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The interaction of proflavine with herring sperm DNA has been investigated by cyclic voltammetry and UV-Vis spectroscopy as well as viscosity measurements. Shifts in the peak potentials in cyclic voltammetry, spectral changes in UV absorption titration, an increase in viscosity of DNA and the results of the effect of ionic strength on the binding constant strongly support the intercalation of proflavine into the DNA double helix. The binding constant for the interaction between proflavine and DNA was K = 2.32 (±0.41) × 10 4 M -1 and the binding site size was 2.07 (±0.1) base pairs, estimated in voltammetric measurements. The value of the binding site size was determined to be closer to that expected for a planar intercalating agent. The standard Gibbs free-energy change is ca. -24.90 kJ/mol at 25˚C, indicating the spontaneity of the binding interaction. The binding constant determined by UV absorption measurements was K = 2.20 (±0.48) × 10 4 M -1 , which is very close to the value determined by cyclic voltammetry assuming that the binding equilibrium is static.

The binding interaction of the antimalarial drug quinacrine with herring sperm deoxyribonucleic acid (DNA) has been studied by square wave voltammetry. The binding parameters, the binding constant K and the binding site size s, were obtained simultaneously by the analysis of bound and free quinacrine concentration corresponding to the limits of slow and fast binding kinetics compared to the experimental timescale. The binding constant and the binding site size for the interaction of quinacrine with DNA were K = 1.59 (+/-0.18)x10(5) M(-1) and s = 7.1 (+/-0.15) base pairs and K = 7.35 (+/-0.83)x10(5) M(-1), s = 6.2 (+/-0.02) base pairs for the limiting conditions of static and mobile binding equilibrium respectively. The standard Gibbs free energy change (Delta G0 = - RT ln K) is approximately -29.67 kJ/mol at 25 degrees C, which highlights the spontaneity of the binding of quinacrine with DNA. The binding of quinacrine to herring sperm DNA results in peak potential shifts in voltammetric and a red shift in UV-absorption measurements. The ionic strength dependence of the binding constant is not large. Furthermore, the relative viscosity of DNA increases in the presence of quinacrine. These characteristics strongly support the intercalation of quinacrine into DNA. The results also show that the intercalation of quinacrine into DNA may occur at approximately every seventh base pair.

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Cyclic voltammetry and differential-pulse voltammetry at mm-sized electrodes were used to measure the decrease in the rate of diffusion of metal complexes upon binding to DNA and to extract the binding constants and effective binding site sizes. A linear correlation was observed between the site size determined electrochemically and the diameter of the complexes [site size: Cu(phen)2(2+) > Fe(phen)3(2+) > Co(bipy)3(3+) approximately Fe(bipy)3(2+) > Ru(NH3)6(3+)]. The binding constants were found to be influenced by the charge of the metal complex, the nature of ligand and the geometry about the metal centre. Competition experiments, in which differential pulse voltammetry was used to observe the release of bound metal complex on addition of a second DNA-binding molecule to the solution, were sensitive to the nature and location of the binding sites for the two species. Steady-state voltammetric experiments at microelectrodes are shown to have a number of advantages over cyclic voltammetry and differential pulse voltammetry at mm-sized electrodes for determination of binding constants. In particular, the steady-state diffusion limited current is directly proportional to the diffusion coefficient, rather than its square root, which improves the discrimination between DNA-bound and freely diffusing metal complex. Further, the kinetics of the binding process do not affect the steady state measurement, whereas for transient techniques, e.g., cyclic voltammetry, only a range of values can be extracted corresponding to the limits of fast and slow binding kinetics compared to the experimental timescale.