The photodecomposition of tryptophan peptides

Aqueous solutions of the four tryptophan peptides, Ala-Trp-Val, Val-Trp-Ala, Ala-Ala-Trp-Val and Ala-Gly-Trp-Leu, have been irradiated with the light of wavelength greater than 295 nm. The major changes were destruction of the tryptophan residue, liberation of ammonia and the formation of photoproducts of increased molecular weight. Up to 40% of Ala-Ala-Trp-Val and Ala-Gly-Trp-Leu were converted to products with molecular weights ranging from two to four times those of the original tetrapeptides. Most of the yellow material formed during irradiation in air was present in the high molecular weight fractions. Irradiation of Ala-Gly-[14C]Trp-Leu gave the following identifiable photoproducts: Ala-Gly-Asp-Leu, Ala-Gly-(N'formyl)Kyn-Leu, Ala-Gly-Oia-Leu, and ammonia, where Kyn means kynurenine and Oia, beta-(3-oxindolyl)alanine.