The double-strand break of DNA induced by visible light and c-rays was studied through a single-molecule observation using fluorescence microscopy. We evaluated the protective effects of water-soluble derivatives of flavonoids, alpha-glucosyl hesperidin (alpha G-HSP) and alpha-glucosyl rutin (alpha G-rutin) in a quantitative manner. With regard to photo-induced damage, alpha G-rutin was found to provide twice as much protection as alpha G-HSP, whereas the protective effect of alpha G-rutin against c-ray irradiation was comparable with that of alpha G-HSP. This observation demonstrates the dependence of the antioxidant potentials of these compounds on the source of irradiation. (C) 2010 Elsevier B.V. All rights reserved.