Riboflavin, photoreduction

Oster [174] proposed the second hypothesis to explain his results on the photopolymerization of acrylonitrile in aqueous solution, buffered at pH 7.0, and sensitized by xanthene dyes and riboflavin using ascorbic acid as the reducing agent. Whereas the monomer is efficiently polymerized when the solution is illuminated in the presence of oxygen, irradiation in its absence leads to photoreduction of the dye to its leuco form but no polymer is formed. Therefore, the author suggests that the leuco dye reacts with atmospheric... 

Riboflavin undergoes photoreduction and photobleaching from its triplet state,476 and the triplet states of several flavins have been studied by ESR techniques.477... 

From an analytical perspective, the single most important physicochemical characteristic of riboflavin is its photosensitivity (80-82). Exposure of this vitamin to ultraviolet and visible light results in irreversible photoreduction to lumiflavin and lumichrome and loss of vitamin activity. In addition, the coenzymes are subject to hydrolysis by endogenous phosphatases that are present in a number of foods. Since these enzymes are generally inactivated by thermal processing, they are a concern only in the analysis of fresh products. 

The nitroblue tetrazolium assay (111) is another indirect method that is used especially for detecting SOD activity on gel electrophoresis. Superoxide radicals are generated by xanthine/xanthine oxidase or by the photoreduction of flavins (typically riboflavin), which oxidize H2O to O2. The gel on which SOD samples have been loaded is then stained with nitroblue tetrazolium chloride. This reagent is reduced by superoxide to the blue-colored formazan. SOD competes with nitroblue tetrazolium and produces colorless zones on the blue gels. This method, which is highly speciflc toward superoxide dismutase, is limited by its low reliability with respect to quantitative determinations. 

Efficient photoreduction of nitrate to nitrite by riboflavin can be effected using molybdate in glacial acetic acid.43... 

A more detailed study of the role of the activator shows that water may be decomposed photochemically by riboflavine, not only in the presence but also in the absence of activator. During an aerobic photoreduction, the following reaction is postulated, in addition to reaction (7). 

Activators increase the quantum yield for the photoreduction of ribo-flavine, and appear to facilitate the cleavage of the 0—H bond of water. Their role may be, first, to provide sites for hydrogen bonding within a complex consisting of riboflavine, water, and activator, and, second, to promote reaction by accepting the oxygen moiety from the water. 

In comparison with riboflavine, the quantum yield for the photoreduction of lumiflavine is very low, indicating that the n-ribityl group also contributes to the lowering of the bond strength of water. Although the structure of the riboflavine-water-activator complex has not yet been elucidated in detail, there is evidence that the 2-hydroxyl group of the D-ribitol residue may be involved. 

The operation of an ionic reduction mechanism certainly would not exclude pesticides from also taking part in the well-known free-radical abstraction of hydrogen atoms. Indeed, it seems inevitable that certain reactions such as the light-energized phenylation of the fungicide Phygon (XVI) (53) must take place exclusively via the free-radical route (Equation 4), and sensitized photooxidations and photoreductions in the presence of natural water constituents such as chlorophyll and riboflavin undoubtedly will be shown to be important in pesticide transformations in the environment. 

Xanthine oxidase/xanthine Electrolytic reduction of Oj Photoreduction of riboflavin Autoxidation of pyrogallol Autoxidation of catecholamines >5°) NADH oxidation by phenazine methosul-fate... 

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