Staffan Sundell

Conformation of phospholipids: Crystal structure of a lysophosphatidylcholine analogue☆

Abstract The conformation and molecular packing of deoxylysophosphatidylcholine monohydrate (3-dodecanoyl-propandiol-1-phosphorylcholine · H2O) has been determined by single-crystal analysis. The lipid crystallizes in the monoclinic space group P2 1 c with unit cell dimensions: a = 24.83, b = 9.53, c = 10.94 A and β = 99.66 ° . The unit cell contains four symmetry-related molecules arranged in pairs of conformational enantiomers. The lipid molecules pack head-to-tail forming a common hydrocarbon layer which is bordered on each side by a region of polar groups. Two hydrocarbon chains are thus accommodated per polar group. The profile of this layer arrangement resembles that of a bilayer except that the thickness of the hydrocarbon region corresponds to the chain length of one fatty acid rather than two. The overall orientation of the phosphorylcholine groups is parallel to the layer surface. The torsion angles of the polar group are very similar to those of phosphatidylethanolamine and glycerylphosphorylcholine. Apparently this preferred polar group conformation is determined by intramolecular forces. The water molecules of hydration link adjacent phosphate groups within the polar group layer to hydrogen-bonded ribbons. Each molecule occupies an area of 52 A2 in the layer surface. The hydrocarbon chains pack according to the monoclinic M ∥ packing mode with all their chain planes parallel and with a cross-section/chain of 19.7 A2. The chains accommodate to the large molecular area by a tilt of 41 ° with respect to the layer normal. The conformational features of the compound are compared with structure data of other membrane lipids. A close resemblance to the gel state of lysophosphatidylcholine and diacylphosphatidylcholine is found which now allows a detailed interpretation of the molecular conformation and packing principles in the fully hydrated lamellar structures of these important membrane lipids.

Glycerol conformation and molecular packing of membrane lipids: The crystal structure of 2,3-dilauroyl-d-glycerol

The single-crystal structure of 2,3-dilauroyl-d-glycerol has been determined by Patterson rotation and translation methods and refined to R = 0.069. 2,3-dilauroyl-d-glycerol crystallizes in the monoclinic space group P21, with unit cell dimensions: a = 5.46 A, b = 7.59 A, c = 34.2 A and β = 93.1 °, and with two molecules per unit cell. The molecules have their hydrocarbon chains aligned parallel, and are arranged in a bilayer structure. The chain stacking is achieved by a bend in the fatty acid. The hydrocarbon chains pack according to the orthorhombic perpendicular chain packing mode, and are tilted 26.5 ° from the layer normal. The structural features of 2,3-dilauroyl-d-glycerol have been analysed with reference to the corresponding hydrophobic moieties in the crystal structures of different membrane lipids. The glycerol group in 2,3-dilauroyl-d-glycerol is oriented parallel to the layer plane, but changes to an approximately layer-perpendicular orientation when a polar group is attached. The molecular conformation of the glycerol-dicarboxylic ester group, however, is identical in both the absence and presence of a head group, indicating extensive conformational restrictions for this group due to both intrinsic properties and chain stacking. The gathered data provide detailed information on the structural properties of the hydrophobic moiety of membrane lipids.

The effect of hydrogen bonds on the conformation of glycosphingolipids. Methylated and unmethylated cerebroside studied by X-ray single crystal analysis and model calculations

The conformation and molecular packing of permethylated beta-D-galactosyl-N-octadecanoyl-D-spingosine (cerebroside) was determined by X-ray single crystal analysis at 185 K (R = 0.16). The lipid crystallizes in the orthorhombic space group P2(1)2(1)2(1) with the unit cell dimensions a = 8.03, b = 7.04 and c = 88.10 A. The four molecules in the unit cell pack in a bilayer arrangement with tilting (48 degrees) hydrocarbon chains. The direction of the chain tilt alternates in the two bilayer halves and in adjacent bilayers. In order to define the effect of hydrogen bonds on the molecular conformation the structural features of the permethylated cerebroside are compared with that of unsubstituted cerebroside (I. Pascher and S. Sundell (1977) Chem. Phys. Lipids 20, 179). It is shown that methylation of the hydrogen donor groups does not affect the conformation of the ceramide part. However, by abolishing the intramolecular hydrogen bond between the amide N--H group and the glycosidic oxygen the galactose ring changes its orientation from layer-parallel to layer-perpendicular. Calculations using molecular mechanics, MM2(87), show that in natural cerebroside the intramolecular hydrogen bond stabilizes the theta 1 = -syn-clinal conformation about the C(1)--C(2) sphingosine bond by 2-2.5 kcal/mol compared to other staggered conformations. The significance of the L shape of the native cerebroside, making both the carbohydrate and polar ceramide groups accessible as a binding epitope in recognition processes, is discussed.

Membrane lipids: preferred conformational states and their interplay. The crystal structure of dilauroylphosphatidyl-N,N-dimethylethanolamine

The conformation and molecular packing of 2,3-dilauroyl-rac-glycero-1-phospho-N,N-dimethylethanolamine (DLPEM2) has been determined by single-crystal analysis (R = 0.079). The lipid crystallizes in a triclinic space group (P1) with a unit cell of a = 5.64, b = 8.20, c = 39.86 A and α = 94.5, β = 90.1, γ = 101.9°. The two molecules of the unit cell are related by centro-symmetry and pack tail to tail in a bilayer structure. The zwitterionic head groups are extended perpendicular to the layer plane and interdigitate with the head group dipoles of the adjacent bilayer. The packing cross-section per molecule is 45.2 A2. The hydrocarbon chains pack with an orthorhombic (O ⊥) subcell and tilt by 33° with respect to the layer normal. The diacylglycerol moiety shows an unusual conformation. The glycerol chain is inclined by 45° with respect to the layer normal and the fatty acid substituted oxygens adopt mutually a — syn-clinal instead of the more common + syn-clinal conformation. Head group interactions and molecular conformation of DLPEM2 are compared with the corresponding structural features of phosphatidic acid, phosphatidylethanolamine and phosphatidylcholine. For the diacylglycerol part three minimum energy conformations are observed, which interconvert by rotation and axial displacement of the hydrocarbon chains.

Polar group interaction and molecular packing of membrane lipids. The crystal structure of lysophosphatidylethanolamine.

Abstract The conformation and molecular packing of 3-palmitoyl- dl -glycerol-1-phosphoryl-ethanolamine has been determined by a single crystal analysis (R = 0.115); it crystallizes in the monoclinic space group P2 1 a with a unit cell of a = 7.66 A , b = 9.08 A , c = 37.08 A and β = 90.2 ° , with four molecules per unit cell. The molecules exist as configurational and conformational enantiomers and pack in a bilayer arrangement. The phosphorylethanolamine groups have an orientation parallel to the layer surface. The hydrocarbon chains are arranged according to the T∥ chain packing mode and adopt an extreme tilt of 57.5 ° with respect to the layer normal. The free glycerol hydroxyl group forms an intramolecular hydrogen bond with, a phosphate oxygen and thus affects the conformation and orientation of the head group. The phosphorylethanolamine dipoles are oriented parallel to each other in double rows, while they are antiparallel and form a continuous network in dilauroylphosphatidylethanolamine (Elder et al., 1977). The area per molecule in 3-palmitoyl- dl -glycerol-1-phosphorylethanolamine (34.8 A2) is less than in diacylphosphatidylethanolamine (38.6 A2), indicating that in the latter the hydrocarbon chains determine the molecular cross-section. The significance of the interaction and space requirement of the phosphorylethanolamine group for the phase behaviour of phosphatidylethanolamine is discussed.

The polar group conformation of a lysophosphatidylcholine analogue in solution: A high-resolution nuclear magnetic resonance study

Abstract The 1H and 13C resonances of 3-lauroyl-propandiol-1-phosphorylcholine in C2H3O2H and 2H2O have been assigned. LPPC ‡ is present as monomers in methanol whereas it forms small micelles in water consisting of about 65 molecules. The vicinal 1H1H, 1H13C and 1H31P spin coupling constants of the polar group resonances were derived from computer simulations of the 1H, 13C and 31P high-resolution nuclear magnetic resonance spectra. From an analysis of these vicinal coupling constants rotamer populations for the CC and CO bonds of the propandiol-3-phosphorylcholine moiety of LPPC were computed using a Karplus treatment (Becker, 1969). In both solvents used there is a preferred conformation in the phosphorylcholine fragment of LPPC whereas the propandiol part is flexible, as is evident from nearly equally populated rotamers around the two CC bonds of propandiol. The motionally averaged conformation of the phosphorylcholine group is characterized by an almost exclusively synclinal conformation of the choline residue (torsion angle α5, OCCN) and by predominantly antiperiplanar conformations about the CCOP bond (torsion angle α1) and the POCC bond (torsion angle α4). Within the error of our conformational analysis there is good agreement between the motionally averaged conformation in solution and the crystal structure of LPPC. The average conformation in solution is independent of the solvent used and of the state of aggregation suggesting that it is mainly determined by intramolecular forces. Electrostatic interaction between the positively charged nitrogen and the anionic phosphate oxygen is responsible for stabilizing the ± synclinal conformation of the choline group. In contrast to the preferred conformation of the phosphorylcholine group, the number of possible conformations in the propandiol group is not restricted. Since, for both torsion angles θ1 and θ3 the three staggered conformations are equally populated, there are essentially nine possible conformational combinations for the propandiol moiety of LPPC. Since the interconversion between these different conformations is rapid on the n.m.r. time scale the LPPC molecule must have considerable flexibility about the two CC bonds of the propandiol fragment. This result indicates that there cannot be any stringent requirements for the packing of the propandiol group or the hydrocarbon chains in LPPC micelles imposing any serious constraints on the segmental motion of that group. In this respect LPPC differs markedly from diacyl phospholipids. Under comparable experimental conditions the latter class of lipids has been reported to have preferred conformations about the two CC bonds of the glycerol (torsion angles θ1 and θ3, Hauser et al., 1978a, 1980). The conformational preference in that part of the molecule is a consequence of the parallel alignment of the two hydrocarbon chains optimizing hydrophobic interactions both intra- and intermolecularly. Apparently, intermolecular chain-stacking in the LPPC micelle does not restrict the number of possible rotamers in the propandiol part of the LPPC molecule.

Molecular arrangements in sphingolipids: crystal structure of the ceramide N-(2d,3d-dihydroxyoctadecanoyl)-phytosphingosine

Abstract The conformation and molecular packing of a ceramide, N-(2 d ,3 d -dihydroxyoctadecanoyl)-phytosphingosine , has been determined by single crystal analysis (R = 0.095). The lipid crystallizes in the space group P21 with unit cell dimensions a = 7.23, b = 7.39, c = 35.80 A and β = 92.9°. The two molecules of the unit cell have extended hydrocarbon chains and pack alternatingly with antiparallel orientation in a single layer arrangement. In the centre of the molecular layer the amide group and hydroxyl groups form a lateral network of hydrogen bonds. The cross-section per molecule in this packing arrangement is 26.7 A2. The hydrocarbon chains extending on both sides of the central polar region pack in accordance with an orthorhombic (0˔) subcell and tilt by 46° with respect to the layer normal.

MOLECULAR ARRANGEMENT AND CONFORMATION OF LIPIDS OF RELEVANCE TO MEMBRANE STRUCTURE

Intact membranes as well as membrane components have been studied extensively by various physical and chemical methods. Important data have accummulated but more specific structural information on the atomic level is still necessary in order to obtain a detailed understanding of lipid-lipid and lipid-protein interactions and of variations in structure and composition of lipids observed in different types of membranes. Only then will it be possible to explain the function of the different constituents and their significance for various membrane properties.

The crystal structure of cholesteryl dihydrogen phosphate

Crystals of cholesteryl dihydrogen phosphate grown from 1,4-dioxane solution are monoclinic, space group C2 with a = 24.40, b = 6.27, c = 40.86 Aand β = 102.7°. The asymmetric unit contains two molecules of cholesteryl phosphate CP and one dioxane molecule of the solvent. The CP molecules pack tail to tail in a bilayer structure. Within the layer they are arranged in double rows with their phosphate groups linked to ribbons by hydrogen bonds. Laterally the double strands of phosphate groups are separated by rows of dioxane molecules. The dioxane serves as hydrogen bond acceptor and as a spacer molecule that compensates the differences in cross-sectional area of the cholesteryl residue (38.4 A2 and the phosphate group (24 A2). In the cholesterol matrix the CP molecules joined to double rows have packing contact with the smooth side of their skeleta and interdigitate with their annular methyl groups with those of molecules of the adjacent double rows. The branched cholesteryl side chains facing the bilayer center are loosely packed and show considerable disorder and/or thermal motion.

On the stereochemistry of 2,3-dihydroxy fatty acids. The single crystal structure of potassium d-erythro-2,3-dihydroxyoctadecanoate

The erythro-2,3-dihydroxyoctadecanoic acid studied is a synthetic homologue of a natural occurring constituent of sphingolipids. The potassium salt of the acid crystallizes in the monoclinic space group P2, with the unit cell dimension: a = 5.39, b = 7.06, c = 26.26 A and β = 94.9°. The crystal structure was solved by direct methods and was refined to R = 0.062. The absolute configuration of the compound was determined by means of anomalous scattering effects, showing that the natural fatty acid has d-erythro configuration. The compound packs tail to tail in an unusual bilayer arrangement. The hydrocarbon chains have an extreme tilt of 60° and opposite inclination in the two halves of the bilayer. Laterally the hydrocarbon chains are arranged according to the monoclinic M∥ packing mode. The carbon chain makes a perpendicular bent at carbon atom 2. This places the 2-hydroxyl group in a preferred co-planar conformation towards the carboxylate group and at hydrogen bond distance to one of the carboxylate oxygens. The carboxylate group and the two hydroxyl groups are co-ordinated to K+ ions and together account for a large molecular cross-ection of 38 A2. Monolayer studies show that the acid forms a phase with this spacious molecular area also in contact with water. On compression above 10 mN m−1 transition to a more condensed state (S = 27 A2) takes place accompanied by marked changes of the surface potential.