Tan Luoxi, Elkins James G, Davison Brian H, Kelley Elizabeth G, Nickels Jonathan
Department of Chemical and Environmental Engineering, University of Cincinnati, 2901 Woodside Drive, Cincinnati, OH 45221, USA.
Biosciences Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831, USA.
J Appl Crystallogr. 2021;54(1). doi: 10.1107/s1600576720015526.
Slab models are simple and useful structural descriptions which have long been used to describe lyotropic lamellar phases, such as lipid bilayers. Typically, slab models assume a midline symmetry and break a bilayer structure into three pieces, a central solvent-free core and two symmetric outer layers composed of the soluble portion of the amphiphile and associated solvent. This breakdown matches reasonably well to the distribution of neutron scattering length density and therefore is a convenient and common approach for the treatment of small-angle scattering data. Here, an implementation of this model within the software suite is reported. The implementation is intended to provide physical consistency through the area per amphiphile molecule and number of solvent molecules included within the solvent-exposed outer layer. The proper use of this model requires knowledge of (or good estimates for) the amphiphile and solvent molecule volume and atomic composition, ultimately providing a self-consistent data treatment with only two free parameters: the lateral area per amphiphile molecule and the number of solvent molecules included in the outer region per amphiphile molecule. The use of this code is demonstrated in the fitting of standard lipid bilayer data sets, obtaining structural parameters consistent with prior literature and illustrating the typical and ideal cases of fitting for neutron scattering data obtained using single or multiple contrast conditions. While demonstrated here for lipid bilayers, this model is intended for general application to block copolymers, surfactants, and other lyotropic lamellar phase structures for which a slab model is able to reasonably estimate the neutron scattering length density/electron-density profile of inner and outer layers of the lamellae.
平板模型是简单且有用的结构描述，长期以来一直用于描述溶致液晶层状相，如脂质双层。通常，平板模型假定中线对称，并将双层结构分解为三部分：一个无溶剂的中央核心以及由两亲分子的可溶部分和相关溶剂组成的两个对称外层。这种分解与中子散射长度密度的分布相当吻合，因此是处理小角散射数据的一种方便且常用的方法。在此，报告了该模型在软件套件中的一种实现方式。该实现旨在通过每个两亲分子的面积以及溶剂暴露外层中包含的溶剂分子数量来提供物理一致性。正确使用此模型需要了解（或对其有良好估计）两亲分子和溶剂分子的体积及原子组成，最终仅通过两个自由参数提供自洽的数据处理：每个两亲分子的横向面积以及每个两亲分子在外层区域中包含的溶剂分子数量。在对标准脂质双层数据集进行拟合时展示了此代码的使用，获得了与先前文献一致的结构参数，并说明了使用单对比度或多对比度条件获得的中子散射数据拟合的典型和理想情况。虽然在此以脂质双层为例进行了演示，但该模型旨在广泛应用于嵌段共聚物、表面活性剂以及其他溶致液晶层状相结构，对于这些结构，平板模型能够合理估计片层内层和外层的中子散射长度密度/电子密度分布。
