Conformation and packing of membrane lipids: Crystal structures of lysophosphatidylcholines

Abstract

The molecular conformation and packing of three lysophosphatidyl-cholines: 3-palmitoyl-D-glycero-1-phosphocholine (PPC), 3-hexadecyl-D-glycero-1-phosphocholine (HPC) and 3-palmitoyl-DL-glycero-1-phospho-N,N-dimethylethanolamine (PPEM2) have been determinated by X-ray single crystal analyses. PPC crystallizes as the monohydrate and HPC as chloroform solvate, both with triclinic unit cells (space group P1) containing two independent molecules in almost identical packing arrangements. The two molecules of PPC/HPC are mirror image conformers with respect to their head groups and pack separately in either half of a bilayer arrangement with interdigitating hydrocarbon chains and interdigitating head groups. The racemic PPEM2 also crystallizes with a very similar interdigitating packing arrangement. The unit cell, however, is monoclinic (space group P21/a) and comprises four molecules, arranged as pairs of centrosymmetric D/L conformers at either side of the bilayer. In PPC and HPC the C16-hydrocarbon chains interdigitate with an overlap of 13 carbon atoms only, leaving a cavity to accommodate the solvate molecules. The chain matrices have identical tilt (45°), but different chain packing modes (O′⊥ and O∥). In PPEM2 the chain ends penetrate with 18-atoms interdigitation to the glycerol region of oppositely oriented molecules and pack in a hybrid matrix with 37° tilt. Despite the differences in hydrocarbon chain attachment (ester/ether) and degree of N-methylation the structures show great similarities, in particular with respect to the head group conformation, which apparently is favored by intrinsic energetics.