Atul N. Parikh, Biophysical Journal 108, 2751-2753, 2015

Commentary, New and Notable

A fluid lipid bilayer in water fluctuates freely. Even in the absence of specific chemical interactions, a complex interplay of a variety of nonspecific forces—attractive and repulsive, short- and long-ranged—determine the equilibrium separation between these well-hydrated bilayers (1). It also plays critical roles in many biological processes, such as cell adhesion and membrane fusion, in which these surfaces are pushed closer together. Contributing to the interplay are coupled influences of the classical DLVO (named after Derjaguin, Landau, Verwey, and Overbeek, who described forces between small, smooth, and charged surfaces in water) and non-DLVO forces. These forces include a) the omnipresent van der Waals force, which provides a weak attraction in a relatively long-ranged manner, and b) the electrostatic double-layer forces between charged membranes. The non-DLVO forces, contributing to interbilayer interactions, include c) the so-called hydration force (a short-range, exponentially decaying repulsive interaction), which is thought to originate from surface-induced perturbation of water dipoles and its propagation away from the interface through water-water interactions (2), and d) the long-range repulsive Helfrich force, which arises from the suppression of the free fluctuations of single bilayers, resulting in entropic loss when two membranes come closer together (3). While the first three direct molecular forces (a-c) can be treated independently and additively, the contributions from the fluctuation-induced Helfrich forces (d) are not readily separable, and couple to the other forces in subtle and complex ways.