Experimental tests for thermally-induced fluctuations in lipid bilayers

For several years researchers have been investigating the interactions between solvated lipid bilayers. Presently there is some disagreement regarding the range, magnitude, and origin of the long-and short-range repulsive and attractive interactions between bilayers. To address this issue experimentally, we have used the osmotic stress/X-ray diffraction method to measure the total repulsive pressure as a function of interbilayer distance for both fluid and solid bilayers. These bilayers were primarily composed of the most common phospholipids found in biological membranes, the zwitterionic lipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). For PC bilayers the pressure-distance data can be explained by the presence of an attractive van der Waals pressure and short- and long-range repulsive pressures. The short-range underlying pressure, which extends about 4 Å into the fluid space from each monolayer, is due to an enthalpically driven, exponentially decaying pressure arising from the work to remove water from the hydrophilic polar head groups, and an excluded volume contribution arising from the non-ideal interactions between head groups from apposing bilayers. This review focusses on experiments designed to determine quantitatively the effects of a long-range repulsive pressure, due to undulations of the entire bilayer arising from thermally induced bending moments. In these experiments pressure - distance relations were measured for bilayers with a range of bending moduli (measured independently) obtained as a function of temperature, the number of double bonds in the lipid acyl chains, and the presence of exogenous compounds such as lysophosphatidylcholine. For PC bilayers the equilibrium fluid spacings can be manipulated in a manner predictable by a theory of continuous unbinding. However, for PE bilayers there is an additional attractive pressure arising from interactions between the PE head groups. This additional interaction, responsible for the large adhesion energy of PE bilayers, gives rise to discontinuous disjoining of PE bilayers.

Duke Scholars

Author Thomas James McIntosh Cell Biology

Author Sidney Arthur Simon Neurobiology