INFLUENCE OF SOLVENT AND COUNTERION ON NA-23+ SPIN RELAXATION IN AQUEOUS-SOLUTION

The contributions of solvent screening and fluctuations to the Na-23+ NMR quadrupolar relaxation rate in a model aqueous ion pair, sodium dimethylphosphate, have been studied using molecular dynamics simulations. Four specific relative interionic geometries are considered, corresponding to models of association of sodium ions with DNA in both inner-sphere and outer-sphere complexes. Analysis of the autocorrelation function of the electric field gradient (efg) at the sodium nucleus indicates correlation times comparable to those for the unpaired ion. The influences of the short-ranged and Coulombic interactions on the efg at the sodium nucleus have been evaluated. It is concluded that the net result for the sodium quadrupolar coupling constant incorporates cancellation between large direct counterion and induced solvent contributions and hence does not sensitively reflect the direct role of Coulombic forces. The quantitative results support the utility of an empirical expression to account for this net contribution for the case of DNA. On the basis of computed average field gradients, we further infer that both contact and solvent-separated ion pairs make potentially important contributions to the NMR observations in DNA solutions.