FIELD-DEPENDENT NA-23 NMR RELAXATION OF SODIUM COUNTERIONS IN ORDEREDDNA

The quadrupolar NMR relaxation of Na-23(+) counterions has been studie d in a solid sample of macroscopically oriented DNA fibers which was e quilibrated at relative humidities of 95 and 98%. The equilibrations r esulted in a sample of relatively high water contents, corresponding t o approximate distances between the DNA surfaces of 1.1 and 1.3 nm, re spectively. Using a combination of relaxation experiments, including t wo-dimensional spin echo and two-dimensional double quantum quadrupola r echo techniques, the spectral densities J(0)(0), J(1)(omega(0)), and J(2)(2 omega(0)) have been determined at different orientations of th e sample with respect to the external magnetic field. The high-frequen cy spectral densities, J(1)(omega(0)) and J(2)(2 omega(0)) were also d etermined at two and four different magnetic field strengths, respecti vely. It was found that they are largely determined by fluctuations of the quadrupolar interaction that occur on a time scale of nanoseconds . The results indicate that the main contribution to J(1)(omega(0)) an d J(2)(2 omega(0)) originates from local motions in the vicinity of th e DNA molecule. Assuming that this contribution can be divided into tw o contributions, one fast (assumed to be a constant frequency-independ ent term) and one slow (assumed to be governed by a Lorentzian functio n), the experimental frequency dependence could be fitted. The effecti ve correlation time for the slow local motion is in the range of 2-3 n s, depending on water content. It is suggested that this slow local mo tion is due to the relative motion of the sodium counterion in the vic inity of a charged phosphate group, caused by local diffusion of the s odium counterion and/or motion of the phosphate group itself.