Influence of solvent and counterion on sodium-23(1+) spin relaxation in aqueous solution

Abstract

The contributions of solvent screening and fluctuations to the 23Na+ N M R 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 N M R observations in DNA solutions.

Sequence-specific binding of counterions toB-DNA

“…For the five central nominal water sites in the primary spine of hydration of A 2 T 2 , we thus obtained ϭ 1.3 Ϯ 0.2 MHz, with no significant difference between two crystal structures (5,7). Although this estimate is crude, it is reassuring to note that QCCs for oxygencoordinated Na ϩ derived from 23 Na NMR studies of cryptands (39) and inorganic solids (40) as well as from molecular simulations (41,42) generally fall in the range of 1-2 MHz.…”

Thermal relaxation and coherence dynamics of spin 3/2. I. Static and fluctuating quadrupolar interactions in the multipole basis

“…Among those probes, the naturally occurring spin quantum number 3/2 nuclei 7 Li, 23 Na, 39 K, 87 Rb, 35 Cl, 81 Br, and 131 Xe constitute an important class. The quadrupolar interaction, which is essentially a tensor property, is determined by the orientation, magnitude, and temporal duration of electric field gradients (EFGs) generated by the surrounding medium (9) and/or the electronic configuration around the nu-cleus. If the motions are rapid and all orientations are equally probable (isotropic system), the quadrupolar interaction is averaged to zero on the timescale of the inverse Larmor frequency.…”

NMR investigations of Co(III) coordination compounds oriented in a dilute lyotropic liquid crystal

“…19 The EFG at the nuclei of monoatomic ions in solution, such as alkali metal cations and halides, is not zero owing to deformations of the solvation sphere. 20 The latter are enhanced by the proximity of the charged aggregates and other ions. The EFG at the water nuclei, in contrast, are larger and mainly a molecular property.…”