Profile Structures of Thin Multilayer Films by X-ray Diffraction Using Direct and Refinement Methods of Analysis

Meridional x-ray diffraction data from Langmuir-Blodgett multilayers containing two to ten molecular monolayers of arachidic acid were analyzed by two independent methods. A Patterson function deconvolution technique uniquely provided the electron density profile (8A resolution) of the average, symmetric bilayer repeated in the multilayer. This average bilayer appeared to disorder as the number of bilayers in the multilayer decreased. A refinement technique, which does not assume a centrosymmetric structure or the existence of a unit cell, uniquely provided the profile structure of the multilayer itself. In particular it could distinguish the individual monolayers in the multilayer. Meridional x-ray scattering data from Langmuir-Blodgett multilayers composed of arachidic acid and either \u27myristic acid or polymerized 10,12 pentacosadiynoic acid were also analyzed by the refinement technique. It found that only the last monolayer in the depostion sequence (ie. the surface monolayer) was disordered and that ordering of the surface monolayer can be induced by the deposition of an additional monolayer. In addition, an application of Langmuir-Blodgett multilayer thin films and their characterization by x-ray scattering is discussed with regards to the structural study of membrane-membrane interactions and the triggering of cellular components of the immune system

Bilayer characteristics of a diether phosphonolipid analog of the major lung surfactant glycerophospholipid dipalmitoyl phosphatidylcholine.

Thermal and lyotropic phase behavior was studied by X-ray diffraction and differential scanning calorimetry for a diether phosphonolipid analog (DEPN-8) of the major lung surfactant glycerophospholipid dipalmitoyl phosphatidylcholine (DPPC). DEPN-8 differs in an ether, rather than an ester, bond at the acyl chain-backbone linkage and a headgroup phosphonate (isosteric methylene substitution) versus phosphate constituent. Analysis of lamellar diffraction maxima demonstrated that at high relative humidity (98%) and temperatures below the liquid crystal phase transition (approximately 45 degrees C), DEPN-8 formed interdigitated bilayers with a characteristic periodicity of 41.9-46.5 A. At low humidity the gel phase DEPN-8 bilayers were characteristic of a normal L beta phase with a periodicity equivalent to DPPC (57-59 A). Above the liquid crystal thermal phase transition, bilayer spacing for both DEPN-8 and DPPC was 51-52 A, characteristic of the L alpha phase. Complete assessments of both lamellar and in-plane X-ray scattering used to construct electron density profiles and structure-factor plots for DEPN-8 defined more fully the interdigitated bilayer state at high humidity and low temperature. Compared to DPPC, it is energetically favorable for DEPN-8 to form interdigitated bilayers under conditions of excess water and low temperature. The flexible character of the ether bonds in DEPN-8 allows increased hydrophobic interactions between acyl chains, without generating a steric penalty from the increased packing density of the molecules. Additionally, the ether bond and the phosphonate moiety may allow for more energetically favorable interactions between the choline portion of the headgroup and water. The DEPN-8 ether linkage may also contribute to the improved adsorption and film respreading found previously for this phosphonolipid compared to DPPC

α-Bungarotoxin Binding to Acetylcholine Receptor Membranes Studied by Low Angle X-Ray Diffraction

The nicotinic acetylcholine receptor (nAChR) carries two binding sites for snake venom neurotoxins. α-Bungarotoxin from the Southeast Asian banded krait, Bungarus multicinctus, is a long neurotoxin which competitively blocks the nAChR at the acetylcholine binding sites in a relatively irreversible manner. Low angle x-ray diffraction was used to generate electron density profile structures at 14-Å resolution for Torpedo californica nAChR membranes in the absence and presence of α-bungarotoxin. Analysis of the lamellar diffraction data indicated a 452-Å lattice spacing between stacked nAChR membrane pairs. In the presence of α-bungarotoxin, the quality of the diffraction data and the lamellar lattice spacing were unchanged. In the plane of the membrane, the nAChRs packed together with a nearest neighbor distance of 80 Å, and this distance increased to 85 Å in the presence of toxin. Electron density profile structures were calculated in the absence and presence of α-bungarotoxin, revealing a location for the toxin binding sites. In native, fully-hydrated nAChR membranes, α-bungarotoxin binds to the nAChR outer vestibule and contacts the surface of the membrane bilayer