Photochemistry of aromatic amino acids in solution. I. DL-phenylalanine, DL-tyrosine and L-dopa

Abstract

Abstract— The action of ultraviolet radiation on dl‐phenyldanine, dl‐tyrosine and l‐dopa has been studied in dilute aqueous solutions (10‐2M–5 × 10‐3M). Irradiation was performed in nitrogen or in air, at a wavelength of 254nm. The photoproducts of low molecular weight (amino acids, carboxylic acids, amines) were isolated either by chromatography on an ion exchange column or by thin‐layer chromatography and identified mainly by ultraviolet absorption and staining with ninhydrin. The isolation of photopolymers was carried out by chromatography on a Biogel column; identification of these photoproducts was mainly achieved by ultraviolet and infrared spectroscopy.With phenylalanine, which is the most photosensitive amino acid, the principal reaction results in splitting of the side chain, giving alanine, glycine, and four other amine compounds whose structure has not been determined. In air, additional minor products resulting from photooxydation are obtained: ortho, meta and paratyrosine. Traces of phenylethylamine were isolated after irradiation in an inert atmosphere. The polymers produced under air and nitrogen are similar. They probably result from rearrangement of aromatic radicals formed by splitting of the side chain.Tyrosine and dopa, irradiated in air, yield mainly melanin; before polymerization, tyrosine is first converted into dopa. In a inert atmosphere, these aromatic aminoacids also polymerize, giving monophenolic and biphenolic compounds with structure close to that of melanin. The biphenolic polymer (thus obtained from dopa) shows the properties of a leuco derivative of melanin. The monophenolic polymer can be converted into melanin by the combined action of oxygen and ultraviolet radiation. Other reactions give only minor products: parahydroxy‐phenyllactic acid and 3,4‐dihydroxyphenyllactic acid by deamination‐hydroxylation of tyrosine and dopa (in air): meta and paratyrosine by dehydroxylation of dopa, either in air or in nitrogen.