Photolysis and photo-oxidation of amino acids and peptides. II. Photo-degradation of cysteine and related amino acids

Forbes, W.F.; Savige, W.E.



Accession: 025234572

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S-methylcysteine (II) or lanthionine (HI) in neutral aqueous solution is partly converted mainly to the corresponding sulphoxide (IV) on irradiation with sunlight or with a sunlamp; cysteine (I) affords cystine and/or products such as the acids (V-for example alaninethiosulphuric acid, V, X = SSO3H) shown to arise from photo-degradation of cystine under similar conditions. In dilute acid or alkaline solution, greater variations occur in the product patterns for cystine and lanthionine than for S-methylcysteine. Cystine in dilute acid solution in the presence of air gives alaninesulphinic acid (V, X = SO2H) and cysteic acid (V, X = SO3H) but not the acids V (X = SSO3H, SSSO3H and X = SO2SH). In the solid state little photo-degradation of the amino acids I, II, III or of cystine is detected. Irradiation of each of the amino acids I, n, or III in solution with 2537 A light affords good yields of alanine and serine in addition to the same products as were obtained with the sunlamp; this behavior contrasts with that of cystine, when only traces of alanine and serine are obtained on irradiation with 2537 A light unless air is excluded when alanine is the main product. Alanine is also obtained from each of the three amino acids I, II, III or cystine on irradiation with 2537 A light in the solid state. In addition, irradiation of cysteine, cystine or lanthionine causes some interconversion. The main reactions of I, II and III with 2537 A light are compatible with homolytic fission of one of the bonds adjacent to the sulphur atom to yield the corresponding free radicals. In conformity with the above type of fission the amino acids I, II, III and also cystine give characteristic ESR absorption spectra on irradiation with 2537 A light. On irradiation with the sunlamp, cystine but not the amino acids I, II or III, gives a strong ESR signal, however the free radical or radicals responsible0do not appear to be identical with those produced by the action of 2537 A light.