A 23Na-NMR study of sodium-DNA interactions in concentrated DNA solutions at low-supporting electrolyte concentration

Aqueous solutions of DNA fragments with a contour length (500 A) near the persistence length at DNA concentrations ranging from 10 to 290 mg/mL solvent and a constant supporting electrolyte concentration of 0.01 M (predominantly NaCl) were examined by 23Na-nmr spectroscopy at temperatures of 20, 40, and 60 degrees C. Over the higher portion of this concentration range (greater than 100 mg/ml) the DNA solutions undergo a complex series of transitions between different anisotropic, liquid crystalline phases (T. E. Strzelecka and R. L. Rill, Biopolymers, in press). Counterions in solutions of strong polyelectrolytes are usually described in terms of a two-state model as free or "bound" (influenced by the electrostatic field of the polyanion). The longitudinal relaxation rate (R1 = 1/T1) at all DNA concentrations decreased with increasing temperature, demonstrating fast exchange between free and bound counterions. R1 increased nearly linearly with increasing DNA phosphate/sodium ratio in the isotropic domain until the onset of anisotropic phase formation, in agreement with similar nmr studies conducted at low DNA concentrations. The value of R1,b = 194 +/- 7 Hz obtained for the isotropic phase from 10 to 100 mg DNA/mL at 20 degrees C was in agreement with values reported previously. A nonlinear increase in R1 with DNA concentration was observed upon onset of anisotropic phase formation, indicating an increase in the product of the fraction of bond ions times their relaxation rate (r.R1,b). The spectral lineshape of all isotropic samples was Lorentzian. Spectra of anisotropic samples exhibited low magnitude quadrupole splitting of less than or equal to 400 Hz correlated with appearance of a cholesteric phase with pitch approximately 2 microns. The magnitude of the quadrupole splitting decreased with increasing DNA concentration at low temperatures and increased with concentration at high temperatures. At all concentrations the quadrupole splitting decreased then increased with temperature. These temperature- and concentration-dependent changes in quadrupole splitting are consistent with an angle between the DNA helix axis and the principal component (VZZ) of the local electric field gradient tensor near the "magic angle" of 54.7 degrees.