Structural transitions in short-chain lipid assemblies studied by 31 P-NMR spectroscopy

Abstract

The self-assembled supramolecular structures of diacylphosphatidylcholine (diCnPC), diacylphosphatidylethanolamine (diCnPE), diacylphosphatidyglycerol (diCnPG), and diacylphosphatidylserine (diCnPS) were investigated by 31P nuclear magnetic resonance (NMR) spectroscopy as a function of the hydrophobic acyl chain length. Short-chain homologs of these lipids formed micelles, and longer-chain homologs formed bilayers. The shortest acyl chain lengths that supported bilayer structures depended on the headgroup of the lipids. They increased in the order PE(C6)< PC(C9)≤ PS (C9 or C10)< PG(C11 or C12). This order correlated with the effective headgroup area, which is a function of the physical size, charge, hydration, and hydrogenbonding capacity of the four headgroups. Electrostatic screening of the headgroup charge with NaCl reduced the effective headgroup area of PS and PG and thereby decreased the micelle-to-bilayer transition of these lipid classes to shorter chain lengths. The experimentally determined supramolecular structures were compared to the assembly states predicted by packing constraints that were calculated from the hydrocarbon-chain volume and effective headgroup area of each lipid. The model accurately predicted the chain-length threshold for bilayer formation if the relative displacement of the acyl chains of the phospholipid were taken into account. The model also predicted cylindrical rather than spherical micelles for ll four diacylphospholipid classes and the 31P-NMR spectra provided evidence for a tubular network that appeared as an intermediate phase at the icelle-to-bilayer transition. The free energy of micellization per methylene group was independent of the structure of the supramolecular assembly, but was-0.95 kJ/mol (-0.23 kcal/mol) for the PGs compared to -2.5 kJ/mol (-0.60 kcal/mol) for the PCs. The integral membrane protein OmpA did not change the bilayer structure of thin (diC10PC) bilayers.