We measured the passive lipid flip-flop of 1,2-dipalmitoyl-
sn-glycero
-3-phosphocholine (DPPC) in solid supported lipid bilayers across their main gel to fluid (L
β
→ L
α
) phase transition. By ...performing time and temperature resolved neutron reflectometry experiments, we demonstrated that asymmetric systems prepared in the gel phase are stable for at least 24 hours. Lipid flip-flop was found to be intrinsically linked to the amount of lipid molecules in the fluid phase. Moreover, the increase of this amount during the broad phase transition was found to be the main key factor for the timing of the flip-flop process. By measuring different temperature scan rate, we could demonstrate that, in the case of supported bilayers and for the temperature investigated, the lipid flip flop is characterised by an activation energy of 50 kJ mol
−1
and a timescale on the order of few hours. Our results demonstrate the origin on the discrepancies between passive flip-flop in bulk systems and at interfaces.
We measured by neutron reflectometry the loss of asymmetry in solid supported lipid bilayers, revealing an intrinsic interplay between passive lipid flip-flop and lipid phase transition.
Emerging therapeutic treatments based on the production of proteins by delivering mRNA have become increasingly important in recent times. While lipid nanoparticles (LNPs) are approved vehicles for ...small interfering RNA delivery, there are still challenges to use this formulation for mRNA delivery. LNPs are typically a mixture of a cationic lipid, distearoylphosphatidylcholine (DSPC), cholesterol, and a PEG-lipid. The structural characterization of mRNA-containing LNPs (mRNA-LNPs) is crucial for a full understanding of the way in which they function, but this information alone is not enough to predict their fate upon entering the bloodstream. The biodistribution and cellular uptake of LNPs are affected by their surface composition as well as by the extracellular proteins present at the site of LNP administration, e.g., apolipoproteinE (ApoE). ApoE, being responsible for fat transport in the body, plays a key role in the LNP’s plasma circulation time. In this work, we use small-angle neutron scattering, together with selective lipid, cholesterol, and solvent deuteration, to elucidate the structure of the LNP and the distribution of the lipid components in the absence and the presence of ApoE. While DSPC and cholesterol are found to be enriched at the surface of the LNPs in buffer, binding of ApoE induces a redistribution of the lipids at the shell and the core, which also impacts the LNP internal structure, causing release of mRNA. The rearrangement of LNP components upon ApoE incubation is discussed in terms of potential relevance to LNP endosomal escape.
Cholesterol, an essential component in biological membranes, is highly unevenly distributed within the cell, with most localized in the plasma membrane while only a small fraction is found in the ...endoplasmic reticulum, where it is synthesized. Cellular membranes differ in lipid composition and protein content, and these differences can exist across their leaflets too. This thermodynamic landscape that cellular membranes impose on cholesterol is expected to modulate its transport. To uncover the role the membrane environment has on cholesterol inter- and intra-membrane movement, we used time-resolved small angle neutron scattering to study the passive movement of cholesterol between and within membranes with varying degrees of saturation content. We found that cholesterol moves systematically slower as the degree of saturation in the membranes increases, from a palmitoyl oleyl phosphotidylcholine membrane, which is unsaturated, to a dipalmitoylphosphatidylcholine (DPPC) membrane, which is fully saturated. Additionally, we found that the energetic barrier to move cholesterol in these phosphatidylcholine membranes is independent of their relative lipid composition and remains constant for both flip-flop and exchange at ∼100 kJ/mol. Further, by replacing DPPC with the saturated lipid palmitoylsphingomyelin, an abundant saturated lipid of the outer leaflet of the plasma membrane, we found the rates decreased by a factor of two. This finding is in stark contrast with recent molecular dynamic simulations that predict a dramatic slow-down of seven orders of magnitude for cholesterol flipping in membranes with a similar phosphocholine and SM lipid composition.
The mechanisms of protein stabilization by uncharged solutes, such as polyols and sugars, have been intensively studied with respect to the chemical thermodynamics of molecular crowding. In ...particular, many experimental and theoretical studies have been conducted to explain the mechanism of the protective action on protein structures by glycerol through the relationship between hydration and glycerol solvation on protein surfaces. We used wide-angle x-ray scattering (WAXS), small-angle neutron scattering, and theoretical scattering function simulation to quantitatively characterize the hydration and/or solvation shell of myoglobin in aqueous solutions of up to 75% v/v glycerol. At glycerol concentrations below ∼40% v/v, the preservation of the hydration shell was dominant, which was reasonably explained by the preferential exclusion of glycerol from the protein surface (preferential hydration). In contrast, at concentrations above 50% v/v, the partial penetration or replacement of glycerol into or with hydration-shell water (neutral solvation by glycerol) was gradually promoted. WAXS results quantitatively demonstrated the neutral solvation, in which the replacement of hydrated water by glycerol was proportional to the volume fraction of glycerol in the solvent multiplied by an exchange rate (β ≤ 1). These phenomena were confirmed by small-angle neutron scattering measurements. The observed WAXS data covered the entire hierarchical structure of myoglobin, ranging from tertiary to secondary structures. We separately analyzed the effect of glycerol on the thermal stability of myoglobin at each hierarchical structural level. The thermal transition midpoint temperature at each hierarchical structural level was raised depending on the glycerol concentration, with enhanced transition cooperativeness between different hierarchical structural levels. The onset temperature of the helix-to-cross β-sheet transition (the initial process of amyloid formation) was evidently elevated. However, oligomerization connected to fibril formation was suppressed, even at a low glycerol concentration.
Complex fluids containing low concentrations of slender colloidal rods can display a high viscosity, while little flow is needed to thin the fluid. This feature makes slender rods essential ...constituents in industrial applications and biology. Though this behavior strongly depends on the rod-length, so far no direct relation could be identified. We employ a library of filamentous viruses to study the effect of rod size and flexibility on the zero-shear viscosity and shear-thinning behavior. Rheology and small-angle neutron scattering data are compared to a revised version of the standard theory for ideally stiff rods, which incorporates a complete shear-induced dilation of the confinement. While the earlier predicted length-independent prefactor of the restricted rotational diffusion coefficient is confirmed by varying the length and concentration of the rods, the revised theory correctly predicts the shear-thinning behavior as well as the underlying orientational order. These results can be directly applied to understand the manifold systems based on rodlike colloids and design new materials.
Abstract
The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li
2
S) via soluble polysulfides (PSs) formation impedes the development of ...high-performance lithium-sulfur (Li-S) batteries with non-aqueous electrolyte solutions. Here, we use operando small and wide angle X-ray scattering and operando small angle neutron scattering (SANS) measurements to track the nucleation, growth and dissolution of solid deposits from atomic to sub-micron scales during real-time Li-S cell operation. In particular, stochastic modelling based on the SANS data allows quantifying the nanoscale phase evolution during battery cycling. We show that next to nano-crystalline Li
2
S the deposit comprises solid short-chain PSs particles. The analysis of the experimental data suggests that initially, Li
2
S
2
precipitates from the solution and then is partially converted via solid-state electroreduction to Li
2
S. We further demonstrate that mass transport, rather than electron transport through a thin passivating film, limits the discharge capacity and rate performance in Li-S cells.
Some styrene/maleic acid (SMA) copolymers solubilise membrane lipids and proteins to form polymer-bounded nanodiscs termed SMA/lipid particles (SMALPs). Although SMALPs preserve a lipid-bilayer core, ...they appear to be more dynamic than other membrane mimics. We used time-resolved Förster resonance energy transfer and small-angle neutron scattering to determine the kinetics and the mechanisms of phospholipid transfer among SMALPs. In contrast with vesicles or protein-bounded nanodiscs, SMALPs exchange lipids not only by monomer diffusion but also by fast collisional transfer. Under typical experimental conditions, lipid exchange occurs within seconds in the case of SMALPs but takes minutes to days in the other bilayer particles. The diffusional and second-order collisional exchange rate constants for SMALPs at 30 °C are k
= 0.287 s
and k
= 222 M
s
, respectively. Together with the fast kinetics, the observed invariability of the rate constants with probe hydrophobicity and the moderate activation enthalpy of ~70 kJ mol
imply that lipids exchange through a "hydrocarbon continuum" enabled by the flexible nature of the SMA belt surrounding the lipid-bilayer core. Owing to their fast lipid-exchange kinetics, SMALPs represent highly dynamic equilibrium rather than kinetically trapped membrane mimics, which has important implications for studying protein/lipid interactions in polymer-bounded nanodiscs.
Recently, reversible cluster formation was identified as an underlying cause of anomalously large solution viscosities observed in some concentrated monoclonal antibody (mAb) formulations, which ...poses a major challenge to the use of subcutaneous injection for some mAbs. A fundamental understanding of the structural and dynamic origins of high viscosities in concentrated mAb solutions is thus of significant relevance to mAb applications in human health care, as well as being of scientific interest. Herein, we present a detailed investigation of an IgG1-based mAb to relate the short-time dynamics and microstructure to significant viscosity changes over a range of pharmaceutically relevant physiochemical conditions. The combination of light scattering, small-angle neutron scattering, and neutron spin echo measurement techniques conclusively demonstrates that, upon addition of Na2SO4, these antibodies form strongly bound reversible dimers at dilute concentrations that interact with each other to form large, loosely bound, transient clusters when concentrated. This hierarchical structure formation in solution causes a significant increase in the solution viscosity.
We review the current literature pertaining to the characterization of soft matter subject to flow utilizing small-angle neutron scattering, flow-SANS, with an emphasis on the simultaneous ...measurement of the rheology, Rheo-SANS. Experimental results are discussed in terms of the flow induced structure and direct connection to the bulk rheology in which we highlight the use of the contrast match method as a unique advantage to neutron scattering techniques. Finally, we discuss specific areas in each field that could benefit from focused flow-SANS experiments, and the projected evolution of specialized flow-SANS sample environments.
Display omitted
► The application of SANS to the characterization of soft matter under deformation. ► The direct measurement of flow induced structural reorganization. ► The simultaneous measure of both structure and rheology. ► The current development of state-of-the-art dynamic SANS sample environments. ► The current approaches to the data analysis of anisotropic scattering patterns.
Adding impurities or defects destroys crystalline order. Occasionally, however, extraordinary behaviour emerges that cannot be explained by perturbing the ordered state. One example is the Kondo ...effect, where magnetic impurities in metals drastically alter the temperature dependence of resistivity. In Type-II superconductors, disorder generally works to pin vortices, giving zero resistivity below a critical current j
. However, peaks have been observed in the temperature and field dependences of j
. This peak effect is difficult to explain in terms of an ordered Abrikosov vortex lattice. Here we test the widespread paradigm that an order-disorder transition of the vortex ensemble drives the peak effect. Using neutron scattering to probe the vortex order in superconducting vanadium, we uncover an order-disorder transition from a quasi-long-range-ordered phase to a vortex glass. The peak effect, however, is found to lie at higher fields and temperatures, in a region where thermal fluctuations of individual vortices become significant.
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