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PHOTOCHEMISTRY AND METAL CATALYSIS: STUDIES ON A FLAVIN SENSITIZED: OXIDATION OF ASCORBATE *
Author(s) -
Homann Peter,
Gaffron Hans
Publication year - 1964
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1964.tb08170.x
Subject(s) - chemistry , flavin group , photochemistry , ascorbic acid , oxygen , substrate (aquarium) , metal ions in aqueous solution , dcmu , metal , organic chemistry , photosystem ii , enzyme , biochemistry , oceanography , food science , geology , photosynthesis
Summary This paper constitutes a study of the interaction between ascorbate, diketogulonate, manganous ion, oxygen and illuminated flavins (or other sensitizers). When ascorbate is chosen as the substrate, the resulting photoxidation assumes a peculiar two‐step course. This happens because traces of ascorbate inhibit the photoxidation of diketogulonate and oxygen retards the photoxidation of small amounts of ascorbate. Sensitized photo‐reactions which are inhibited by high concentrations of oxygen are affected by the addition of various organic compounds. Of these, the herbicides 3‐(p‐clilorophenyl)‐1,1‐dimethylurea (CMU) and 3‐(3, 4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) interfere nearly exclusively with flavin sensitized photoreactions, while other inhibiting compounds tested, such as tryptophane and tyrosine, do not display a similar selectivity for only one dye. A second even more rigid specificity concerns the role of manganous ions. Diketogulonate is oxidized through the action of light excited flavin or of traces of alkali in the dark only when Mn ++ is available. No other metal tested had this ability. The response of flavin sensitized photoreactions to the addition of CMU or DCMU varies with the substrate being oxidized. With ascorbate and 2, 3‐diketogulonate as substrates in the presence of manganous ions, CMU reverses the inhibition by oxygen, but with ethylenediamine tetraacetic acid (EDTA) or triethylene tetramine (TETA) as substrates CMU acts as an inhibitor itself. With one exception all effects of CMU on flavin sensitized reactions, be they anaerobic bleaching or aerobic oxidations, were inhibitory. The one exception, the apparent promotion of manganese catalyzed photoxidations, may come about through the inhibition of competing reactions such as the quenching of light‐excited dyes by oxygen. At the same time it IS noteworthy that the manganese dependent photoxidation is refractory against CMU as inhibitor. The conclusion is inescapable that herbicides like CMU form stable charge transfer complexes with excited flavin molecules. But simple complex formation is not sufficient to explain all of our findings on the basis of the two most frequently proposed reaction mechanisms for sensitized light reactions. We have had to modify them to make them fit the experiments here summarized.