Emanuel Schneck

Hydration repulsion between biomembranes results from an interplay of dehydration and depolarization

Hydration repulsion dominates the interaction between polar surfaces in water at nanometer separations and ultimately prevents the sticking together of biological matter. Although confirmed by a multitude of experimental methods for various systems, its mechanism remained unclear. A simulation technique is introduced that yields accurate pressures between solvated surfaces at prescribed water chemical potential and is applied to a stack of phospholipid bilayers. Experimental pressure data are quantitatively reproduced and the simulations unveil a rich microscopic picture: Direct membrane–membrane interactions are attractive but overwhelmed by repulsive indirect water contributions. Below about 17 water molecules per lipid, this indirect repulsion is of an energetic nature and due to desorption of hydration water; for larger hydration it is entropic and suggested to involve water depolarization. This antagonistic nature and the presence of various compensating contributions indicate that the hydration repulsion is less universal than previously assumed and rather involves finely tuned surface-water interactions.

Insight into the Molecular Mechanisms of Protein Stabilizing Osmolytes from Global Force-Field Variations

A prominent class of osmolytes that are able to stabilize proteins in their native fold consist of small highly water-soluble molecules with a large dipole moment and hydrophobic groups attached to the positively charged end of the molecule, for which we coin the term dipolar/hydrophobic osmolytes. For TMAO, which is a prime member of this class, we perform large-scale water-explicit MD simulations and determine the bulk solution activity coefficient as well as the affinity to a stretched polyglycine chain for varying TMAO dipolar strength and hydrophobicity. Double optimization with respect to experimental values for the activity coefficient and the polyglycine transfer free energy is achieved. The resulting optimal TMAO force field shows excellent transferability to different concentrations and also reproduces transfer free energies of various amino acids, including the tryptophan anomaly, for which TMAO acts as a denaturant. By globally analyzing the thermodynamic and structural properties of suboptimal TMAO force fields, we identify the frustration between dipolar and hydrophobic interactions as the working mechanism and the design principle of dipolar/hydrophobic osmolytes.

Quantitative determination of ion distributions in bacterial lipopolysaccharide membranes by grazing-incidence X-ray fluorescence

A model of the outer membrane of Gram-negative bacteria was created by the deposition of a monolayer of purified rough mutant lipopolysaccharides at an air/water interface. The density profiles of monovalent (K+) and divalent (Ca2+) cations normal to the lipopolysaccharides (LPS) monolayers were investigated using grazing-incidence X-ray fluorescence. In the absence of Ca2+, a K+ concentration peak was found in the negatively charged LPS headgroup region. With the addition of CaCl2, Ca2+ ions almost completely displaced K+ ions from the headgroup region. By integrating the experimentally reconstructed excess ion density profiles, we obtained an accurate measurement of the effective charge density of LPS monolayers. The experimental findings were compared to the results of Monte Carlo simulations based on a coarse-grained minimal model of LPS molecules and showed excellent agreement.

Calcium ions induce collapse of charged O-side chains of lipopolysaccharides from Pseudomonas aeruginosa

Lipopolysaccharide (LPS) monolayers deposited on planar, hydrophobic substrates were used as a defined model of outer membranes of Pseudomonas aeruginosa strain dps 89. To investigate the influence of ions on the (out-of-plane) monolayer structure, we measured specular X-ray reflectivity at high energy (22 keV) to ensure transmission through water. Electron density profiles were reconstructed from the reflectivity curves, and they indicate that the presence of Ca2+ ions induces a significant change in the conformation of the charged polysaccharide head groups (O-side chains). Monte Carlo simulations based on a minimal computer model of LPS molecules allow for the modelling of 100 or more molecules over 10−3 s and theoretically explained the tendency found by experiments.

First order melting transitions of highly ordered dipalmitoyl phosphatidylcholine gel phase membranes in molecular dynamics simulations with atomistic detail

Molecular dynamics simulations with atomistic detail of the gel phase and melting transitions of dipalmitoyl phosphatidylcholine bilayers in water reveal the dependency of many thermodynamic and structural parameters on the initial system ordering. We quantitatively compare different methods to create a gel phase system and we observe that a very high ordering of the gel phase starting system is necessary to observe behavior which reproduces experimental data. We performed heating scans with speeds down to 0.5 K/ns and could observe sharp first order phase transitions. Also, we investigated the transition enthalpy as the natural intrinsic parameter of first order phase transitions, and obtained a quantitative match with experimental values. Furthermore, we performed systematic investigations of the statistical distribution and heating rate dependency of the microscopic phase transition temperature.

Neutron Reflectometry Elucidates Density Profiles of Deuterated Proteins Adsorbed onto Surfaces Displaying Poly(ethylene glycol) Brushes: Evidence for Primary Adsorption

The concentration profile of deuterated myoglobin (Mb) adsorbed onto polystyrene substrates displaying poly(ethylene glycol) (PEG) brushes is characterized by neutron reflectometry (NR). The method allows to directly distinguish among primary adsorption at the grafting surface, ternary adsorption within the brush, and secondary adsorption at the brush outer edge. It complements depth-insensitive standard techniques, such as ellipsometry, radioactive labeling, and quartz crystal microbalance. The study explores the effect of the PEG polymerization degree, N, and the grafting density, σ, on Mb adsorption. In the studied systems there is no indication of secondary or ternary adsorption, but there is evidence of primary adsorption involving a dense inner layer at the polystyrene surface. For sparsely grafted brushes the primary adsorption involves an additional dilute outer protein layer on top of the inner layer. The amount of protein adsorbed in the inner layer is independent of N but varies with σ, while for the outer layer it is correlated to the amount of grafted PEG and is thus sensitive to both N and σ. The use of deuterated proteins enhances the sensitivity of NR and enables monitoring exchange between deuterated and hydrogenated species.

Hydration repulsion between membranes and polar surfaces: Simulation approaches versus continuum theories

A review of various computer simulation approaches for the study of the hydration repulsion between lipid membranes and polar surfaces is presented. We discuss different methods and compare their advantages and limitations. We consider interaction pressures, interaction thermodynamics, and interaction mechanisms. We take a close look at the influence of the experimental boundary conditions and at repulsion mechanisms due to the unfavorable overlap of interfacial water layers. To this end, we analyze several distinct water order parameters in simulations of interacting polar surfaces and compare the results to the predictions of simple continuum theories.

Hydration interaction between phospholipid membranes: Insight into different measurement ensembles from atomistic molecular dynamics simulations

Using the novel thermodynamic extrapolation technique in molecular dynamics simulations, we investigate the interaction between phospholipid bilayers subject to various boundary conditions that correspond to established experimental methods for the determination of pressure-distance curves: the osmotic stress method, the hydrostatic method, and the surface force apparatus method. We discuss the roles of van der Waals and Helfrich undulation pressures in the force balance and find that they do not play a major role in the distance range below 28 water molecules per lipid as considered by us. We address the influence of experimental boundary conditions on bilayer structural changes as well as the consequences on interaction pressures. Significant discrepancies are observed between pressures obtained in osmotic stress and hydration methods on one hand and the surface force apparatus method on the other hand. We quantify the contribution of lipid volume compressibility to the total work of dehydration and find it to be substantial for high pressures. In a wide hydration range, the interaction pressure is mostly determined by the area per lipid molecule. This means that the influence of fatty acid chemistry on experimental pressure-distance curves is indirect and mediated by the area per lipid.

Native supported membranes on planar polymer supports and micro-particle supports

To bridge soft biological materials and hard inorganic materials is an interdisciplinary scientific challenge. Despite of experimental difficulties, the deposition of native biological membranes on supports is a straightforward strategy. This review provides an overview of advances in the fabrication and characterization of native biological membranes on planar polymer supports and micro-particles.

Membrane Adhesion via Homophilic Saccharide-Saccharide Interactions Investigated by Neutron Scattering

Solid-supported membrane multilayers doped with membrane-anchored oligosaccharides bearing the LewisX motif (Le(X) lipid) were utilized as a model system of membrane adhesion mediated via homophilic carbohydrate-carbohydrate interactions. Specular and off-specular neutron scattering in bulk aqueous electrolytes allowed us to study multilayer structure and membrane mechanics at full hydration at various Ca(2+) concentrations, indicating that membrane-anchored Le(X) cross-links the adjacent membranes. To estimate forces and energies required for cross-linking, we theoretically modeled the interactions between phospholipid membranes and compared this model with our experimental results on membranes doped with Le(X) lipids. We demonstrated that the bending rigidity, extracted from the off-specular scattering signals, is not significantly influenced by the molar fraction of Le(X) lipids, while the vertical compression modulus (and thus the intermembrane confinement) increases with the molar fraction of Le(X) lipids.