Solid-state deuterium NMR investigation of internal motion in 2'-deoxythymidine

Abstract

/sup 2/H NMR spectroscopy is used to characterize the dynamics of 2'-deoxythymidine in the solid state. Three different specifically deuteriated nucleosides were prepared, with labels at the methyl group, at the C6 position, and at the 5'-methylene group, respectively. The /sup 2/H NMR quadrupole echo line shapes and spin-lattice (T/sub 1/) relaxation times were analyzed to determine the mechanism and rate of motion of the labeled moieties. Only the methyl group undergoes large-amplitude motion, which consists of 3-fold jumps about the C/sub 3/ symmetry axis with a room-temperature correlation time /tau//sub c/ approx. 3.0 ps. The apparent activation energy (E/sub a/) for this process is 5.9 kJ/mol. The data also indicate that the entire pyrimidine base undergoes a rapid, small-amplitude motion, which the authors have modeled as a /plus minus/9/degrees/ libration about the equilibrium position. At 298 K, the libration occurs with correlation time /tau//sub c/ approx. 1.0 ps. The motion of the pyrimidine base also affects the methyl group, but the added motion causes only a small variation in the room-temperature methyl jump correlation time, /tau//sub c/ approx. 4.2 ps. The librational motion makes only a small contribution to the spin-lattice relaxation of the methyl deuterons, and this likelymore » does not contribute significantly to the unusually low value of the apparent activation energy that is observed. The 5'-methylene group also appears to be undergoing a similar libration (/plus minus/ 10/degrees/) as the pyrimidine base but with a much longer correlation time, /tau//sub c/ approx. 200 ns at room temperature. From the models of motion and the experimental line shapes, they are able to determine the static values of the asymmetry parameter (/eta/) and the quadrupole coupling constant for deuterons at the various sites.« less