A STUDY OF THE QUADRUPOLAR NMR SPLITTING OF LI-7+, NA-23+, AND CS-133+ COUNTERIONS IN MACROSCOPICALLY ORIENTED DNA FIBERS

The hydration and temperature dependencies of the Na-23+, Cs-133+, and Li-7+ 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 Na-23+ and Cs-133+ quadrupolar splittings decrease as the temperature increases, while the Li-7+ splitting shows the opposite behavior. At high water contents the Na-23+ and Cs-133+ 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 Li-7+ splitting vanishes, indicating counterion condensation. The behavior of the Li-7+ 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.