The photochemical degradation of cystine in aqueous solution in the presence of air

Abstract

1. 1.Ultraviolet irradiation of cystine in air-saturated aqueous solution at pH 1 and 10 has been investigated. 2. 2. The quantum yield for destruction of cystine has been shown to be independent of pH, the increase in destruction rate at pH 10 being explained by the increased absorption of energy at this pH. The reaction shows first-order characteristics when the intensity of the incident is in excess of that absorbed by cystine. 3. 3. Degradation of cystune occurs only when the wavelength of the incident radiation is below 300 nm. 4. 4. A large number of products have been confirmed, notably pyruvic acid, cysteine, ammonia, cysteic acid, alanine 3-sulphinic acid, cysteine S-sulphonic acid, alanine, serine, glycine, SO 4 2− and, in acid solution, lanthionine, bis-(2-amino-2-carboxyethyl)trisulphide, bis-(2-amino-2-carboxyethyl)_tetrasulphide and H 2S. NH 2OH has been identified at both pH values. 5. 5. It has been deduced that the difference in products formed in acid and alkali is due to secondary reactions rather than to any difference in the origical fission of the cystine links. 6. 6. Evidence suggests that fission of C-S, S-S and C-N links occur in acid and alkaline solution. 7. 7. No decarboxylation products of cystine have been detected. 8. 8. A reaction mechanism has been postulated which accounts for the products identified and for the amounts formed.