SPECTROSCOPIC STUDIES OF LIPIDS AND BIOLOGICAL-MEMBRANES - C-13 AND PROTON MAGIC-ANGLE SAMPLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE STUDY OF GLYCOLIPID WATER-SYSTEMS

We have obtained H-1 and C-13 magic-angle sample-spinning (MAS) nuclear magnetic resonance (NMR) spectra of three glycosyldiacylglycerol-water (1:1, weight ratio) mesophases, at 11.7 T, as a function of temperature, in order to probe lipid headgroup, backbone, and acyl chain dynamics by using natural-abundance NMR probes. The systems investigated were monogalactosyldiacyldiglyceride [MGDG; primarily 1,2-di[(9Z,12Z,15Z)octadec-9,12,15-trienoyl]-3-beta-D-galactopyranosyl-sn-glycerol]; digalactosyldiacyldiglyceride [DGDG; primarily 1,2-di[(9Z,12Z,15Z)octadec-9,12,15-trienoyl]-3-(alpha-D-galactopyranosyl-1-6-beta-D-galactopyranosyl)-sn-glycerol]; and sulfoquinovosyldiacyldiglyceride [SQDG; primarily 1[(9Z,12Z,15Z)octadec-9,12,15-trienoyll-2-hexadecanoyl-3-(6-deoxy-6-sulfono-alpha-D-glucopyranosyl)-sn-glycerol]. At approximately 22-degrees-C, all three lipid-water systems give well-resolved C-13 and H-1 MAS NMR spectra, characteristic of fluid, liquid-crystalline mesophases. C-13 spin-lattice relaxation times of the headgroup and glycerol backbone carbons of all three materials give, within experimental error, the same NT1 values (approximately 400 ms), implying similar high-frequency motions, independent of headgroup size and charge. Upon cooling, pronounced line broadenings are observed, due to an increase in slow motional behavior. For each lipid, the onset of line broadening is seen with the glycosyl headgroup, glycerol backbone, and the first two or three carbons of the acyl chains. By approximately 20-degrees, all headgroup carbon resonances are broadened beyond detection. Both galactose moieties in DGDG "freeze out" together, implying a rigid-body motion of the disaccharide unit. Upon further cooling, the bulk polymethylene chain resonances in all three systems (in both C-13 and H-1 MAS) broaden greatly, followed by the olefinic and allylic carbon resonances. The 9,10-Z carbons, in each lipid, are the first to broaden, followed at lower temperatures by the 12,13-Z carbons. In proton-coupled C-13 MAS NMR spectra, there are differential line broadenings of the individual C9 and CIO doublets, due to interference between the dipolar and chemical shift anisotropy interactions: the C9 doublet is broader than the C10 doublet, due to a very small C-H dipolar interaction for C10, because of tilt of the 9,10-double bond, and a resultant magic-angle effect. Overall, the H-1 and C-13 MAS NMR lire width results show a gradual freezing-out of motion beginning in the polar headgroup region of each glycolipid, which is transmitted over about a 60-degrees-C range along the hydrocarbon chains to the chain methyl termini. The freezing behavior is consistent with a broad thermal transition, for each of the glycolipids.