Relative binding affinities of monovalent cations for double-stranded DNA

Abstract

The competition bewteen Na+ and various other monovalent cations that bind to helical DNA in aqueous solution has been studied by 23Na NMR. Variations in the Na linewidth with the concentration of the other ion were analyzed with an equation that describes the competitive binding in terms of 2 parameters: r, the total extent of counterion binding, and D, a measure of the binding affinity of a cation relative to Na. The concentration dependence of these parameters was minimal. In the absence of a competing cation the constancy of r was 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: NH4+ > Cs+ > K+ > Li+ > Na+.