Reconstruction and interpretation of lipid bilayer structure from X-ray scattering often rely on assumptions regarding the molecular distributions across the bilayer. It is usually assumed that changes in head-head spacings across the bilayer, as measured from electron density profiles, equal the variations in hydrocarbon thicknesses. One can then determine the structure of a bilayer by comparison to the known structure of a lipid with the same headgroup. Here we examine this procedure using simulated electron density profiles for the benchmark lipids DMPC and DPPC. We compare simulation and experiment in both real and Fourier space to address two main aspects: (i) the measurement of head-head spacings from relative electron density profiles, and (ii) the determination of the absolute scale for these profiles. We find supporting evidence for the experimental procedure, thus explaining the robustness and consistency of experimental structural results derived from electron density profiles. However, we also expose potential pitfalls in the Fourier reconstruction that are due to the limited number of scattering peaks. Volumetric analysis of simulated bilayers allows us to propose an improved, yet simple method for scale determination. In this way we are able to remove some of the restrictions imposed by limited scattering data in constructing reliable electron density profiles.

中文翻译：

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由脂质双层分子动力学模拟指导的小角度X射线测量的解释。

X射线散射对脂质双层结构的重建和解释通常取决于关于双层分子分布的假设。通常假定，从电子密度分布图测得，跨双层的头顶间距的变化等于碳氢化合物厚度的变化。然后，可以通过与具有相同头基的脂质的已知结构进行比较来确定双层的结构。在这里，我们使用基准脂质DMPC和DPPC的模拟电子密度曲线检查了此程序。我们在真实空间和傅立叶空间中对仿真和实验进行了比较，以解决两个主要方面：（i）从相对电子密度分布图测量头-头间距，以及（ii）确定这些分布图的绝对比例。我们找到了实验程序的支持证据，从而解释了从电子密度分布图得出的实验结构结果的鲁棒性和一致性。但是，由于散射峰的数量有限，我们还会在傅立叶重构中暴露出潜在的陷阱。模拟双层的体积分析使我们能够提出一种改进的但简单的规模确定方法。这样，我们就可以消除在构建可靠的电子密度分布图时由有限的散射数据所施加的一些限制。模拟双层的体积分析使我们能够提出一种改进的但简单的规模确定方法。这样，我们就可以消除在构建可靠的电子密度分布图时由有限的散射数据所施加的一些限制。模拟双层的体积分析使我们能够提出一种改进的但简单的规模确定方法。这样，我们就可以消除在构建可靠的电子密度分布图时由有限的散射数据所施加的一些限制。