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 x 10(9)) is stabilized by intercalation in the DNA (binding constant, K([quercetin-Cu(II)-DNA])=(1.82+/-0.20) x 10(5) 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 (K(R)/K(O)) 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 x 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 x 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.