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A study has been made of the effects of eight amino acids and a group of compounds related to tryptophan upon the photofading of methylene blue under anaerobic conditions and upon the excited species generated by flash photolysis of the dye. These effects are discussed in relation to the ease of sensitized photooxidation of the amino acids by methylene blue, in order to test the hydrogen-abstraction mechanism proposed for this reaction.In general, amino acids susceptible to sensitized oxidation also act as reducing agents in the photoreduction of methylene blue, while the insensitive compounds do not reduce the lightexcited dye molecule. Methionine, histidine and tyrosine are exceptions, but this is probably due to our choice of pH for the photofading experiments.The flash-photolysis experiments give no clear-cut evidence for a reaction between amino acid and dye triplet excited state leading to an enhanced yield of the semi-reduced radical, thought to be an intermediate of the photofading reaction. The lifetime of the semi-oxidized radical is reduced by many of the amino acids, and this may be the reactive intermediate in their oxidation.

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Abstract— Seven chromatographically separable products were shown to be formed when an aqueous solution of tryptophan was exposed to the light of a 100‐W bulb at pH 9 in the presence of methylene blue and oxygen. Some of these products were detected, though in much smaller quantities, even when tryptophan was irradiated in the absence of methylene blue and/or oxygen. Contrary to reports in the literature, none of the common derivatives of tryptophan, such as tryptamine, indole acetic acid, indole aldehyde, anthranilic acid or kynurenine, were detected on irradiation of the amino acid by visible light. Such irradiation of tryptamine and indole acetic acid gave 1–2 components which were chromatographically identical with those obtained from tryptophan; irradiation of indole aldehyde gave no detectable breakdown products. Exposure of tryptophan to ultraviolet light or when treated with hydrogen peroxide did not result in the formation of any of the products obtained with visible light. The results presented here suggest that during exposure of tryptophan to visible light, the indole ring is first oxygenated resulting in the formation of dioxindole derivatives. One of the products of irradiation of tryptophan with visible light was tentatively identified as dioxin‐dolylalanine.

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FOR U N D E R S T A~I N G the role of histidine in the biological activity of enzymic and hormonal proteins, photooxidation technique has been frequently employed.(1-6) The main interest in these experiments has rested in the quantitative aspect of the disappearance of histidine in the protein molecule. Weil(?* e, has shown that photooxidation of free histidine results in destruction of the imidazole group. Lukton and colleagues(s) and Weilo measured the rate constants for the destruction of this amino acid by visible light at varying pH and temperature. Information in regard to the chemical nature of the resultant products is not available.The experiments presented in this report show that when an aqueous solution of histidine containing methylene blue (MB), is irradiated with visible light, four chemically distinct products are formed which can be detected by paper chromatography. It was found, using labelled histidine, that these products retain carbon-2 of the imidazole ring. None of them were formed when a solution of histidine was subjected to the action of either hydrogen peroxide or u.v.-light. Experimental details have been described earlier. (lo) MB was removed by adsorption on active carbon; contrary to the observation made with tryptophan,(lO) histidine and its irradiation products were not adsorbed on active carbon. Irradiation at pH 7 or 9 in the presence of MB resulted in a decrease in the u.v.-absorption at all wave lengths between 200-230 mp (Figs. 1 and 2). It was shown by Saidel and associates(ll) that the imidazole group has characteristic maximum at 210-212 mp in acid but not in neutral media. An irradiated solution of histidine did not show this maximum in an acid medium ( Fig. l), suggesting that the observed decrease in the intensity of absorption at 210-212 mp on irradiation is due to the disappearance of the imidazole group. Irradiation of histidine in the absence of MB resulted in a slight increase in the absorption in the above mentioned range, which could be due to slight evaporation during the irradiation period.Separation of the products obtained by irradiation of a solution of histidine in the presence of MB was attempted using paper chromatography in two different solvents.

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