Oxidation electron donor

In an attempt to overcome the problem of accumulation of the oxidized electron donor, we have incorporated a recyclable surface-active electron donor in DODAC vesicles (12). This electron donor contains a sulfide moiety which dimerizes upon light-induced oxidation. Simultaneously, hydrogen is evolved via vesicle-stabilized, catalyst-coated, colloidal CdS particles. The dimer could be chemically reduced for additional hydrogen formation. Figure 9 is an idealized view of this cyclic process (12). 

The electronic structure of a semiconductor is characterized by a filled conduction band and an empty valence band separated by a bandgap of energy (EG). When the catalyst is illuminated with photons whose energy is equal to or greater than this bandgap, the promotion of an electron from the valence band to the conduction band occurs with the creation of electron-hole pairs (e —h ). The valence-band hole can oxidize electron-donor molecules (water or hydroxyl ions) to produce oxidizing hydroxyl radicals, whereas the conduction-band electron can reduce acceptor molecules such as 02 (to yield a superoxide ion) or a metal ion (reduced to its lower valence states) (Figure 15.1). 

Fraction of reduced photoproduct resisting recombination with oxidized electron donor... 

Illumination at low temperature (200K) allowed us to reduce pheophytin and to trap the material for ESR studies at 20K. The ESR spectrum indicates two species which are tentatively interpreted as reduced pheophytin and an oxidized electron donor. The putative donor spectrum has a high g value (2.0045) and it could be a modified state of the physiological donor Z (Frank et al., 1987). If this interpretation is correct, it would mean that Z is carried by the Db D2 couple, like the other partners in primary photochemistry. 

Figure 1. Schematic representation of regenerative and photo-electrosynthetic cells. S is the sensitizer excited state, D is an electron donor, and D is the oxidized electron donor. Regenerative cells convert light into electricity while photoelectrosynthetic cells convert light into electricity and also produce chemical products.

Thus different mechanisms for the oxidation of substituted aromatics can be proposed, depending on the oxidant resulting in the formation of species B or C. Species C is well known to oxidize electron donor molecules such as olefines ... 

Although substituted aromatic molecules can be oxidized under mild conditions over a variety of substituted molecular sieves and using different oxidants, the choice of the latter is very important to direct the nature of the products. In the presence of transition metal ions, hydrogen peroxide forms a cyclic metalloperoxide which is known to oxidize electron donor molecules. Therefore, H202 will preferentially lead to the hydroxylation of the aromatic ring. 

The integrated N2-fixing reaction catalyzed by N2ase is represented in its simplest form in Figure 2(6). Reactants are (1) an electron donor, (2) adenosine triphosphate (ATP) and divalent cation (M ), and (3) an electron acceptor. Protons of the aqueous media as well as any one of six additional classes of reducible substrates can function in this capacity. Products are (1) oxidized electron donor, (2) adenosine diphosphate (ADP), inorganic phosphate (Pi), and (3) H2 and the reduced product characteristic of any added reducible substrates, e.g., NH3 from... 

Positively charged vesicles prepared from dioctadecyldimethylammonium chloride (DODAC) [Ci5H3iC02(CH2)2N+(CH3)(CH2C6H4CH = CH2Cr [Fig. 5(b)], as well as from its polymerized counterpart, were found to be better media for CdS-mediated water photoreduction than those prepared from DHP [Fig. 5(c)] [11,12]. Unfortunately, the quantum efficiency of the hydrogen formation was lower than 0.5% and the accumulated oxidized electron donor diphenyl disulfide PhSSPh led rapidly to the ultimate destruction of the vesicles. 

In such partial systems it is inevitable that one of the components be oxidized (electron donor in the case of water reduction) or reduced (electron acceptor in cases of water oxidation) irreversibly. In principle, reversible electron donors and acceptors can be used but cross reactions (as observed... 

Alteration of the energy level of the oxidized electron donor, P, results in changes in the energies of the charge separated states P Ha, P" Qa and P Ba". Due to the changes... 

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