Measurement of the interactions between membranes in a stack

Abstract

INTERACTIONS between membranes are important in many biological processes and are of increasing interest to biophysicists and physicists1. The forces that determine the stability of a given configuration include van der Waals, electrostatic double-layer2, hydration2–4, undulation5,6, ion–ion correlation7 and surface dipole-dipole8 forces, and their interplay depends critically on the system. Direct measurement of forces between two unsupported membranes in solution is a formidable task, but the interactions operating in a stack of membranes, such as a multilamellar mesophase, are not so inaccessible. The compressibility of the stack normal to the layers is related directly to the free energy density of interlayer interactions, so that measurements of the compressibility allow the type and contribution of the various forces to be inferred. Here we report the measurement of layer compressibility in a lamellar mesophase using a surface force apparatus9. The molecular resolution of this device also yields structural information on the confined mesophase, such as the reticular spacing and the nature and dynamics of packing defects.