Direct evidence for the partial dehydration of phosphatidylethanolamine bilayers on approaching the hexagonal phase

Abstract

X-ray diffraction studies on oriented multilayers of 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE) in the lamellar gel (L beta) and inverted hexagonal (HII) phases at various temperatures (5-50 degrees C) and relative humidities (0-100%) are reported. One-dimensional electron density profiles of the L beta phase bilayers were constructed to a resolution of better than 4 A 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 beta 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-A decrease in repeat spacing. The lower temperature hydrated L beta phase POPE electron density distribution resembles that obtained from the L beta phase 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) bilayers and is unlike the partially dehydrated POPE bilayers.

X-ray diffraction studies of oriented lipid bilayers

“…Typical lipid-water samples used in X-ray diffraction studies can either be oriented onto a curved glass substrate (e.g., Torbet and Wilkins 1976;Katsaras et al 1993) or unoriented, whereby a lipid-water dispersion is deposited into a thin-wall (10 pm) glass or quartz capillary tube (diameter = 1-2 mm) and flame sealed (e.g., Franks and Lieb 1981). Oriented mul- Fig.…”

“…tion pattern is recorded (Katsaras et al 1992b(Katsaras et al , 1993. From the reflections along the c* axis, the bilayer repeat distance d can easily be determined using Bragg's law; hA = 2d sin 0,, where 0, is the Bragg angle, h is the order number, and A is the wavelength of the X-ray radiation.…”

“…In the plane of the bilayer, the hydrocarbon chains are packed in such a fashion that they are s&ounded by six neighbors. If the hydrocarbon chains of the lipid molecules were not tilted with respect to the bilayer normal (termed the Lp phase), then only two wide-angle reflections centered on the equatorial axis (the axis that is perpendicular to the c* axis) would be observed (Katsaras et al 1993) and would indicate hexagonal symmetry. In Fig.…”

“…in A; -, reflections not In Fig. 5, we present data from a series of X-ray swelling experiments for oriented multibilayers of POPE (Katsaras et al 1993). As we swelled the bilayers, a different d is obtained, effectively sampling the Fourier transform FR at different points x = hld.…”

“…As such, X-ray diffraction has proven to be one of the most powerful and direct methods for characterizing lipid-water systems (e.g., Smith et al 1988;Hentschel and Rustichelli 1991;Katsaras et al 1 9 9 2~;1992b;Weiner and White 1992;Katsaras et al 1993;Tristram-Nagle et al 1993;McIntosh and Simon 1993). One of the most comprehensive studies on the structure and polymorphism of lipid-water systems using X-ray diffraction was performed by Tardieu et al (1 973).…”