Headgroup organization and hydration of methylated phosphatidylethanolamines in Langmuir monolayers

Subtle differences in the molecular conformation of fully hydrated phospholipids, and in their interaction with the water reservoir, were assessed as functions of headgroup methylation with surface-sensitive X-ray scattering. To achieve such a structural and functional comparison, diacylphosphatidylethanolamines (PEs) and their mono-, di- and trimethylated (diacylphosphatidylcholine, PC) derivatives in surface monolayers on water have been studied. While the molecular structures of these lipids are quite similar, their subtle distinctions lead to surprisingly large differences in their overall organization. Independent of the surface pressure, π, the amine function in PE extends 1–2 Å further into the subphase than those of the methylated headgroups. Not only is the exposure of the amine moiety to water in PE thus larger than that of the other lipids, but also the phosphate and lipid backbone of PE are more hydrated than that of PC. Overall, the PE headgroup hydration is ≈25% larger than that of PE-N-Me, PE-N-Me2 or PC. The main reason for these differences resides in their distinct capabilities to donate hydrogen bonds, but differences in the hydrophobicities of the amine functions on the lipid headgroups may also play a role. While the impact of amine methylation on the headgroup interaction with the water subphase appears rather straightforward, there are also differences in lipid backbone organization and acyl chain packing. The results presented here provide a deeper understanding of lipid conformation as the hydrophobicity of the terminal headgroup fragment is systematically altered and may also impact on our understanding of the molecular details of membrane fusion.