Primary radiation damage in tryptophan and tryptamine. A pulse-radiolysis and low-temperature ESR study

The transient optical absorption spectra of tryptamine and tryptophan single crystals have been obtained by pulse radiolysis. Two components were identified. One, denoted as A, exhibiting a λsub(max) value at 450 nm, was attributed to an excited triplet. The other component, B, absorbing in the u.v., was assigned to the radical anion. Both components decayed by first-order kinetics and the decay proceeded without activation energy. For component B the decay is ascribed to protonation of the anion at the indole ring. Crystals irradiated at 15 K were studied using ESR spectroscopy. The ESR spectrum consisted of two components. One single line was attributed to the radical anion with the unpaired spin density localized on the indole moiety. The other multiline resonance was assigned to a dienyl type radical which became more prominent at higher temperatures and radiation doses. This species is thought to be formed by protonation of the anion at the indole ring. It is stable up to room temperature. The differences in structure and reactions between radical anions of amino acids with aliphatic and aromatic side chains are pointed out. Radical pairs were found in tryptophan but not in tryptamine after irradiation at 15 K. (author)