Direct Evidence for the Partial Dehydration of Phosphatidylethanolamine Bilayers on Approaching the Hexagonal Phase

X-ray diffraction studies on oriented multilayers of 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE) in the lamellar gel (L_β) and inverted hexagonal (H_II) phases at various temperatures (5–50°C) and relative humidities (0–100%) are reported. One-dimensional electron density profiles of the phase bilayers were constructed to a resolution of better than 4 Å using direct methods to solve for the phase problem. In addition, the electron density profiles were fitted favorably using a model in which the atomic groups were assumed to be Gaussian distributed [Wiener, M. C., & White, S. H. (1992) Biophys. J. 61, 434–447]. The X-ray data clearly demonstrate that, at 100% relative humidity (RH), POPE samples exist in two distinct L_β phases, differing primarily in the amount of water between the lamellae. As the hexagonal phase transition temperature is approached, 100% RH POPE samples partially dehydrate, releasing approximately 5 water molecules per phospholipid and experiencing on average a 3-Å decrease in repeat spacing. The lower temperature hydrated L_β phase POPE electron density distribution resembles that obtained from the L_β phase 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) bilayers and is unlike the partially dehydrated POPE bilayers.