The effect of polydispersity, shape fluctuations and curvature on small unilamellar vesicle small-angle X-ray scattering curves

Abstract

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-q features, whereas thermal fluctuations predominantly affect the scattering at larger q, related to membrane bending rigidity. Finally, it is shown that simulation of fluctuating vesicle ensembles can be used for analysis of experimental SAXS curves.

Computational Reverse Engineering Analysis for Scattering Experiments (CREASE) on Vesicles Assembled from Amphiphilic Macromolecular Solutions

“…Further, in the ELP-CLP systems, if we use the more sophisticated coarse-grained model of CLP triple helix that can capture the melting of CLP triple helix into CLP single strands, , we can also quantify the extent of melted CLP helices in the inner vs outer solvophilic layers. Lastly, the reconstructed equilibrated structure can also provide insight into the membrane bending rigidity and associated thermal fluctuations of the vesicle structure …”

“…Lastly, the reconstructed equilibrated structure can also provide insight into the membrane bending rigidity and associated thermal fluctuations of the vesicle structure. 57 The above pieces of information, that are useful in designing vesicles for delivery, are not available to a researcher who is using traditional fitting of the scattering profile with analytical models to interpret structure of vesicles formed in amphiphilic polymer solutions.…”

Pore-Spanning Plasma Membranes Derived from Giant Plasma Membrane Vesicles

“…One approach to obtain molecular information about the organization and function of plasma membranes is to apply artificial lipid bilayers with an enormously reduced number of membrane components. Designed for a particular biological question to be addressed, small, large or giant unilamellar vesicles are frequently used as well as membranes that are attached to a support. − For some applications, the latter ones are superior over vesicles, as they are planar and form quasi two-dimensional structures with a large surface area that is readily accessible by microscopy methods such as fluorescence and atomic force microscopy as well as other surface sensitive techniques. , With the invention of pore-spanning membranes (PSMs), which are lipid bilayers attached to a highly ordered porous substrate, a new planar chip-based membrane system has been developed greatly expanding our repertoire of model membranes. − PSMs have the advantage that they combine supported lipid membrane parts with freestanding ones, which introduces higher mobility in the membrane components and partially overcomes the challenges that come along with a membrane that is in direct contact to the support.…”

3D printed SAXS chamber for controlled in-situ dialysis and optical characterization

“…Small angle X-ray scattering (SAXS) is a powerful technique for studying soft matter and biological materials at the nanometer scale (Hura et al ., 2013; Jacoby et al ., 2015; Kornreich et al ., 2015; Safinya et al ., 2015; Kornreich et al ., 2016; Kikhney & Svergun, 2015). Apart from the high resolution, the key advantage of SAXS is its ability to resolve structures in solution under physiological conditions (Kler et al ., 2012; Mertens & Svergun, 2010; Chappa et al ., 2021; Brennich et al ., 2019). To this end fluid samples are traditionally filled in X-ray capillaries made of quartz glass.…”