Conformation and motion of the choline head group in bilayers of dipalmitoyl-3-sn-phosphatidylcholine

Abstract

The conformation and motion of the choline head group in lipid bilayers above and below the gel-to-liquid crystal transition point are studied by means of deuterium and phosphorus magnetic resonance. For this purpose dipalmitoyl-3-sn-phosphatidylcholine is selectively deuterated at various positions on the choline and glycerol constituents. The residual deuteron quadrupole couplings and the phosphorus chemical-shift anisotropy of the corresponding lipid-water mixtures yield quantitative information on the segmental motions. The choline methyl group is only slightly hindered in its movement, but the motional freedom becomes increasingly restricted the closer the segment is located to the glycerol backbone. The average value of the OC-CN bond rotation angle changes with temperature. Increasing the temperature rotates the choline methyl group into the vicinity of the phosphorus atom. The choline group as a whole is thus characterized by a flexible, temperature-dependent structure. Its orientation in space is not fixed, either parallel or perpendicular to the bilayer surface. Instead all segments execute angular oscillations with varying degrees of restriction around the normal on the bilayer surface. The gel-to-liquid crystal phase transition at 41 degrees is clearly reflected in the deuterium and phosphorus resonance spectra of the choline moiety, while no change is observed at 34 degrees. The calorimetric pretransition at 34 degrees seems not to be associated with a conformational change in the choline group.

Hydration-optimized oriented phospholipid bilayer samples for solid-state NMR structural studies of membrane proteins

“…1a, for comparison. This motionally averaged powder pattern is characteristic of phospholipids in liquid crystalline bilayers [10][11][12], as expected for DOPC well above its chain melting transition temperature of −18 °C [13,14]. In this spectrum, the 45 ppm chemical shift anisotropy reflects an average orientation of the phosphocholine headgroups nearly parallel to the bilayer surface [15].…”

Lipid polymorphism and the functional roles of lipids in biological membranes

“…Given that biological membranes such as that of the erythrocyte do, in fact, exhibit largely bilayer structures (witness the 'bilayer' 31p NMR lineshapes obtained for erythrocyte ghosts [52]) however, and that this structure appears dictated by the lipid component (as indicated by the bilayer arrangement of model membrane systems composed of extracted lipids [52]) it is clear that at least some endogeneous lipid does play a structural 'bilayer stabilizing' role. Phosphatidylcholines are logical contenders for such a role, in view of their preference for bilayer structure in model liposomal systems as indicated by the bilayer 31p NMR lineshapes obtained [35][36][37][38]. This behaviour appears to be independent of the fatty acid composition or other biologically relevant variables [35].…”

NMR Studies of Membrane Structure and Dynamics

“…Direct measures of the order in the polar regions of phospholipid multilayer systems have recently been obtained from analysis of 31p chemical shift anisotropy and deuterium quadrupolar splittings (23,(29)(30)(31)(32)(33)(34). Phosphorus-31 NMR has been previously used to investigate the phospholipid head group (6,8,(103)(104)(105)(106), but most 31p experiments have been performed on sonicated vesicles, under which conditions the chemical shift anisotropy is rotationally averaged to zero, and hence most of the structural information that may be gleaned from the alp shielding tensor is lost.…”

“…For phospholipid systems, the measured ~aC chemical shiR anisotropies are quite small (27) and no detailed interpretations in terms of segmental order have been attempted. However, ~P chemical shift anisotropies are larger, and these chemical shift anisotropy effects have, in fact, been observed for the more ordered head group (28)(29)(30)(31)(32)(33)(34). The observed spectra are characteristic of system with axial symmetry, even though the static shielding tensor for the a~P nucleus is not axially symmetric.…”