloride complex. Also shown are the properties of elastomers made from partially crystalline copolymers prepared with zinc diethyl-water catalyst. Of particular interest are the lower room-... 

Figure 28 Proposed role of the Mn4/Yz center in water oxidation. (From Ref. 154a.)...

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