Conformation of phospholipids: Crystal structure of a lysophosphatidylcholine analogue

Abstract

The conformation and molecular packing of deoxylysophosphatidylcholine monohydrate (3-dodecanoyl-propandiol-1-phosphorylcholine · H 2O) 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 Å 2 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 Å 2. 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.