The mechanisms of the binding of quercetin•2H2O and LaL3•6H2O to ctDNA

The phenomenon of quercetin . 2H(2)O (3', 3, 4', 5, 7-pentahydroxyflavone) and its complex LaL3. 6H(2)O [L=quercetin (3-OH group deprotonated)] exhibiting different antitumour activities has been examined by the interactions of quercetin . 2H(2)O and complex LaL3. 6H(2)O with calf thymus DNA (ctDNA). The mechanisms for the binding of quercetin . 2H(2)O and LaL3. 6H(2)O to ctDNA have been studied by fluorometric and electrochemical methods. The complex interferes with the usual fluorescence enhancement of ethidium bromide (EB) by competing for intercalation sites on ctDNA. The intrinsic binding constant, K is 2.98x10(4) M-1 at 20 degreesC, and the binding site size n(s) of the complex to ctDNA is 5, as determined by absorption titration in 5 mM Tris-HCl, 50 mM NaCl buffer (pH 7.0). The results show that quercetin . 2H(2)O and complex can both bind to DNA, but the binding modes and binding affinities are different. The complex LaL3. 6H(2)O binds to DNA mainly by intercalation and quercetin . 2H(2)O does so mainly by electrostatic attraction.