Origin of Short-Range Repulsion between Hydrated Phospholipid Bilayers:  A Computer Simulation Study

Abstract

The grand canonical Monte Carlo technique is used to simulate the pressure-distance dependence for supported dilauroylphosphatidylethanolamine (DLPE) membranes. The intra- and intermolecular interactions in the system are described with a combination of an AMBER-based force field for DLPE and a TIP4P model for water. To improve the balance between the pair interactions of like and unlike molecules, the water-lipid interaction potentials are scaled to reproduce the hydration level and intermembrane separation at full hydration. It is found that the short-range water-mediated repulsion originates from the hydration component of the intermembrane pressure, whereas the direct interaction between the membranes remains attractive throughout the pressure range studied (0-5 kbar).

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

“…2A one sees that Π dir is strongly attractive, whereas Π ind is repulsive and overcompensates the direct attraction throughout the studied hydration range. Such a near-cancellation is known from simple continuum models of van der Waals interactions between hydrocarbon assemblies in water and is also typical for charge interactions in aqueous solution due to dielectric effects (24); it has been seen in previous simulation studies at low hydration (17) and immediately rules out the direct interaction between bare lipid headgroups (be it steric or electrostatic) as an explanation for the hydration repulsion.…”

“…Although several theoretical (9)(10)(11) and simulation (12)(13)(14)(15)(16)(17)(18) studies elucidated partial aspects of the HR, none treated the full complexity of the problem and could quantitatively reproduce and explain experimental pressure-distance curves, meaning that the HR mechanism remained essentially unclear. The reason for this is obvious: Theory typically only treats one part of the problem, be it the water-water interactions, the water-surface binding, or the configurational entropy of bilayer molecules, whereas current simulation strategies account for the constant water chemical potential either in the form of a large reservoir (13)(14)(15) or by grand-canonical simulations (16,17). Due to limitations in the numerical accuracy, however, both approaches do not enable quantitative comparison of the HR pressure with experimental data.…”

“…The main question we address with our simulations in essence is: What is it that keeps the bilayers separated even at high pressures of 10 8 Pa or 1,000 atm? To make progress in this direction, the pressure Π ¼ Π dir þ Π ind is first decomposed into the direct membrane-membrane contribution and all other water-mediated forces, in the following denoted as the indirect contribution (13,17). Note that this decomposition is indepen-dent of the position of the surface through which the pressure is calculated as long as it lies entirely inside the water phase.…”

From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces

“…These so-called hydration forces arise from the complex interplay of surface group configurational degrees of freedom, desorption of hydration water from polar surface groups, and ordering of the intersurface water film (4). The understanding of hydration forces has recently been advanced by computer simulations that include explicit water molecules (12)(13)(14).In the absence of direct surface-surface interactions, the transition between cavitation-induced attraction and hydration repulsion should coincide with the contact angle characterizing the border between hydrophilic and hydrophobic surface properties; i.e., θ = 90°. In contrast, experiments probing the interactions between similar neutral surfaces with well-defined contact angles demonstrated that even hydrophilic surfaces exhibit short-range attractions not accountable by vdW forces down to typical adhesive contact angles of θ adh = 65°-80°(15-18).…”

“…These so-called hydration forces arise from the complex interplay of surface group configurational degrees of freedom, desorption of hydration water from polar surface groups, and ordering of the intersurface water film (4). The understanding of hydration forces has recently been advanced by computer simulations that include explicit water molecules (12)(13)(14).…”