Vitamin B2 has been studied as a conventional
antioxidant (in the dark) since its discovery in 1926. The effect of visible light on
vitamin B2-containing food has a long history of scientific investigation. Although photodegradation of the
vitamin producing several photoproducts is evident in certain experimental conditions,
phototoxicity revealing an additional oxidative stress in the medium is also clear from some reports. Here we report the photosensitized
antioxidant effect of the
vitamin, which is found to be greater than 2 orders of magnitude more efficient than that in the dark condition. The photoinduced
antioxidant property is apparently paradoxical compared to the reported phototoxic effect of the
vitamin. Our present study unravels a unified picture underlying the difference in character of
vitamin B2 under visible light irradiation. UV-vis absorption and fluorescence studies in a number of physiologically relevant nanoscopic environments (
micelles and reverse
micelles) reveal the
antioxidant activity to a well-known oxidative stress marker
2,2-diphenyl-1-picrylhydrazyl (DPPH) as well as a
phototoxicity effect resulting in self-degradation of the
vitamin. Picosecond-resolved Förster resonance energy transfer (FRET) from the
vitamin to the marker DPPH in the biomimetic environments clearly reveals the role of proximity of an
oxidizing agent in the photoinduced effect of the
vitamin. Our systematic and detailed studies unravel a simple picture of the mechanistic pathway of the photosensitized
vitamin in the physiologically important environments leading to the
antioxidant/
phototoxicity effect of the
vitamin. The excited
vitamin transfers its electron to the
oxidizing agent in proximity for the
antioxidant effect, but otherwise it employs
oxygen to generate
reactive oxygen species (ROS), resulting in
phototoxicity/self-degradation.