A study of the quadrupolar NMR splittings of ⁷Li⁺, ²³Na⁺, and ¹³³Cs⁺ counterions in macroscopically oriented DNA fibers

Abstract

The hydration and temperature dependencies of the 23Na+, 133Cs+, and 7Li+ quadrupolar splitting have been determined in hydrated, macroscopically oriented DNA fibers. At low water contents the quadrupolar splitting is found to decrease as the water content increases, regardless of counterion, while at high water contents the hydration dependence is reversed. The 23Na+ and 133Cs+ quadrupolar splittings decrease as the temperature increases, while the 7Li+ splitting shows the opposite behavior. At high water contents the 23Na+ and 133Cs+ splittings decrease, and then, after passing zero splitting, increase as the temperature increases. The interpretation of the temperature dependence is discussed in terms of a two-site model (free and bound ions) and a three-site model (free ions and specifically or nonspecifically bound ions). It is suggested that a three-site model is more consistent with the data for the present system. At high water contents, the temperature dependence of the 7Li+ splitting vanishes, indicating counterion condensation. The behavior of the 7Li+ splitting is confirmed by measurements on DNA fibers in equilibrium with a C2H5OD-D2O-LiCl solution. The salt dependence in this system is weak. The counterion quadrupolar splitting is seen to be very sensitive to structural transitions in double-helical DNA.

Optical microscopy and multinuclear NMR investigation of the liquid crystalline netropsin–DNA complex

“…In parallel, since changes of the counterion population are induced by thermal treatment, a redistribution of counterions among the heterogeneous binding sites accounts also for the observed temperature dependence of the quadrupolar splitting in all of the systems and mainly the occurrence of a zero splitting point. 9,27…”

23Na NMR study of the effect of organic osmolytes on DNA counterion atmosphere

“…To verify this hypothesis directly, we studied, by 23Na NMR, the modification of DNA-ion interaction when organic osmolytes are added to the solution. This technique has frequently afforded a direct and sensitive probe of the interactions of small ligands with macromolecules (Braunlin et al, 1987;Schultz et al, 1992;Wright and Lerner, 1994). In particular, 23Na has often been utilized owing to its advantageous NMR properties from technical and theoretical viewpoints.…”

Interactions of spermidine and methylspermidine with DNA studied by nuclear magnetic resonance self-diffusion measurements

“…Sodium was the counter-ion of choice in the NMR studies mentioned above. Lithium as counter-ion to DNA has been studied by NMR in only a few cases (Nilsson et al, 1985;Stilbs, 1987;Einarsson et al, 1990;Einarsson et al, 1991;Schultz et al, 1992;Andreasson et al, 1994). Comparative studies of lithium and sodium ion binding to double-helical DNA (Ross and Scruggs, 1964a;Ross and Scruggs, 1964b;Bleam et al, 1980) have shown that lithium binds with a somewhat larger affinity to DNA than does sodium.…”