Effects of hydration on purine motion in solid DNA

Deuterium quadrupole echo spectra and spin-lattice relaxation rates measured at 76.8 and 38.4 MHz as a function of relative humidity are reported for calf thymus DNA deuterated at positions A8 and G8. The amplitude of base pair motion is observed to increase slightly with increasing degree of hydration (up to approximately 20 mol of H2O/nucleotide), and the onset of motion is associated with a more than 100-fold drop in T1. This observed decrease in T1 parallels that observed previously for the phosphate backbone and appears to be characteristic of collective modes of motion. Above approximately 20 mol of H2O/nucleotide, the amplitude of the base motion increases substantially up to a point where slow components of motion lead to a complete loss of the quadrupole echo.