Alexander Schlaich

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.

Water-Mediated Interactions between Hydrophilic and Hydrophobic Surfaces

All surfaces in water experience at short separations hydration repulsion or hydrophobic attraction, depending on the surface polarity. These interactions dominate the more long-ranged electrostatic and van der Waals interactions and are ubiquitous in biological and colloidal systems. Despite their importance in all scenarios where the surface separation is in the nanometer range, the origin of these hydration interactions is still unclear. Using atomistic solvent-explicit molecular dynamics simulations, we analyze the interaction free energies of charge-neutral model surfaces with different elastic and water-binding properties. The surface polarity is shown to be the most important parameter that not only determines the hydration properties and thereby the water contact angle of a single surface but also the surface-surface interaction and whether two surfaces attract or repel. Elastic properties of the surfaces are less important. On the basis of surface contact angles and surface-surface binding affinities, we construct a universal interaction diagram featuring three different interaction regimes-hydration repulsion, cavitation-induced attraction-and for intermediate surface polarities-dry adhesion. On the basis of scaling arguments and perturbation theory, we establish simple combination rules that predict the interaction behavior for combinations of dissimilar surfaces.

Tight cohesion between glycolipid membranes results from balanced water–headgroup interactions

Membrane systems that naturally occur as densely packed membrane stacks contain high amounts of glycolipids whose saccharide headgroups display multiple small electric dipoles in the form of hydroxyl groups. Experimentally, the hydration repulsion between glycolipid membranes is of much shorter range than that between zwitterionic phospholipids whose headgroups are dominated by a single large dipole. Using solvent-explicit molecular dynamics simulations, here we reproduce the experimentally observed, different pressure-versus-distance curves of phospholipid and glycolipid membrane stacks and show that the water uptake into the latter is solely driven by the hydrogen bond balance involved in non-ideal water/sugar mixing. Water structuring effects and lipid configurational perturbations, responsible for the longer-range repulsion between phospholipid membranes, are inoperative for the glycolipids. Our results explain the tight cohesion between glycolipid membranes at their swelling limit, which we here determine by neutron diffraction, and their unique interaction characteristics, which are essential for the biogenesis of photosynthetic membranes.

Hydration friction in nano-confinement: from bulk via interfacial to dry friction

The viscous properties of nanoscopically confined water are important when hydrated surfaces in close contact are sheared against each other. Numerous experiments have probed the friction between atomically flat hydrated surfaces in the subnanometer separation regime and suggested an increased water viscosity, but the value of the effective viscosity of ultraconfined water, the mechanism of hydration layer friction, and the crossover to the dry friction limit are unclear. We study the shear friction between polar surfaces by extensive nonequilibrium molecular dynamics simulations in the linear-response regime at low shearing velocity, which is the relevant regime for typical biological applications. With decreasing water film thickness we find three consecutive friction regimes: For thick films friction is governed by bulk water viscosity. At separations of about a nanometer the highly viscous interfacial water layers dominate and increase the surface friction, while at the transition to the dry friction limit interfacial slip sets in. Based on our simulation results, we construct a confinement-dependent friction model which accounts for the additive friction contributions from bulklike water, interfacial water layers, and interfacial slip and which is valid for arbitrary water film thickness.

Hydration Repulsion Difference between Ordered and Disordered Membranes Due to Cancellation of Membrane–Membrane and Water-Mediated Interactions

Hydration repulsion acts between all sufficiently polar surfaces in water at small separations and prevents dry adhesion up to kilobar pressures. Yet it remained unclear whether this ubiquitous force depends on surface structure or is a sole water property. We demonstrate that previous deviations among different experimental measurements of hydration pressures in phospholipid bilayer stacks disappear when plotting data consistently as a function of repeat distance or membrane surface distance. The resulting pressure versus distance curves agree quantitatively with our atomistic simulation results and exhibit different decay lengths in the ordered gel and the disordered fluid states. This suggests that hydration forces are not caused by water ordering effects alone. Splitting the simulated total pressure into membrane-membrane and water-mediated parts shows that these contributions are opposite in sign and of similar magnitude, thus they are equally important. The resulting net hydration pressure between membranes is what remains from the near-cancellation of these ambivalent contributions.

Effects of Urea and TMAO on Lipid Self-Assembly under Osmotic Stress Conditions

Most land-living organisms regularly experience dehydration. In nature, one commonly applied strategy to protect against this osmotic stress is to introduce small polar molecules with low vapor pressure, commonly called osmolytes. Two examples of naturally occurring small polar compounds are urea and trimethylamine N-oxide (TMAO), which are known to have counteracting effects on protein stability. In this work, we investigate the effects of urea and TMAO on lipid self-assembly at varying water contents, focusing on dehydrated conditions. By using complementary experimental techniques, including sorption microcalorimetry, NMR, and X-ray scattering, together with molecular dynamics simulations in model systems composed of phosphatidylcholine lipids, water, and solute, we characterize interactions and self-assembly over a large range of hydration conditions. It is shown that urea and TMAO show qualitatively similar effects on lipid self-assembly at high water contents, whereas they have clearly different effects in dehydrated conditions. The latter can be explained by differences in the molecular interactions between the solutes and the lipid headgroups. TMAO is repelled from the bilayer interface, and it is thereby expelled from lipid lamellar systems with low water contents and narrow inter-bilayer regions. In these conditions, TMAO shows no effect on the lipid phase behavior. Urea, on the other hand, shows a slight affinity for the lipid headgroup layer, and it is present in the lipid lamellar system at all water contents. As a result, urea may exchange with water in dry conditions and thereby prevent dehydration-induced phase transitions. In nature, urea and TMAO are sometimes found together in the same organisms and it is possible that their combined effect is to both protect lipid membranes against dehydration and still avoid denaturation of proteins.