We have synthesized unique siloxane phosphocholines and characterized their aggregates in aqueous solution. The siloxane phosphocholines form nearly monodisperse vesicles in aqueous solution without ...the need for secondary extrusion processes. The area/lipid, lipid volume, and bilayer thickness were determined from small-angle X-ray scattering experiments. The impetus for the spontaneous formation of unilamellar vesicles by these compounds is discussed.
Using dilatometry and small-angle X-ray diffraction, we have studied under bulk conditions the structural changes and elastic response of dioleoyl phosphatidylcholine bilayers to alamethicin. With ...increasing peptide concentration, we found a progressive thinning of the membrane. However, in contrast to previously published reports, this thinning exhibits exponential behavior. Furthermore, an increase in alamethicin content resulted in an increased lateral area per lipid and a swelling of the multibilayers which can be attributed to a decrease in the bilayer's bending rigidity by approximately 50%. At the same time, hydration and van der Waals forces remained unaffected by the presence of the peptide. Interestingly, all elastic and structural parameters followed the same exponential form found for the membrane thickness, implying a common underlying mechanism for all of these structural parameters. Our results can be understood by introducing an additional entropy term into the free-energy description of peptide incorporation, a term previously not considered. As a result, we have been able to reconcile recent controversies regarding the effect of peptides on membrane thinning.
We mimicked the effect of sphingomyelinase activity on lipid mixtures of palmitoyl-oleoyl-phosphatidylcholine, sphingomyelin, ceramide, and 10 mol % cholesterol. Using x-ray diffraction experiments ...in combination with osmotic stress we found, in agreement with previous studies, that ceramide induces a coexistence of Lα and Lβ domains. A detailed structural analysis of the coexisting domains demonstrated an increase of lipid packing density and membrane thickness in the Lα domains upon increasing overall ceramide levels. This provides evidence for a ceramide-driven accumulation of cholesterol in the Lα domains, in support of previous reports. We further determined the bending rigidities of the coexisting domains and found that the accumulation of cholesterol in the Lα domains stabilizes their bending rigidity, which experiences a dramatic drop in the absence of cholesterol. Deriving experimental estimates for the spontaneous curvature and Gaussian modulus of curvature, we show, using a simple geometric model for ion channels, that in this way changes in the conformational equilibrium of membrane proteins can be kept small.
A previously reported multi‐scale model for (ultra‐)small‐angle X‐ray (USAXS/SAXS) and (very) small‐angle neutron scattering (VSANS/SANS) of live Escherichia coli was revised on the basis of ...compositional/metabolomic and ultrastructural constraints. The cellular body is modeled, as previously described, by an ellipsoid with multiple shells. However, scattering originating from flagella was replaced by a term accounting for the oligosaccharide cores of the lipopolysaccharide leaflet of the outer membrane including its cross‐term with the cellular body. This was mainly motivated by (U)SAXS experiments showing indistinguishable scattering for bacteria in the presence and absence of flagella or fimbrae. The revised model succeeded in fitting USAXS/SAXS and differently contrasted VSANS/SANS data of E. coli ATCC 25922 over four orders of magnitude in length scale. Specifically, this approach provides detailed insight into structural features of the cellular envelope, including the distance of the inner and outer membranes, as well as the scattering length densities of all bacterial compartments. The model was also successfully applied to E. coli K12, used for the authors' original modeling, as well as for two other E. coli strains. Significant differences were detected between the different strains in terms of bacterial size, intermembrane distance and its positional fluctuations. These findings corroborate the general applicability of the approach outlined here to quantitatively study the effect of bactericidal compounds on ultrastructural features of Gram‐negative bacteria without the need to resort to any invasive staining or labeling agents.
Structural and compositional information about Escherichia coli cells is summarized and translated into an analytical multi‐length‐scale scattering form factor model of live bacterial suspensions.
Biomimetic lipid vesicles are indispensable tools for gaining insight into the biophysics of cell physiology on the molecular level. The level of complexity of these model systems has steadily ...increased, and now spans from domain-forming lipid mixtures to asymmetric lipid bilayers. Here, we review recent progress in the development and application of elastic neutron and X-ray scattering techniques for studying these systems in situ and under physiologically relevant conditions on the nanometer to sub-nanometer length scales. In particular, we focus on: (1) structural details of coexisting liquid-ordered and liquid-disordered domains, including their thickness and lipid packing mismatch as a function of a size transition from nanoscopic to macroscopic domains; (2) membrane-mediated protein partitioning into lipid domains; (3) the role of the aqueous medium in tuning interactions between membranes and domains; and (4) leaflet-specific structure in asymmetric bilayers and passive lipid flip-flop.
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