Lateral compressibility and penetration into phospholipid monolayers and bilayer membranes

Abstract

THE phospholipid bilayer¹ in certain biological membranes is maintained so that the range of temperature over which the gel-to-liquid crystal transition occurs includes the environmental temperature². As a result, clusters of phospholipid molecules in crystalline and liquid-crystalline states¹ coexist in the membrane and the isothermal lateral compressibility of the membrane lipids is enhanced³. Any increase in compressibility should facilitate insertion of foreign molecules into the bilayer thereby affecting transport across the membrane. Indeed, there is evidence that transport of ions^4,5 and sugars³, and penetration of an enzyme⁶ is increased when the chain-melting transition of the lipids occurs. The lateral compressibility of phospholipid bilayers has not been measured, however, and there is no direct evidence for an increase in compressibility at the point where the gel-to-liquid crystal phase transition occurs. In order to measure lateral compressibility, compression solely in the plane of the bilayer is required. Such directed compression of bilayers will be difficult but it can be readily achieved with monolayers at the air–water interface. Since the molecular packing in such monolayers of phospholipids is equivalent to that in bilayers dispersed in excess water^1,7, this model system is particularly convenient for exploring the role of lateral compressibility. Here we show (1) how the compressibility of lecithin mono-layers varies with packing density and changes at the chain-melting transition, and (2) how penetration of a hydrophobic protein is dependent on the compressibility.