Photoreduction, selective

Recent results include the isolation of a clone, 104G5 with 14 times higher tolerance to 02 than the WT (obtained by the application of one round of photoreductive selective pressure) and the isolation of first and second generation mutants with, respectively, 3-4 and 10 times higher tolerance to 02 than the WT (by application of two rounds of the H2-evolution selective pressure). 

Several papers in the last two years attempted to separate the photoreceptor-flavin (and/or the associated b-type cytochrome) from the overwhelming amount of bulk-flavin which is involved in metabolism rather than photoreception. However, a clear-cut assay for the bluelight receptor has not yet been found. Specific photoaffinity labeling and the considerable selectivity in the photoreduction of a b-type cytochrome, as mediated by suitable dyes, are the most promising attempts. 

The continuous flow method is still necessary when one must use probe methods which respond only relatively slowly to concentration changes. These include pH, Oj-sensitive electrodes, metal-ion selective electrodes,thermistors and thermocouples, " epr and nmr detection. Resonance Raman and absorption spectra have been recorded in a flowing sample a few seconds after mixing horseradish peroxidase and oxidants. In this way spectra of transients (eompounds I and II) can be recorded, and the effext of any photoreduction by the laser minimized. ... 

Figure 15.2 Schematic representation of selective fabrication of Pt nanowires and Pt nanoparticles by wet photoreduction and by dry H2 reduction of H2PtCl6/FSM-16, respectively.

Photoreduction was quenched by high concentrations of biacetyl, slightly retarded by iodonaphthalene, but not affected by azulene or anthracene.113 These observations led to the unsatisfying conclusion that reduction proceeded via a triplet state which could be only selectively quenched. However, later work114 using flash photolysis showed that the benzophenone ketyl radical was generated upon irradiation of solutions of benzophenone and acridine, and that its predominant mode of disappearance was by reaction with... 

Photocatalytic reduction of C02 can be accomplished by suspending photosensitive semiconductor powders in aqueous solutions under irradiation, usually using UV light.129156 Photoreduction of C02, however, is in competition with H2 formation due to water decomposition, and leads to mixtures of reduced carbon products. Selectivity, therefore, is one of major problems of these processes. 

Almost all photoreactions of porphyrins and phthalocyanines occur via the triplet state whose lifetime (ca. 1 ms at T = 300K) and yields of formation are greatest for porphyrins. The redox potentials of porphyrins are also more amenable to water photoreduction via oxidative or reductive cycles and some selected values are found in Table 11. 

No attention is given to the mechanistic importance of a reaction rather, an attempt has been made to concentrate on reactions that have an actual (potential) synthetic role. This is not always an obvious selection, because photochemistry has not been sufficiently used in such syntheses, and mechanistic studies are not necessarily a reliable guide towards this aim. As an example, hydrogen abstraction by ketones (Scheme 1.3), which probably is the most thoroughly studied photochemical reaction, is not mechanistically discussed. Neither is presented the resultant photoreduction of ketones (Scheme 1.3, path a), because this will hardly ever become a sensible synthetic alternative for the reduction. However, other reactions arising from the same primary photoprocess, namely bimolecular reduction (path b) and... 

Goren Z., Willner I., Nelson A.J., Frank A.J. (1990) Selective Photoreduction of CO2/HCO3 to Formate by Aqueous Suspensions and Colloids of Pd-Ti02, J. Phys. Chem. 94(9), 3784-3790. 

Takashima H, Shinkai S, Hamachi I. Ru(bpy)3-based artificial receptors towards a protein surface selective binding and efficient photoreduction of cytochrome c. J Chem Soc Chem Commun 1999 2345-6. 

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

While indeed a clean photoreduction of COa to CO in the complex (prophos)Cu (C03)Cu (prophos) was observed the concomitant oxidation did not yield oxygen or hydrogen peroxide but phosphine oxide (30). In order to prevent this dead end, phosphines have to be replaced by ligands that do not act as oxygen atom acceptors. For this purpose we selected the tridentate ligand hydrotris (3,5-dimethyl-l-pyrazolyl)borate (Tp ) as a suitable ligand (40). Various Cu(I) and Cu(II) complexes of Tp or other Tp derivatives have been prepared and characterized (41-45). Unfortunately, we were not able to obtain simple carbonate complexes of the Cu Tp moiety (Structure 1). In the presence of COs, Cu Tp underwent a decomposition. 

Selective production of enzymatically active NADH by visible light and without formation of dimers has been achieved by the photoreduction of NAD using [Ru(bipy>3]S04 and [Ru(bipy)3]2(S04)3 as sensitizers, and triethanolamine as electron donor.The synthesis, photophysical properties, and excited-state redox behaviour of [(CN)(bipy)2Ru(CN)Pt(dien)X 104 and [(dien)PtNC Ru(bipy)2 CNPt(dien)]C104 (dien = diethylenetriamine) have been reported, as have the light-induced electron-transfer reactions between [Ru(bipy) (thpy)3 f [where thpy = 2-(thiazol-2 -yl)pyridine, n=2,1, or 0] and methyl viologen. ... 

An analysis of the time course of luminescence emission following selective laser excitation has enabled the forward and reverse rate constants to be obtained for the equilibrium between the Eu" complex of 1,2-diaminocyclohexanetetraacetate, [Eu(DCTA)], and iminodiacetate, IMDA, to form the ternary complex [Eu(DCTA)(IMDA)] . Some other systems in which chemical interconversion processes occur at rates much greater than, or much slower than, the reciprocal excited-state lifetime of the Eu" -containing species were also examined. A matrix isolation e.s.r. study has appeared of the photoreduction of the formato-complex of Eu " in a formic acid-sodium formate buffer. A three-step cyclic scheme has been suggested to explain the observations, namely photoreduction of by Eu, radical alternation from H to C02 , and oxidation of CO2 by Eu ". ... 

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