Interactions in Lyotropic Lamellar Phases: A High Resolution X-Ray Study

Abstract

The understanding of interactions between membranes has attracted a lot of attention in the last decade, the reason being the relevance of these interactions in biological processes such as cell-cell interactions. The basic forces between membranes are the van der Waals attraction and the electrostatic repulsion [1]. Besides these two well known forces PARSEGIAN, RAND and coworkers [2] have shown in the last decade that there is another short range interaction between phospholipid bilayers embedded in water. This repulsive interaction is attributed to the hydration of the head groups and decays exponentially with a typical length of 3 Å. More recently, HELFRICH [3] has proposed that thermally induced out of plane fluctuations of membranes in a multilayer system can lead to a long range repulsive interaction. The crucial elastic modulus which controls the strength of this interaction is k_c associated with the bending energy of a single membrane. For this steric interaction to compete with the microscopic forces (that is, van der Waals and electrostatics), k_c has to be of order k_BT simply because the interaction is induced by thermal fluctuations. Steric repulsion is also known to be the dominant interaction associated with wandering walls of incommensurate phases [4]. Despite much theoretical [5,6] and experimental [7,8] effort the experimental relevance of this interaction to biological membranes remains controversial.