ELUCIDATION OF MOTIONAL MODES IN GLYCOGLYCEROLIPID BILAYERS - A H-2 NMR RELAXATION AND LINE-SHAPE STUDY

In the present study, a combination of 2H spin-lattice relaxation and line-shape analysis demonstrates that two motions are sufficient to describe the spectral and relaxation behavior of the glycolipid 1,2-di-O-tetradecyl-3-O-(β-D-glucopyranosyl)-sn-glycerol (β-DTGL). In the gel phase, at 25 °C, the powder and oriented-sample spectral line shapes for the glycolipid, specifically labeled at the C3-position of the glycerol backbone, are characteristic of fast-limit axially asymmetric motions. In particular, the oriented-sample spectra have powder line shapes characteristic of a system with a motionally averaged asymmetry parameter ηeff close or equal to unity. Moreover, the Zeeman spin-lattice relaxation times were dependent on both the polar and azimuthal angles, θ and ø describing the orientation of the motional axis relative to the magnetic field direction. Inspection of the partially relaxed line shapes of powder spectra of gel-phase lipid clearly revealed the θ-dependence of the spin-lattice relaxation times. Furthermore, for oriented samples with a given θ, a ø-dependence of the relaxation times was also observed. This effect was most evident at the magic-angle (θ = 54.7°) orientation. The nature of this θ-dependence puts severe constraints on the motional model and the motional rates used to simulate the gel-phase line shape and T1, anisotropy (T1Z(θ,ø)). Theline-shape and relaxation features were best simulated with a 3-site jump model with relative site populations of 0.46, 0.34, and 0.20 and a correlation time of 6.7 × 10-10 s. These results indicate that a single internal motion is sufficient to describe the line shape and relaxation in the gel phase. However, a second motion, namely rotation about the long axis of the molecule as a whole, is needed to account for the observed variation in the quadrupolar echo amplitude and the spectral lineshape over the temperature range of 25-60°C. This motion does not significantly influence the line shape in the gel phase at 25 #x00B0;C or the spin-lattice relaxation behavior in the gel and liquid-crystalline phases. From the line-shape study, an activation energy of about 150 kJ mol-1 was determined for this motion. © 1990, American Chemical Society. All rights reserved.