Conformational Study of Phospholipids in Crystalline State and Hydrated Bilayers by 13C and 31P CP-MAS NMR

Abstract

Crystalline forms of the racemic and enantiomerically pure saturated zwitterionic phospholipids, phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidyl-N-methylethanolamine (PEM1), and phosphatidyl-N,N-dimethylethanolamine (PEM2) have been investigated using 13C and 31P cross-polarization magic angle spinning (CP-MAS) NMR. The observed spectra indicate two types of packing behavior depending on the optical purity of phospholipids and the structure of a head group. The crystalline lattices of the enantiomerically pure PC, PE, PEM1, and PEM2 contained two conformations, whereas the corresponding racemic phospholipids contained only a single conformational species. On the basis of the known crystal structures of phospholipids, these results can be explained in terms of different ways the various zwitterionic phospholipids achieve packing into lattices with a minimal residual dipole moment. In enantiomerically pure PC and PE, the presence of two conformations, combining one configurational isomer of the diacyl glycerol moiety with two conformational mirror images of the head group, is necessary to eliminate the residual in-plane dipole moment of head groups and still achieve tight packing of the chiral diacylglycerol residue. In racemates, the in-plane dipole moment is eliminated by packing a single conformation into a true centrosymmetric lattice. The hydration of the crystalline phospholipid samples produced significant spectral alterations such as changes in the 13C chemical shifts of the diacylglycerol C2 carbon and carbonyl groups, and 31P NMR signals, suggesting conformational changes. The hydrated bilayers of both racemic and enantiomerically pure dipalmitoyl PC (DPPC) gave rise to identical 13C NMR spectra regardless the phase state, except for the most rigid Lc subgel phase. The invariance of chemical shifts in these gel phases indicated that no major conformational changes occurred during phase transitions of hydrated bilayers of PC. The NMR data presented in this work provide evidence that conformations of phospholipids in the crystalline state are strongly affected by packing forces and the tendency of the head groups to pack in true- or quasi-centrosymmetric lattices. On the other hand, conformations in hydrated bilayers are largely governed by the intrinsic conformational properties of monomeric molecules. The data presented address the problem of the relevance of the X-ray structures of crystalline phospholipids for evaluation of the conformations in the less ordered hydrated bilayers.