AN NMR SELF-DIFFUSION STUDY OF LITHIUM IONS IN MACROSCOPICALLY ORIENTED LI-(B)DNA FIBERS

Quadrupolar ion NMR splittings and self-diffusion of 7Li+ (I = 3/2) ions have been studied in macroscopically oriented Li-(B)DNA fibers. The Li splittings were measured with a double quantum filter pulse sequence, followed by subsequent fitting of the out-of-phase doublet spectrum. Lithium(I) self-diffusion was measured with the pulsed field gradient NMR spin-echo method. Diffusion was found to be restricted in both the parallel and perpendicular direction with respect to the fiber axis. Apparent self-diffusion coefficients D∥ and D⊥ were obtained by fitting the echo attenuation to the Stejskal-Tanner equation. Quadrupolar splittings decrease and diffusion coefficients increase with increasing salt and/or water content of the fibers. The morphology and composition (salt and water content) of the system, which is very heterogeneous on a microscopic as well as on a macroscopic scale, are found to be important. The data can be rationalized by considering a dynamically averaged state where lithium ions sample many different environments. Lithium ion diffusion is considerably retarded as compared to dilute isotropic solutions of Li-DNA, but the mobility along and around the DNA surface is still quite high. The cylindrical Poisson-Boltzmann cell model, which is based on a continuum description of the system, is not applicable for describing the ion-DNA interactions in this highly compact system that basically lacks bulk water. © 1990 American Chemical Society.