Sensitized Photooxidation of Amino Acids: Effects on the Reactivity of Their Primary Amine Groups With Fluorescamine and O‐phthalaldehyde

The photooxidation of his, met, trp and tyr with methylene blue and eosin as sensitizers resulted in a loss of primary amine reactivity as measured with fluorescamine and o‐phthalaldehyde. The rates of oxygen uptake and of loss of primary amine reactivity with illumination was characteristic for each amino acid. Glycine did not photooxidize and showed no change in primary amine reactivity. Primary amine reactivity of the N ‐acetyl derivatives of these amino acids was negligible and during photooxidation the reactivity increased slowly with N ‐acetylhis and N ‐acetyltrp while the reactivity of N ‐acetylmet and N ‐acetyltyr did not change. Imidazole and indole also showed a slow increase in primary amine reactivity during sensitized photooxidation. The rate of oxygen uptake and primary amine reactivity loss during the sensitized photooxidation of his and met was markedly increased in deuterium oxide. This indicated that singlet oxygen was involved. Radical scavengers did not affect the rates indicating that free radical intermediates were not involved. Primary amine reactivity loss is probably due to intra‐ and/or inter‐molecular secondary dark reactions between unknown initial oxygenation products and primary amino groups.