²⁵ Mg NMR in DNA solutions: Dominance of site binding effects

Abstract

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...

The morphology of adsorbed extracellular matrix assemblies is critically dependent on solution calcium concentration

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Deuterium NMR of water in immobilized protein systems

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Interaction of Z form poly(dG-dC). poly(dG-dC) with divalent metal ions: localization of the binding sites by I.R. spectroscopy

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