A study of the photo-oxidation of the 2,6-dichlorophenol indophenol-ascorbic couple was undertaken with a view to investigating the postulated mechanism of oxidation by a component of the oxygen evolving sequence operative in photosynthesis. A spectrophotometric method for the measurement of the progress of photo-oxidation was developed. Temperature and pH dependence were determined and the effect of various common inhibitors and physiological substances tested. None of the common Hill reaction inhibitors were effective in the ascorbic acid photo-oxidation in the system used. KCN and catalase as well as a few other high molecular weight proteins were able to inhibit dark auto-oxidation of DPIP, presumably through the absorption or complexing of metal ions. The presence of catalase or peroxidase at levels of 5 x 10-7 and 1 x 10-6 Molar respectively was necessary for photo-oxidation of DPIP when KCN was added to the system in a concentration of 1 x 10-3 Molar. Many of the effects of catalase, peroxidase and KCN are unexplained. It was found that riboflavin mono-nucleotide sensitization accounted for all the photo-oxidation observed in the system used. The optimum concentration was 1 x 10-4 M. Neither chlorophyll nor chloroplasts had any effect on the reaction. It was also found that FMN could sensitize the photo-oxidation of mammalian cytochrome c Catalase, peroxidase and KCN were without effect in this reaction. The effect of ethanol concentration on the chlorophyll a sensitized photo-oxidation of DPIP and it was shown that this reaction probably does not take place in an aqueous medium as photo-oxidation decreases rapidly with ethanol concentration. The effect of light intensity on the FMN sensitized photo-oxidation of DPIP was studied and found to exhibit no inhibitory effect at levels as high as 1600 foot-candles. The possible implication of FMN in light inhibited photo-synthesis was noted. The possible implication of FMN in previously reported photo-oxidations was discussed as well as its possible role in photo-synthesis.



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Physical and Mathematical Sciences; Chemistry and Biochemistry



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