Displacement of sodium ions by surfactant ions from DNA. A 23Na-NMR investigation.

23Na-NMR probes the ionic composition in the immediate vicinity of the DNA molecule, in the presence of a series of quaternary ammonium bromides, of varying hydrocarbon chain length. The 23Na-NMR line shows two Lorentzian components, in accordance with quadrupolar relaxation theory for S = 3/2 nuclei under slow modulation. Deconvolution of the observed lineshape provides, in a reliable manner, the relative fraction of sodium counterions neutralizing the phosphate sites on DNA. This quantity (p B chi 2) serves as an index of the relative affinities of various surfactant ions toward DNA, Na+ being the reference cation. The results are consistent with site binding of detergent ions to the nucleic acid, an interaction dominated by hydrophobic forces.