Spectroscopic Studies of Lipids and Biological Membranes: Carbon-13 and Proton Magic-Angle Sample-Spinning Nuclear Magnetic Resonance Study of Glycolipid-Water Systems

We have obtained ¹H and ¹³C 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-β-D-galactopyranosyl-sn-glycerol]; digalactosyldiacyldi-glyceride [DGDG; primarily 1, 2-di[(9Z, 12Z, 15Z)octadec-9, 12, 15-trienoyl] -3-(α-D-galactopyranosyl-1 -6-β-D-galactopyranosyl)-1-6-β-D-glycerol]; and sulfoquinovosyldiacyldiglyceride [SQDG; primarily 1-[(9Z, 12Z, 15Z)octadec-9, 12, 15-trienoyl]-2-hexadecanoyl-3-(6-deoxy-6-sulfono-a-D-glucopyranosyl)-sn-glycerol]. At ~22 °C, all three lipid-water systems give well-resolved ¹³C and ¹H MAS NMR spectra, characteristic of fluid, liquid-crystalline mesophases. ^I3C spin-lattice relaxation times of the headgroup and glycerol backbone carbons of all three materials give, within experimental error, the same N T₁ values (~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 ~-20°, 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 ¹³C and 'H 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 ¹³C 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 CIO doublet, due to a very small C-H dipolar interaction for CIO, because of tilt of the 9, 10-double bond, and a resultant magic-angle effect. Overall, the 'H and ¹³C MAS NMR line 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 °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.