Riboflavin as a photocatalyst and hydrogen carrier in photochemical reduction

Abstract

1. 1. In the presence of certain metal-chelating agents riboflavin was reversibly reduced by visible light. This reduction has been followed visually, spectrophotometrically and polarographically. 2. 2. Riboflavin not only absorbs the light but is reduced in the process, and in turn can transfer its electrons to suitable hydrogen acceptors added to the system. 3. 3. Polarographic data were used to establish the reversibility of the photoreduction system. Semiquinone formation was strongly suggested. 4. 4. From the half-wave potential ( E 1 2 ) for the polarographic reduction of riboflavin, the reduction potential at pH 7.3 was calculated to be −0.208 volts, and −0.194 volts at pH 7.0. 5. 5. Oxygen inhibited the photoreduction of riboflavin and also protected the riboflavin from photolysis. Na 2EDTA not only protected riboflavin, but also promoted the photoreduction. 6. 6. Certain metal ions, dinitrophenol, p-nitrobenzaldehyde, and sodium azide inhibited the photoreduction of riboflavin. This inhibition could be reversed by excess riboflavin. 7. 7. Evidence points to a metal-mediated complex between riboflavin or light-activated riboflavin and the metal-chelating agent.