Relative binding affinities of monovalent cations for double-stranded DNA

Abstract

The competition between sodium and various other monovalent cations that bind to helical DNA in aqueous solution has been studied by 23Na 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: NH4+ > Cs+ > K+ > Li+ > Nat. The utility of 23Na NMR as a means of studying the interactions of counterions with polyions in solution has been demonstrated in a wide variety of systems, including soluble RNA (1), polyacrylate (2-4), polyphosphate (5), polystyrenesulfonate (6), mucopolysaccharides (7), polymethacrylate (4), and phosphatidylserine vesicles (8). Previous studies (9, 10) of doublestranded DNA by using 23Na relaxation rate measurements have been principally concerned with quantifying the extent to which monovalent counterions are bound in order to test the applicability of different theories of polyelectrolyte binding. The purpose of the research communicated here is to establish the relative binding affinities for helical DNA of a series of monovalent cations whose binding interactions are anticipated to be predominantly electrostatic. The effectiveness with which these cations compete with sodium is determined by analyzing measurements of the 23Na linewidth as a function of solution composition. This analysis incorporates the assumption that r, the extent of counterion association with DNA, does not depend on the identities or concentrations of the competing monovalent cations. The results conform to this assumption, which is one of the basic features of Manning's model for counterion-polyion interactions (11, 12).THEORETICAL BACKGROUND The 23Na NMR spectrum of an aqueous sodium chloride solution containing double-stranded DNA is a single peak whose lineshape, under the conditions of the present study, is Lorentzian. (The criterion is given in the next section.) Therefore, the condition of extreme narrowing applies to the quadrupolar relaxation rates of all nuclei contributing to the signal, and chemical exchange among all significantly populated magnetic environments accessible to the sodium ions is fast in comparison to the 23Na relaxation rates in each of these environments (4, 13). It follows that the linewidth of the peak at half height,The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.AvI/2, is a direct measure of R, the trans...

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