Abstract
SARS-CoV-2 spike proteins are responsible for the membrane fusion event, which allows the virus to enter the host cell and cause infection. This process starts with the binding of the spike ...extramembrane domain to the angiotensin-converting enzyme 2 (ACE2), a membrane receptor highly abundant in the lungs. In this study, the extramembrane domain of SARS-CoV-2 Spike (sSpike) was injected on model membranes formed by supported lipid bilayers in presence and absence of the soluble part of receptor ACE2 (sACE2), and the structural features were studied at sub-nanometer level by neutron reflection. In all cases the presence of the protein produced a remarkable degradation of the lipid bilayer. Indeed, both for membranes from synthetic and natural lipids, a significant reduction of the surface coverage was observed. Quartz crystal microbalance measurements showed that lipid extraction starts immediately after sSpike protein injection. All measurements indicate that the presence of proteins induces the removal of membrane lipids, both in the presence and in the absence of ACE2, suggesting that sSpike molecules strongly associate with lipids, and strip them away from the bilayer, via a non-specific interaction. A cooperative effect of sACE2 and sSpike on lipid extraction was also observed.
Atomic force microscopy (AFM) is used to carry out rheology measurements on the nanoscale and to determine the mechanical properties of poly(l-lysine) (PLL)/hyaluronic acid (HA) multilayer films. ...Storage (G′) and loss modulus (G″) of the films are calculated and compared with the values obtained from quartz crystal microbalance with dissipation monitoring measurements (QCM-D). A predominant elastic behavior independently of the applied frequencies (5–100 Hz) is observed for native HA/PLL films consisting of 36 double layer. If the layers are cross-linked, the value of G′ increases by 2 orders of magnitude, while the loss modulus becomes negligible, making these films a purely elastic chemical gel. The values of G′ and G′′ extracted from QCM-D measurements on native films are much higher, due to the different frequency regime of the applied shear stress. However, the viscoelastic ratio from the two methods is the same and proves the elastic dominated response of the multilayer in both frequency regimes.
Bare interfaces between water and hydrophobic media like air or oil are of fundamental scientific interest and of great relevance for numerous applications. A number of observations involving ...water/hydrophobic interfaces have, however, eluded a consensus mechanistic interpretation so far. Recent theoretical studies ascribe these phenomena to an interfacial accumulation of charged surfactant impurities in water. In the present work, we show that identifying surfactant accumulation with X-ray reflectometry (XRR) or neutron reflectometry (NR) is challenging under conventional contrast configurations because interfacial surfactant layers are then hardly visible. On the other hand, both XRR and NR become more sensitive to surfactant accumulation when a suitable scattering length contrast is generated by using fluorinated oil. With this approach, significant interfacial accumulation of surfactant impurities at the bare oil/water interface is observed in experiments involving standard cleaning procedures. These results suggest that surfactant impurities may be a limiting factor for the investigation of fundamental phenomena involving water/hydrophobic interfaces.
The myelin sheath-a multi-double-bilayer membrane wrapped around axons-is an essential part of the nervous system which enables rapid signal conduction. Damage of this complex membrane system results ...in demyelinating diseases such as multiple sclerosis (MS). The process in which myelin is generated
is called myelination. In our study, we investigated the adhesion process of large unilamellar vesicles with a supported membrane bilayer that was coated with myelin basic protein (MBP) using time-resolved neutron reflectometry. Our aim was to mimic and to study the myelination process of membrane systems having either a lipid-composition resembling that of native myelin or that of the standard animal model for experimental autoimmune encephalomyelitis (EAE) which represents MS-like conditions. We were able to measure the kinetics of the partial formation of a double bilayer in those systems and to characterize the scattering length density profiles of the initial and final states of the membrane. The kinetics could be modeled using a random sequential adsorption simulation. By using a free energy minimization method, we were able to calculate the shape of the adhered vesicles and to determine the adhesion energy per MBP. For the native membrane the resulting adhesion energy per MBP is larger than that of the EAE modified membrane type. Our observations might help in understanding myelination and especially remyelination-a process in which damaged myelin is repaired-which is a promising candidate for treatment of the still mostly incurable demyelinating diseases such as MS.
Abstract
Soft, stratified, amphiphilic systems are recurrent motifs in nature, e.g., in myelin sheaths or thylakoid stacks, and synthetic analogues are increasingly being exploited in the areas of ...biocatalysis, biosensing, and drug delivery. The synthesis of such complex multilayered systems usually requires lengthy preparation protocols. Here, we demonstrate the formation of multilayered fatty acid/polysaccharide thin films prepared via a single step protocol, which exploits the spontaneous self-assembly of the components into vesicular systems in aqueous solution. The solutions are characterized by light and neutron scattering experiments and the thin films by neutron reflectometry, optical ellipsometry, atomic force microscopy, and x-ray diffraction. The thin films exhibit structural features with sub-10 nm dimensions, stemming from the ordered sequence of hydrophilic and hydrophobic layers and respond strongly to changes in ambient humidity. Using this approach, films with a total thickness varying from tens to hundreds of nanometers can be easily prepared.
Lipopolysaccharides (LPSs) are a constitutive element of the cell envelope of Gram-negative bacteria, representing the main lipid in the external leaflet of their outer membrane (OM) lipid bilayer. ...These unique surface-exposed glycolipids play a central role in the interactions of Gram-negative organisms with their surrounding environment and represent a key element for protection against antimicrobials and the development of antibiotic resistance. The biophysical investigation of a wide range of different types of in vitro model membranes containing reconstituted LPS has revealed functional and structural properties of these peculiar membrane lipids, providing molecular-level details of their interaction with antimicrobial compounds. LPS assemblies reconstituted at interfaces represent a versatile tool to study the properties of the Gram-negative OM by exploiting several surface-sensitive techniques, in particular X-ray and neutron scattering, which can probe the structure of thin films with sub-nanometer resolution. This review provides an overview of different approaches employed to investigate structural and biophysical properties of LPS, focusing on studies on Langmuir monolayers of LPS at the air/liquid interface and a range of supported LPS-containing model membranes reconstituted at solid/liquid interfaces.
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•Gram-negative bacteria resist many antimicrobials thanks to a layer of lipopolysaccharides (LPS) in their cell envelope•In vitro model membranes containing LPS provide unique tools for biophysical studies of the bacterial surface•Molecular-level understanding of antibiotic interactions with LPS layers can aid the design of new effective drugs
Chitosan is one of the most studied cationic polysaccharides. Due to its unique characteristics of being water soluble, biocompatible, biodegradable, and non-toxic, this macromolecule is highly ...attractive for a broad range of applications. In addition, its complex behavior and the number of ways it interacts with different components in a system result in an astonishing variety of chitosan-based materials. Herein, we present recent advances in the field of chitosan-based materials from a physico-chemical perspective, with focus on aqueous mixtures with oppositely charged colloids, chitosan-based thin films, and nanocomposite systems. In this review, we focus our attention on the physico-chemical properties of chitosan-based materials, including solubility, mechanical resistance, barrier properties, and thermal behaviour, and provide a link to the chemical peculiarities of chitosan, such as its intrinsic low solubility, high rigidity, large charge separation, and strong tendency to form intra- and inter-molecular hydrogen bonds.
The outer membrane of Gram-negative bacteria is of great scientific interest because it mediates the action of antimicrobial agents. The membrane surface is composed of lipopolysaccharide (LPS) ...molecules with negatively charged oligosaccharide headgroups. To a certain fraction, LPSs additionally display linear polysaccharides termed O-side chains (OSCs). Structural studies on bacterial outer surfaces models, based on LPS monolayers at air-water interfaces, have so far dealt only with rough mutant LPSs lacking these OSCs. Here, we characterize monolayers of wild-type LPS from Escherichia coli O55:B5 featuring strain-specific OSCs in the presence of defined concentrations of monovalent and divalent ions. Pressure-area isotherms yield insight into in-plane molecular interactions and monolayer elastic moduli. Structural investigations by x-ray and neutron reflectometry reveal the saccharide conformation and allow quantifying the area per molecule and the fraction of LPS molecules carrying OSCs. The OSC conformation is satisfactorily described by the self-consistent field theory for end-grafted polymer brushes. The monolayers exhibit a significant structural response to divalent cations, which goes beyond generic electrostatic screening.
While the temperature responsive behavior of nonionic polymers has been extensively studied and is nowadays one of the key mechanisms of smart materials, the pressure response of thin films remains ...basically unexplored. We investigate the conformational transition of nonionic brushes and semidilute solutions induced by hydrostatic pressure and temperature variations. Interestingly, the pressure–temperature phase diagram for the coil-to-globule transition of brushes, probed by neutron reflectometry, nearly coincides with that in semidilute solutions. We also show that the phase behavior can be understood and predicted with simple thermodynamic concepts employed so far for the denaturation of proteins. Fully atomistic molecular dynamics simulations provide molecular insight into the pressure-responsive behavior. Combining all three approaches allows us to demonstrate that pressure-induced hydration of nonionic polymers at low pressure is universal as it is dictated by water and is polymer-independent. In contrast, the pressure-induced dehydration at high pressure is strongly polymer-specific. The outcomes apply to a wide class of nonionic polymers and can aid the design of responsive coatings with the desired pressure-responsive behavior.
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