Ultraviolet- and x-ray-induced radicals in frozen polar solutions of L-tryptophan

Acid, neutral, and alkaline matrices containing the aromatic amino acid L-tryptophan have been uv and x irradiated at 77 K, and the irradiation products examined by ESR and optical absorption spectroscopy. Three types of tryptophan radicals have been detected: (1) T1ox, supposedly the tryptophan cation, which has a characteristic optical absorption band around 600 nm and a singlet ESR spectrum at g=2.004 of width around 25 G, (2) T2ox, probably the radical formed by H abstraction at N1, which absorbs light around 550 nm and has a narrow singlet ESR spectrum (g=2.004, H=13–14 G), and (3) Tred, assumed to be the radical formed by H addition at C7, showing an ESR spectrum which is a triplet of triplets with hfs constants a1=39 G and a2=12 G. T1ox is formed by photoionization of tryptophan or by the reaction of tryptophan molecules with x-ray induced oxidized solvent radicals such as Cl2− in matrices containing LiCl or HCl, O− in alkaline solutions, and sulphuric acid radicals in matrices containing H2SO4. T2ox is formed by radical conversion of T1ox in acid matrices, and is probably the main oxidized tryptophan radical present in alcohol matrices. Tred is formed by the reaction of electrons as well as H atoms with tryptophan molecules.