The Aerobic Riboflavin-Sensitized Photodecomposition of Tryptophan, Tryptamine and Indole Acetic Acid. Ground-State and Photopromoted Flavin-Indole Interactions

The interaction between riboflavin (RF) and the indole derivatives tryptophan, tryptamine and indole-3-acetic acid at visible light irradiation was studied in water and water/ethanol mixed solvent. Besides a dark indole-RF association, important only at indole concentration higher than 10-3 mol dm-3, a complex picture of competitive reactions operate. Triplet excited RF is quenched by the indoles in concentrations ca 10-4 mol dm-3 (rate constants of the order of 109 mol-1 dm3 s -1) and by dissolved oxygen. Both, singlet molecular oxygen and superoxide anion (in a further reaction step) are produced. As a consequence the rate of indol photodecomposition decreases in order: indole-3-acetic acid > tryptophan >> tryptamine, and the rate of RF autofading suffers an important delay compared with that in the absence of indoles.