Direct X-Ray Study of the Molecular Tilt in Dipalmitoyl Lecithin Bilayers

Abstract

Direction and angle of molecular tilt in an ordered multilayer system of dipalmitoyl lecithin and water were directly determined by X-ray diffraction as a function of temperature in the gel phase with ca. 20 wt. °/o of water.Early X-ray studies by Levine [1] and Tardien et al. [2] indicated that the hydrocarbon chains are tilted with respect to the layer normal in the gel phase of lecithin-water multilayer systems. Janiak et al.[3] obtained a temperature dependence of the tilt angle. Except the work of Levine, all these experiments were done with dispersions ("powders") and the tilt angle was deduced from the stacking period and the concentration of water. The latter was usually large enough ( > 20% by weight) to keep the pretransition and the main transition at the temperatures characteristic of the single bilayer, i. e. at 34 °C and 43 °C [4]. Stamatoff et al. [5] determined the tilt angle directly, using free-standing monodomain samples. However, the maximum concentration of water was only 10wt.% and the data were analyzed on the assumption that all tilt directions were equally probable.The following investigations were carried out with a "multisandwich" sample consisting of 100 mylar films alternating with multilayer systems of dipalmitoyl lecithin (DPL), both 5 [xm thick. The multisandwich was prepared at 60 °C by compressing a mixture of DPL and 20 wt.% of water. At room temperature the 1 mm high sample was cut into a 1x10 mm strip. Polarizing microscopy showed the bilayers to be well-aligned parallel to the mylar films [6]. The sample was then allowed to stand for 50 h in water-saturated air at 4 °C, sealed in a thin quartz capillary, and finally mounted on the sample holder of an X-ray pinhole camera. When the temperature was changed, measurements were resumed only after at least one day. The diffraction patterns confirmed the good alignment. With horizontal layers there were several orders of small-angle reflections indicating the stacking period along the vertical and wide-angle reflections from the hydrocarbon chain lattice near the equator. A density map of wide angle reflections is given in Figure 1. It suggests that the chains are tilted along one of the basic vectors of a hexagonal lattice (Case I) rather than along one of the bisectors (Case II). The two cases which are the most likely are sketched in Figure 2 The tilt angle derived for Case I, the stacking period, and the spacing of the hydrocarbon chain lattice are 0340-4811 / 80 / 0600-0647 5 01.00/0. -Please order a reprint rather than making your own copy.

X-ray diffraction studies of oriented lipid bilayers

“…The wide-angle reflections can be explained in terms of a centered rectangular lattice (in which the hexagon is slightly distorted from regularity) with the DPPC molecules tilted towards their nearest neighbors (Leadbetter et al 1979;Smith et al 1988). The tilt angle 0, to a first approximation, can then be calculated relatively easily using sin IJJ = cos 30" sin 0, where IJJ is the angle made with the equatorial axis by the 4. l -A wide-angle reflection and is measured directly from the diffraction pattern (Levine 1973;Hentschel et al 1980). If the hydrocarbon chains were tilted in a direction between nearest neighbors, referred to as the LpF p.hase, then each quadrant of the diffraction pattern would contain two offequatorial reflections and 0 would simply be equal to the angle made by the (2 0) reflection (Leadbetter et al 1979;Smith et al 1988;Katsaras et al 1992b).…”

“…The geometrical relationships for the example in Detailed structural information without intensity measurements section have been worked by various groups (e.g. Levine 1973;Hentschel et al 1980;Hentschel and Rustichelli 1991) and the reader only needs to apply them. The results obtained for the example presented in the structure of DPPC subgel phase bilayer section may not be obvious to the reader with no background in fiber diffraction but are presented to further demonstrate the detail made available by this technique.…”

Hydration of single crystals of dipalmitoylphosphatidylcholine.

“…X-ray diffraction studies of lecithin/water phases have been carried out by a number of workers (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22), and the general properties of the phase diagrams are well established. There exists a transition temperature T, (see, for example, ref.…”

“…For T < T, and low water concentrations, the equilibrium phases are crystalline-i.e., periodic in.three dimensions. At higher water concentrations, for T < Tc, x-ray diffraction measurements on partially oriented samples indicate rows of scattering along the (h,0,l) axis, h (4.2 A)-1, suggestive ofphases that have well-developed periodicity within each-bilayer but in which successive bilayers are not correlated (19,20). Thus, these phases can be simply interpreted as two-dimensional crystals stacked on top of one another but without any correlation in their relative positions.…”

Ein Kinkenblock-Modell für die Hauptumwandlung in Phospholipidschichten

“…Wir vernachl~ssigen also eine kollektive Kippung der Ketten, wie sie bekanntlich in der kristallinen Phase der Lecithine auftritt [19]. Die beiden monomolekularen Schichten nehmen ihre thermodynamischen GleichgewichtszustSnde unabh~ingig voneinander einl).…”