Double-strand break of giant DNA: protection by glucosyl-hesperidin as evidenced through direct observation on individual DNA molecules.

We have performed a real-time observation of photo-induced breakage on individual long DNA molecules stained by a cyanine dye, YOYO, under the intense illumination (lambda=450-490 nm) in solution. It was shown that the double-strand breakage is accelerated by Fenton's reagent (Fe2+/H2O2). In addition, it was found that the breakage reaction is protected in the presence of a water-soluble flavonoid, glucosyl-hesperidin. To interpret the kinetic process of the double-strand breakage reaction, a two-step mechanism is proposed: under light illumination on giant DNA molecules, single-strand breaks are generated to induce nicks in a stochastic manner, and then reactive oxygen attacks these nicks causing the double-strand break.