Voltammetric measurements of the interaction of metal complexes with nucleic acids

Cyclic voltammetry and differential-pulse voltammetry at mm-sized electrode s were used to measure the decrease in the rate of diffusion of metal compl exes upon binding to DNA and to extract the binding constants and effective binding site sizes. A linear correlation was observed between the site siz e determined electrochemically and the diameter of the complexes [site size : Cu(phen)(2)(2+) > Fe(phen)(3)(2+) > Co(bipy)(3)(3+) approximate to Fe(bip y)(3)(2+) > Ru(NH3)(6)(3+)]. The binding constants were found to be influen ced by the charge of the metal complex, the nature of ligand and the geomet ry about the metal centre. Competition experiments, in which differential p ulse voltammetry was used to observe the release of bound metal complex on addition of a second DNA-binding molecule to the solution, were sensitive t o the nature and location of the binding sites for the two species. Steady- state voltammetric experiments at microelectrodes are shown to have a numbe r of advantages over cyclic voltammetry and differential pulse voltammetry at mm-sized electrodes for determination of binding constants. In particula r, the steady-state diffusion limited current is directly proportional to t he diffusion coefficient, rather than its square root, which improves the d iscrimination between DNA-bound and freely diffusing metal complex. Further , the kinetics of the binding process do not affect the steady state measur ement, whereas for transient techniques, e.g., cyclic voltammetry, only a r ange of values can be extracted corresponding to the limits of fast and slo w binding kinetics compared to the experimental timescale.