Dynamics of glycolipids in the liquid-crystalline state. 2H NMR study

It has been shown previously that two types of motion are adequate to describe the partially relaxed 2H NMR line shapes (inversion recovery experiment) for the backbone portion of the glycolipid 1,2-di-O-tetradecyl-3-O-(beta-D-glucopyranosyl)-sn-glycerol (beta-DTGL) in the highly ordered gel phase (Auger, M.A., D. Carrier, I.C.P. Smith, and H. C. Jarrell. 1990. J. Am. Chem. Soc. 112:1373-1381). This study extends the latter investigation to the more fluid liquid-crystalline phase, where more complex motions are anticipated. Analyses of the powder line shapes and oriented sample relaxation data for both the glycerol backbone and head group regions of this lipid have been performed. The dynamics of glycerol at the C3 position in the gel state have been described by large angle jumps about the C2-C3 bond with a correlation time in the fast-limit motional regime (omega o tau c much less than 1) and site populations 0.46, 0.34, and 0.20. The present data show that in the liquid-crystalline phase the internal jump rate is maintained, and two additional motions are necessary to describe the dependence of the relaxation rate on the orientation of the director with respect to the magnetic field direction. These are rotation about the molecular long axis with a correlation time in the slow-limit motional regime very near to the T1 minimum (omega o tau c approximately 0.65), and molecular fluctuations about the order director (modeled by a Maier-Saupe restoration potential). This treatment was also extended to the glucose head group where additional segmental motion about the glycosidic bond has been reported previously. While the two motions dominating relaxation at the glycerol C3 segment reproduce the general relaxation features of the glucose head group, the results suggest that additional motion about the glycosidic linkage must be present. This study is a stringent test of the motional model chosen earlier because relaxation data were obtained at two 2H NMR frequencies using two relaxation experiments (T1Z and T1Q) and two types of sample preparation (oriented and dispersed multibilayers). The results strongly uphold the choice of model and indicate the utility of both oriented samples and the T1Q experiment.