7Li and 133Cs spin relaxation in macroscopically oriented Li- and Cs-DNA fibers

A7Li and 133Cs NMR relaxation study of macroscopically oriented hydrated Li- and Cs-DNA fibers at three different magnetic fields is presented. Two-dimensional quadrupolarecho and spin-echo techniques were used to determine the homogeneous linewidths free from the broadening due to magnetic field inhomogeneity and to a small distribution of the orientation of DNA helix axes. The relaxation rates obtained show the presence of quadrupolar, dipolar, and paramagnetic relaxation processes in Li-DNA. The separation of these relaxation contributions was made by studying H2O/ D2O exchanged samples and (for Li-DNA) 6Li relaxation. For Cs-DNA, the dipolar and paramagnetic contributions are negligible, but relaxation caused by site-exchange modulation of the Cs chemical shift can be observed. The observed relaxation behavior of the two ions is interpreted in terms of two molecular motions on different time scales.