Dark and photoinduced interactions between riboflavin and indole auxins

Interactions between riboflavin (RF) and indole auxins in the darkness and under conditions of irradiation with visible light were carried out. Dark complexation takes place in buffered (pH 7) aqueous solution. The association constants determined by static fluorescence quenching range from 70 to 150 mol-1 dm3. The complexation is significant at relatively high indole auxin concentrations (above 10-3 mol dm-3). It is governed by an additive effect of charge transfer, known for flavine-indole systems, and of the hydrophobic bond formation. The latter appears in the case of auxins as the driving force of the interaction. No quenching of singlet-excited RF was detected up to indole auxin concentrations of 5 . 10-3 mol dm-3. Under irradiation with visible light a complex mechanism of competitive reactions is operative: both RF and especially the auxins are decomposed by a combination of type I and II photooxidations at indole auxins concentration of 10-4 mol dm-3. The rate constants of the order of 109 mol-1 dm3 s-1 were estimated for the quenching of triplet excited RF by the indoles. For comparative purposes Rose Bengal sensitized photooxidation of a series of indolecarboxylic acids of indole-3-acetic, -propionic and –butyric acid was examined. The rate constants of the photooxidative process, mediated by singelt molecular oxygen, were of the order of 107 mol-1 dm3 s-1 for the auxins. As a result of the interaction, RF is protected against photobleaching and the auxins decompose via a different mechanism, depending on the relative flavin-auxin concentrations.