Interaction between quercetin–copper(II) complex and DNA with the use of the Neutral Red dye fluorophor probe

The interaction of quercetin–Cu(II) complex with calf thymus DNA was investigated with the use of Neutral Red (NR) dye as a spectral probe by the application of UV–vis spectrophotometry, cyclic voltammetry and synchronous fluorescence spectroscopy. The results showed that both quercetin–Cu(II) complex and the NR molecule can intercalate into the double helix of the DNA. The 2:1 quercetin:Cu(II) complex (estimated binding constant = 2.85 × 10^9) is stabilized by intercalation in the DNA (binding constant, K [quercetin–Cu(II)–DNA] = (1.82 (plus-minus) 0.20) × 105 M^-1), and displaces the NR dye from the NR–DNA complex in a competitive reaction. Cyclic voltammetry studies confirm the intercalation reaction and show that the ratio (KR/KO) of binding constants for the reduced and oxidized forms of the metal complex is 2.05. Furthermore, the alternative least squares (ALS) method was applied to resolve a complex two-way array of the absorption spectra data. This yielded the equilibrium concentration profiles of each component in the reaction (NR, NR–DNA and quercetin–Cu(II)) as well as the corresponding pure spectra. The extracted profiles showed that at equilibrium the [NR–DNA] and [NR] trends decreased and increased symmetrically, respectively, with approximately linear behaviour being observed below 10 × 10^-6 mol L^-1 of the added quercetin–Cu^2+ complex. Thereafter, these trends converged asymptotically. The free [quercetin–Cu(II)] trend-line at equilibrium was linear over the whole range of the complex added. It was possible to estimate the approximate value of the equilibrium constant of the exchange process (approximately 5 × 10^-1) involving the intercalation of the quercetin–Cu(II) complex. It was also found that about 35% of the bound complex was unaccounted by the intercalation reaction, presumably being stabilized at an alternative site.