Oxygen photoproduction from water

Metalloporphyrins electrical properties, 144 in oxygen production from water, 522 photochemical properties, 510 in photoproduction of hydrogen from water, 511, 510-513 structure, 615 Mctallothioncins cobalt(II), 673 copper and zinc storage, 672 EXAFS, 673 mammals... 

Fig. 1 Schematic drawing of hydrogen peroxide photoproduction by the biological photosynthetic apparatus with electrons either from water or from an exogenous electron donor. A redox catalyst (RC) transfers electrons from the terminal acceptor of photosystem I to molecular oxygen.

The pyrimidine nucleobases have the highest quantum yields for photoreactivity, with thymine uracil > cytosine. The purine nucleobases have much lower quantum yields for photochemistry, but can be quite reactive in the presence of oxygen. As can be seen from Figure 9-3, thymine forms primarily cyclobutyl photodimers (ToT) via a [2ir + 2tt cycloaddition, with the cis-syn photodimer most prevalent in DNA. This is the lesion which is found most often in DNA and has been directly-linked to the suntan response in humans [65]. A [2Tr + 2Tr] cycloaddition reaction between the double bond in thymine and the carbonyl or the imino of an adjacent pyrimidine nucleobase can eventually yield the pyrimidine pyrimidinone [6 1]-photoproduct via spontaneous rearrangement of the initially formed oxetane or azetidine. This photoproduct has a much lower quantum yield than the photodimer in both dinucleoside monophosphates and in DNA. Finally, thymine can also form the photohydrate via photocatalytic addition of water across the C5 = C6 bond. 

Indoles were high (0.02-1.2), evidence that auto-catalysls occurred In distilled water. In aqueous CRM-1, Indoles underwent sensitized photolysis, possibly through a superoxide or singlet-oxygen Intermediate. The same photoproduct of 3-MI, o-(N-formyl)amlnoacetophenone, was found In both distilled water and In aqueous CRH-1. Carbazole underwent direct photolysis In both distilled water and aqueous CRM-1, Indicating that It has a different photolysis mechanism from that of Indoles In the two systems studied. 

It IS likely that lawsone is a "second-generation" photoproduct. One mechanism by which it could be formed is the decomposition of 1,2-naphthoquinone with water. The source of the 1,2-quinone is presumed to be the reaction of 1-naphthol and superoxide radical, produced from dissolved oxygen by electron transfer from an excited state of 1-naphthol (9) ... 

The distal arrangement of the substituents in 24 prevents cyclization to a benzoxazole-type intermediate. While stable in alcohols, this intermediate can undergo hydrolysis (in water) to give hydroxyformanilide (25). Further analysis of the reaction revealed that 24 is a secondary photoproduct of the first-formed azirine 26, in contrast with the thermally induced azirine conversion to isonitrile that is observed for 18. Triplet quenchers such as oxygen and methyl acrylate quench the reaction efficiently. The authors propose a mechanism for conversion of 23 to 26 that involves a deprotonation-reprotonation sequence preceded by intramolecular electron transfer from the triplet-excited state of 23. 

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