Lipid monolayer states and their relationships to bilayers.

Abstract

Uncommon methods of formation and analysis of lipid monolayers have enabled the recognition of several monolayer states and the identification of that in which molecular organization corresponds closely to that of the bilayer. Monolay-ers were formed by continuously adding a solution ofphospholipid [dimyristoyl phosphatidylcholine in hexane/ethanol, 9:1 (vol/ vol)] to the air/water interface of a constant-area trough. This procedure generates unconventional surface pressure (X7)-surface concentration (1) isotherms, which for liquid-crystalline monolayers consist of straight lines with three prominent intersections, two of which are not apparent in conventional n-A isotherms. The regions of linear change of v are explicable in terms of the area dependence of alkyl chain entropy. The two breaks at lower a delimit states in which both chains lie parallel to the surface. The third occurs at collapse, which corresponds to a true equilibrium for unstressed liposomes. Mechanical and thermodynamic properties of bilayers, particularly phase-transition parameters, correspond closely to those of monolayers with which they are in equilibrium.Monolayers of amphiphilic substances at air/water interfaces are of interest in a variety of disciplines. In physics and chemistry, the interest is in understanding the origin and magnitude of the molecular interactions of a single layer of complex molecules positioned between two fluids (1, 2). In biology, monolayers have been important since Gorter and Grendel first proposed the bilayer as the foundation ofbiological membranes (3). Their conclusion was based on an entirely arbitrary compression of a monolayer of lipid extracted from red cells. The critical question of the appropriate compression and, hence, of the relationship between molecular packing in monolayers and that in bilayers remains unanswered.Using an uncommon constant-area procedure to generate surface pressure (7r)-surface concentration (F) isotherms of dimyristoyl phosphatidylcholine (Myr2-PtdCho) monolayers, we have found that the monolayer states of this lipid are characterized by linear regions in the isotherms. There are four such states, and these can be analyzed in terms of their molecular organization. The most condensed state exists in equilibrium with bilayers, and its properties correspond closely to half of a bilayer. This identification is critical for understanding many bilayer properties and is important for membrane reconstitution (4). MATERIALS AND METHODSMyr2-PtdCho, purchased from Avanti Polar Lipids and from Sigma, was dissolved in 9:1 (vol/vol) hexane or pentane/ ethanol to a concentration of 0.5 mg/ml. Lipid solution concentrations given are nominal; accurate values were determined by phosphate assay according to a modification of the Bartlett procedure (5). Absolute ethanol was treated with activated charcoal and distilled. Hexane and pentane were passed through a column of activated alumina. Distilled and deionized water was charcoal-filtered, redeionized, and redistilled. Reagent grade ...

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