J. Schultz et al., FIELD-DEPENDENT NA-23 NMR RELAXATION OF SODIUM COUNTERIONS IN ORDEREDDNA, Journal of physical chemistry, 98(34), 1994, pp. 8507-8518
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.