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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25Mg NMR spectroscopy is applied to a study of magnesium ion interactions with DNA, which is considered as a model for a linear polyelectrolyte. It is demonstrated that the magnesium ion spectrum is complicated by a non-Lorentzian line shape and is dominated by the effects of chemical exchange with macromolecule binding sites. A distinction is made between specific-site interactions in which the magnesium ion loses a water molecule from the first coordination sphere on binding and those interactions, referred to as territorial binding, in which the ion maintains its first coordination sphere complement of solvent. The first type of site-binding interactions are shown to dominate the magnesium ion NMR spectrum, based on a consideration of the magnitudes of the observed 25Mg relaxation rates compared with 3Na relaxation rates, the clear contributions of chemical exchange-limited relaxation, and an ion displacement experiment employing sodium.Magnesium ion is essential for a number of critically important life-sustaining processes (1). Magnesium ion interactions with polyanions are not understood at an atomic level (2, 3); however, two types may be anticipated: specific interactions involving metal-ligand association in the classical sense and nonspecific interactions dominated by purely electrostatic forces (3).An association between oppositely charged particles is expected even in solutions of simple electrolytes (4). The effects may be amplified in the presence of a polyelectrolyte. Regardless of the theoretical approach to the polyelectrolyte case, electrostatic considerations predict that: (i) there will be a gradient in the concentration of counterion extending radially from the surface of the polyelectrolyte, considered to be an infinitely long charged cylinder; (ii) the local concentration of counterion may be high at the polyelectrolyte surface (in excess of 1 M for a highly charged polyion), even when the total concentration of counterion in the solution is low; and (Mi) the outer boundary of the region of high counterion concentration is somewhat arbitrary and depends on ionic strength but is of the order of 10 A from the polyion surface. Ions that are close to the polyelectrolyte are often called bound ions. Manning (5) provided a useful and clear distinction between bound species; he describes those ions that retain a full hydration sphere when associated with the polyion as "territorially bound" and describes those ions that lose one or more first coordination sphere water molecules in the interaction with the polyelectrolyte as "site bound." For divalent ions interacting with a polyelectrolyte like DNA, which has a low surface charge density but a high axial charge density, Manning states (5) that ion association should be almost exclusively territorial in spite of the fact that equilibrium constants for magnesium ions with phosphate moieties are significant, frequently as large as 100 M-1 (6).Experimental distinction between territorially bound and EXPERIMENTAL 25Mg NMR spectra were accumul...