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 10 9 mol -1 dm 3 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.