Photochemical oxidation-reduction

Oxidation-reduction reactions in water are dominated by the biological processes of photosynthesis and organic matter oxidation. A very different set of oxidation reactions occurs within the gas phase of the atmosphere, often a consequence of photochemical production and destruction of ozone (O3). While such reactions are of great importance to chemistry of the atmosphere - e.g., they limit the lifetime in the atmosphere of species like CO and CH4 - the global amount of these reactions is trivial compared to the global O2 production and consumption by photosynthesis and respiration. 

As already mentioned, macular zeaxanthin comprises two stereoisomers, the normal dietary (3/(,37()-/caxanthin and (3f ,3 S)-zeaxanthin(=(meyo)-zeaxanthin), of which the latter is not normally a dietary component (Bone et al. 1993) and is not found in any other compartment of the body except in the retina. The concentration of (tneso)-zeaxanthin in the retina decreases from a maximum within the central fovea to a minimum in the peripheral retina, similar to the situation with (3/ ,37 )-zeaxanthin. This distribution inversely reflects the relative concentration of lutein in the retina and gave rise to a hypothesis (Bone et al. 1997) that (meso)-zeaxanthin is formed in the retina from lutein. This was confirmed by an experiment in which xanthophyll-depleted monkeys had been supplemented with chemically pure lutein or (3/ ,37 )-zeaxanthin (Johnson et al. 2005). (Meyo)-Zeaxanthin was exclusively detected in the retina of lutein-fed monkeys but not in retinas of zeaxanthin-fed animals, demonstrating that it is a retina-specific metabolite of lutein only. The mechanism of its formation has not been established but may involve oxidation-reduction reactions that are mediated photochemically, enzymatically, or both. Thus, (meso)-zeaxanthin is a metabolite unique to the primate macula. 

This reaction is also an oxidation-reduction process whereby the oxygen atom is oxidized from the —2 oxidation state to the zero oxidation state as the chlorine atom is reduced from the +1 to —1 oxidation state. As diatomic oxygen is an effective disinfectant, pool owners should avoid the loss of O2 via the decomposition of the hypochlorite ion. Adding hypochlorite-containing disinfectant in the evening hours reduces the loss of the ion from photochemical decomposition. 

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

H. B. Gray We must remember that we are still in the initial stages of systematic study of inorganic oxidation-reduction photochemistry. Nature has indeed some slick ways to optimize photochemical energy conversion. I am confident that inorganic chemists will do as well or better, perhaps even before the turn of the century ... 

We have been investigating the oxidation-reduction reactions of the binuclear iron site in the protein matrix (27-32). The methemerythrin form contains both irons in the +3 oxidation state and can be reduced in two steps (by dithionite ion (27, 31), reduced methylviologen, and photochemically using a riboflavin/EDTA mixture (28)) to the deoxy form in which both irons are in the +2 oxidation state. The intermediate (semi-met), in which one iron is +3 and the other iron +2, has been... 

Iron oxide dissolution can proceed by a variety of pathways, viz. protonation, com-plexation and reduction, photochemical and biological. 

General chemical properties of triazolopyridines, such as oxidations, reductions, reaction with electrophiles, reactions with nucleophiles, homolytic reactions, ring-opening reactions, and photochemical reactions can be found in <2002AHC(83)2>. 

In the case of two flavoenzyme oxidase systems (glucose oxidase (18) and thiamine oxidase s where both oxidation-reduction potential and semiquinone quantitation values are available, semiquinone formation is viewed to be kinetically rather than thermodynamically stabilized. The respective one-electron redox couples (PFl/PFl- and PFI7PFIH2) are similar in value (from essential equality to a 50 mV differential) which would predict only very low levels of semiquinone (32% when both couples are identical) at equilibrium. However, near quantitative yields (90%) of semiquinone are observed either by photochemical reduction or by titration with dithionite which demonstrates a kinetic barrier for the reduction of the semiquinone to the hydroquinone form. The addition of a low potential one-electron oxidoreductant such as methyl viologen generally acts to circumvent this kinetic barrier and facilitate the rapid reduction of the semiquinone to the hydroquinone form. 

A major group of photochemical reduction reactions are oxidation-reduction processes. As typical examples, phenazine (CXXI) and alloxan (CXXIII) are reduced by ethanol to give dihydrophenazine (CXXIl)/ 2 and alloxantin (CXXIV).42 Isatin (CXXV) in the presence of ace-naphthene (CXXVI) is reduced to isatide (CXXVII).204 The photoreaction proceeds at the expense of the alcohol, or (CXXVI) acetaldehyde and acenaphthylene (CXXVIII), are formed as by-products respectively. The formation of CXXVII may be due to the interaction of CXXV with the intermediate oxindole (CXXIX). 

Another approach to wards photocatalysis is to use dy as a sensitizer instead of a semiconductor as in photosynthesis. It is not the aim of this book to cover all the aspects of the sensitized photochemical conversion system, but typical sensitized systems for photocatalytic reactions of water are described in Chapter 18 The concept of a photochemical conversion system using a sensitizer and water oxidation/reduction catalysts is mentioned in Chapter 19, accompanied by a discussion on the sensitization of semiconductors. 

Besides the photochemical dissociation, ozone decays in oxidation-reduction reactions with different species. The stratospheric 03 reacts rapidly with nitric oxide and products of photodissociation of halogenated hydrocarbons (Figure 9.5). 

Photoelectrochemistry A term applied to a hybrid field of chemistry employing techniques which combine photochemical and electrochemical methods for the study of the oxidation-reduction chemistry of the ground or excited states of molecules or ions. In general, it is the chemistry resulting from the interaction of light with electrochemical systems. 

Photogalvanic cell An electrochemical ceU in which current or voltage changes result from photochemically generated changes in the relative concentrations of reactants in a solution phase oxidation-reduction couple. Compare photovoltaic cell. 

When dte oxidation-reduction equilibria in equation (6a) ate included, the thermal activation of elec-tron-transfn oxidation in equation (3b) follows a course that is akin to charge-transfn activation in equation (5). In both, the A cotiq>lex [RH,A] is the important precursor which is directly converted into the critical contact ion pair [RH, A ]. Such an involvonent of reactive intermediates in common does widen the scqte of electron-transfer oxidations to include both thermal and photochemical pro-... 

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