High Resolution Membrane Structures within Hybrid Lipid‐Polymer Vesicles Revealed by Combining X‐Ray Scattering and Electron Microscopy

Abstract

Hybrid vesicles consisting of phospholipids and block‐copolymers are increasingly finding applications in science and technology. Herein, small angle X‐ray scattering (SAXS) and cryo‐electron tomography (cryo‐ET) are used to obtain detailed structural information about hybrid vesicles with different ratios of 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (POPC) and poly(1,2‐butadiene‐block‐ethylene oxide) (PBd22‐PEO14, Ms = 1800 g mol−1). Using single particle analysis (SPA) the authors are able to further interpret the information gained from SAXS and cryo‐ET experiments, showing that increasing PBd22‐PEO14 mole fraction increases the membrane thickness from 52 Å for a pure lipid system to 97 Å for pure PBd22‐PEO14 vesicles. Two vesicle populations with different membrane thicknesses in hybrid vesicle samples are found. As these lipids and polymers are reported to homogeneously mix, bistability is inferred between weak and strong interdigitation regimes of PBd22‐PEO14 within the hybrid membranes. It is hypothesized that membranes of intermediate structure are not energetically favorable. Therefore, each vesicle exists in one of these two membrane structures, which are assumed to have comparable free energies. The authors conclude that, by combining biophysical methods, accurate determination of the influence of composition on the structural properties of hybrid membranes is achieved, revealing that two distinct membranes structures can coexist in homogeneously mixed lipid‐polymer hybrid vesicles.

Unraveling the Phase Behavior, Mechanical Stability, and Protein Reconstitution Properties of Polymer-Lipid Hybrid Vesicles

“…Minor asymmetries were reported, and attributed to curvature effects at the membrane; 22 as the simulation profiles are based on planar bilayers, this behavior was not seen, supporting the hypothesis of the authors. As the polymer fraction increases, the peak-to-peak distance becomes longer, indicating a larger membrane thickness.…”

“…Fluorescence experiments were performed to evaluate the leakage of hybrid vesicles in the presence of detergent, aiming to determine if increasing the concentration of polymers enhances the stability of the membrane when surfactants are added. This work helps to explain experimentally observed phenomena, such as the presence of different spatial configurations in homogeneous membranes 22 and improved incorporation of membrane proteins into HVs. 18,19…”

“…Recently, the electron density profiles (EDPs) of hybrid vesicles composed of POPC and PBd 22 -b-PEO 14 were determined using small-angle X-ray scattering (SAXS) and cryo-electron tomography techniques. 22 To validate the simulation results, the EDPs at different temperatures were compared with the experimental data. The density profiles of both leaflets were found to be symmetrical and in agreement with the experiments, as depicted in Figure 1.…”

“…Experimental observations have shown the existence of two distinct populations of HVs, with different membrane thickness. 22 One of these has an EDP similar to that of polymersomes ('thick' membrane), while the other resembles that of pure liposomes ('thin' membrane). This is an interesting observation given that earlier fluorescence studies have shown that POPC and PBd 22 -b-PEO 14 form well mixed and homogeneous vesicles containing both polymers and lipids in their composition.…”

“…20 Therefore, despite the distinct structural characteristics of lipids and copolymers, HVs have demonstrated various examples of equivalent/superior performance when compared to pure liposomes and polymersomes, showcasing their unique advantages. These observations have led to an increased interest in the mixing behavior of lipids and polymers, 2,21,22 and a need to optimize the vesicle environment through a better understanding of the relationship between a hybrid membrane's chemical composition and its material properties.…”