Small-and Wide Angle X-ray Scattering of Oriented Lecithin Multilayers

Abstract

It is the aim of this paper to demonstrate that much information is to be obtained about lameller systems when diffraction work is performed on well aligned samples and paracrystal methods are applied. There is no need to examine “crytallized” monohydrates in order to get more diffractio spots. Contrary to this, a water cotent above 20% is of higherbiological relevance. The water content of 23 wt.% wasd introduced by swelling in the gel-phase, giving a bigger stackign period than usually reported, when swelling is performed in the L_α-phase. At various temperatures oriented “multi-sandwiches” of dipalmitoyl lecithin (DPL) multilayers were analyzed by small-and wide angle X-ray diffraction as party reported before. Thus, in the gel-phase the tilt angle θ of the hydrocarbon chains can directly be measured, and moreover it is proved that the chains are tilted by 7° to 15° in b-direction of the orthorhombic and the hexagonal lattice. 012-[paraffine netplanes sablize a well-defined membrane surface like the 201-planes in Cerebrosides. Contary to paraffins the c/2 steps of the CH₂-chains along the b-direction are only a statistical average. These steps roughen the lamellae surfaces and produce paracrystalline distortions of g ∼ 2.4% of the long period in the gel-phase. In the P_β-phase and the CH₂-chains are laterally paracrystallien distored with g ∼ 10% and the membrane surfaces become smooth because molecular dynamics hinder the chains from latching on to eachother, and the long period, before therefore, is crystalline-like. Astbury's remarks on the interplay of order-disorder between molecular and supermolecular structures is fulfilled inm a special way. In addition the a*-law, which combines the size of the lamellae bundles withtheir g-values also is fully confirmed.

Below 35°C the hydrocarbon lattice is orthorhombic with a 1 % deviation from the hexagonal case. Only near 0°C undergoing the “subtransition” is the lattice similar to paraffins, and the tilt angle is reduced. The P_s-phase is characterized by a hexagonal hydrocarbon lattice accompanied by the wellknown “ripples” which take the direction of the a-axis, making them orthogonal to the molecular tilt direction. Contraty to the findings of others our “weak” ripples produce no equoatorial reflection in the small angle region, explained by the lack of longitudinal density fluctuations. A profound theoreical calculation of the weights of the humps at the long period tails yeilds ripple waves with a mean wave length of the humps at the long period tails yields ripple waves with a mean wave lenght of 310 ± 30 and an amplitude (4.5 ± 0.5) Å. The wave lingths here vary to such a large amount that the small angle pattern offers no distinct isolated lateral spots. This is explained by the multi-sandwich preparation which hinders their free development. The analysis, with help of uncorrelated step dislocations, leads to a paracrystalline value of g_exp = 10%.