Effect of pressure on bilayer phase behavior of N-methylated di-O-hexadecylphosphatidylethanolamines: relevance of head-group modification on the bilayer interdigitation

Abstract

The phase transitions of N-methylated di-O-hexadecylphosphatidylethanolamines (DHPE, DH-N-methyl-PE (DHMePE) and DH-N,N-dimethyl-PE (DHMe2PE)) were observed by differential scanning calorimetry (DSC) and fluorometry under atmospheric pressure and by light-transmittance measurements under high pressure. The DSC thermograms showed that the N-methylated DHPE bilayers underwent the phase transition from the gel phase to the liquid crystalline (Lα) phase under atmospheric pressure. The gel phase was identified by fluorometry as the lamellar gel (Lβ) phase, and not interdigitated gel (LβI) phase. The gel/Lα transition temperature increased with pressure while decreased stepwise with increasing polar head-group size. This stepwise depression of the transition temperature may be caused by the inverse-proportional hydrogen-bonding capabilities of the head-group to the head-group size. The thermodynamic quantities of the gel/Lα transition were comparable for the N-methylated DHPE bilayers. The pressure-induced LβI phase was not found in these bilayers although the bilayer of di-O-hexadecylphosphatidylcholine (DHPC), which is a kind of N-methylated DHPEs, forms the LβI phase only by hydration under atmospheric pressure. Taking into account that the bilayers of diacyl-homologs of N-methylated DHPEs, N-methylated dipalmitoyl-PEs except for dipalmitoylphosphatidylcholine (DPPC), do not form the LβI phase in the whole pressure range investigated but the DPPC bilayer forms the LβI phase under high pressure, we can say that the interdigitation requires weaker interaction between large-sized head groups like the bulky choline group.