A Metastable Intermediate in the Anaerobic Photolysis of Riboflavin

Using flash photolysis and kinetic spectroscopy, a metastable intermediate has been detected in the anaerobic photolysis of riboflavin (D) in neutral aqueous solution. Judging from its reactions and its absorption spectrum, the intermediate appears to be the semiquinone, DH·. The fading can then be written where AH2 is any reducing agent (for instance the activator or the side chain of the dye itself). Quenchers like KI or O2 affect step (1). In presence of oxygen measurable amounts of the intermediate are produced only in presence of activators. In this case the intermediate disappears much faster, and by a first order reaction. This is consistent with a competing step (3) DH· + O2 → D + HO2·. Absolute values can be estimated for the concentration of the intermediate, its absorptivity, and its decay rate. This is found to be slightly lower than the maximum rate allowed by diffusion considerations. The fading process is known to be fundamentally different in presence and in absence of activators (in the latter case only, degradation of the riboflavin side chain takes place). Nevertheless, the intermediate reported here seems to be almost the same in the two cases.