Chappa Veronica, Smirnova Yuliya, Komorowski Karlo, Müller Marcus, Salditt Tim
Faculty of Physics, University of Göttingen, Friedrich-Hund-Platz 1, Göttingen 37077, Germany.
J Appl Crystallogr. 2021 Mar 25;54(Pt 2):557-568. doi: 10.1107/S1600576721001461. eCollection 2021 Apr 1.
Small unilamellar vesicles (20-100 nm diameter) are model systems for strongly curved lipid membranes, in particular for cell organelles. Routinely, small-angle X-ray scattering (SAXS) is employed to study their size and electron-density profile (EDP). Current SAXS analysis of small unilamellar vesicles (SUVs) often employs a factorization into the structure factor (vesicle shape) and the form factor (lipid bilayer electron-density profile) and invokes additional idealizations: (i) an effective polydispersity distribution of vesicle radii, (ii) a spherical vesicle shape and (iii) an approximate account of membrane asymmetry, a feature particularly relevant for strongly curved membranes. These idealizations do not account for thermal shape fluctuations and also break down for strong salt- or protein-induced deformations, as well as vesicle adhesion and fusion, which complicate the analysis of the lipid bilayer structure. Presented here are simulations of SAXS curves of SUVs with experimentally relevant size, shape and EDPs of the curved bilayer, inferred from coarse-grained simulations and elasticity considerations, to quantify the effects of size polydispersity, thermal fluctuations of the SUV shape and membrane asymmetry. It is observed that the factorization approximation of the scattering intensity holds even for small vesicle radii (∼30 nm). However, the simulations show that, for very small vesicles, a curvature-induced asymmetry arises in the EDP, with sizeable effects on the SAXS curve. It is also demonstrated that thermal fluctuations in shape and the size polydispersity have distinguishable signatures in the SAXS intensity. Polydispersity gives rise to low- features, whereas thermal fluctuations predominantly affect the scattering at larger , related to membrane bending rigidity. Finally, it is shown that simulation of fluctuating vesicle ensembles can be used for analysis of experimental SAXS curves.
小单层囊泡（直径20 - 100纳米）是强弯曲脂质膜的模型系统，特别是用于细胞器的模型系统。通常，小角X射线散射（SAXS）被用于研究它们的大小和电子密度分布（EDP）。目前对小单层囊泡（SUVs）的SAXS分析通常采用将其分解为结构因子（囊泡形状）和形状因子（脂质双层电子密度分布），并引入了额外的理想化假设：（i）囊泡半径的有效多分散分布，（ii）球形囊泡形状，以及（iii）对膜不对称性的近似考虑，这一特征对于强弯曲膜尤为重要。这些理想化假设没有考虑热形状波动，并且在强盐或蛋白质诱导的变形以及囊泡粘附和融合的情况下也会失效，这使得脂质双层结构的分析变得复杂。本文展示了从粗粒化模拟和弹性考虑推断出的具有实验相关大小、形状和弯曲双层EDP的SUVs的SAXS曲线模拟，以量化大小多分散性、SUV形状的热波动和膜不对称性的影响。据观察，即使对于小囊泡半径（约30纳米），散射强度的分解近似也成立。然而，模拟表明，对于非常小的囊泡，EDP中会出现曲率诱导的不对称性，对SAXS曲线有显著影响。还证明了形状的热波动和大小多分散性在SAXS强度上有可区分的特征。多分散性导致低波特征，而热波动主要影响较大q处的散射，这与膜弯曲刚度有关。最后，表明波动囊泡系综模拟可用于分析实验SAXS曲线。
