Boundary Potential and the Energy of Lipid Monolayer Compression at the Liquid Expanded State

Abstract—

Compression diagrams were obtained and analyzed for monolayers composed from DOPC, POPC, DMPC, DPPC, DPhPC, and DMPS at the surface of solutions containing KCl, CaCl₂, or BeCl₂ and in the presence of polylysine (PLL) or chlorpromazine (CPZ). Elastic properties of monolayers are characterized by the area of lipid molecules, presented as an incompressible area with a soft shell, the size of which exponentially depends on the lateral pressure and the coefficient of elasticity. This assumption describes well the shape of the lipid compression diagrams, including lipids with saturated hydrocarbon chains (DPPC and DMPS) in the region, where they exhibit liquid crystal properties (liquid expanded state, LE). All lipids show changes in the interfacial Volta potential in this region; these changes linearly depend on the effective value of the work applied to compress the monolayer. Choosing for zero value of the Volta potential its magnitude at the point with a lateral pressure of about 1 mN/m, the slope of linear section of this dependence was estimated. The slope of this dependence makes it possible to identify different types of membrane-active compounds affecting the elastic and electrostatic characteristics of the monolayer. It turned out that this slope is practically independent of the pH and ionic composition of the aqueous subphase but decreases upon adsorption of PLL polypeptides on the surface of the DMPS monolayer. The adsorption of small positively charged CPZ molecules on this monolayer leads to the deviation in the potential vs work dependence from the linear. A quantitative description of this deviation is in a good agreement with the assumption that CPZ molecules are incorporated into the monolayer. Their contribution to the change in the energy of the monolayer and the Volta potential is determined by the amount of incorporated CPZ molecules, the effect of which on lateral pressure can be approximated by a function similar to the Boltzmann relation.