Relative binding affinities of monovalent cations for double-stranded DNA.

The competition between sodium and various other monovalent cations that bind to helical DNA in aqueous solution has been studied by (23)Na NMR. Variations in the sodium linewidth with the concentration of the other ion have been analyzed with an equation that describes the competitive binding in terms of two parameters: r, the total extent of counterion binding, and D, a measure of the binding affinity of a cation relative to sodium. The concentration dependence of these parameters was found to be minimal. In the absence of a competing cation the constancy of r has been demonstrated over a range of DNA phosphate concentrations (0.0025-0.015 M) and NaCl concentrations (0.003-1.3 M). For the cations investigated the range in D values is small (0.5-0.9), and the relative binding affinities follow the order: NH(4) (+) > Cs(+) > K(+) > Li(+) > Na(+).