A number of amino acids in solution have been irradiated by ultraviolet light and the rate of yellowing measured. With the exception of cystine, the production of color depends on the presence of oxygen, while the decomposition of the amino acid takes place even in the absence of oxygen, although at a slower rate. Cystine produces color both in the presence and absence of oxygen. A number of ultraviolet-absorbing soluble proteins were found to "yellow" on irradiation even in the absence of oxygen. Based on the amino acid composition, it is postulated that the sulfur-containing amino acids plus tryptophan are responsible for the color produced. The degree of coloration cannot be correlated with any single amino acid.
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Leaver, I.H. and Lennox, F.G., Studies on the Photodegradation of Tryptophan , Photochem. Photobiol.4, 491-497 (1965).
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