The Alkali Ion-DNA Interaction as Reflected in the Nuclear Relaxation Rates of Na and Rb.

Longitudinal relaxation rates of (23)Na and (87)Rb in aqueous solutions of DNA were measured and interpreted in terms of contributions of "bound" and free species. The frequency dependence of the (23)Na relaxation rate sets an upper limit of 5.5 nsec for the correlation time characteristic of the alkali ion-DNA interaction. The plausible description of the alkali ion-DNA interaction emerging from the results is one in which the "bound" ions comprise a highly mobile ionic cloud around the lattice of phosphate groups with a population characterized by an apparent dissociation constant of 10.9 mM. Any specific binding to phosphate groups involving severe restrictions of motional freedom must be much (at least two orders of magnitude) weaker and of a very short duration (<5.5 nsec).