Self-consistent field theoryof compressible phospholipidmembranes at ambient pressure

Abstract

We present a microscopic theory of compressible, fully hydratedphospholipid membranes in equilibrium with excess solvent, apply it toan idealized physical model of these systems, and calculate a broadrange of their thermodynamic and structural properties. An essentialfeature of the theory is the anisotropy of the effective fields actingon each of the hydrocarbon segments, which arises as a naturalconsequence of the hard-core repulsions between segments and theconnectivity of the lipid molecules. These fields, along with theinhomogeneous particle and bond density distributions throughout thebilayer interior, are determined via numerical, self-consistent fieldcalculations. In addition to the gel and liquid crystal phases, themodel naturally includes the fully intercalated LβI phase. Theincorporation of compressibility effects enables us to calculatevarious effects of pressure and the density change at the transitionand in the liquid crystal phase. In this paper, we focus on theproperties of fully hydrated (DPPE) in the liquid crystal phase,as well as properties of the main transition, at atmosphericpressure. To a lesser extent, we also discuss predicted dependencesof several key properties on chain length and head group size. Despiteits simplifications, the model provides a quantitative description ofmany of the measured properties of DPPE, and the correct qualitativetrends for the other lipids. In an accompanying paper [the following article], we examine the effects ofpressure, and obtain quantitative agreement with recent observations onoverall density and bilayer thickness.PACS Nos.: 87.22.Bt, 87.10.+e, 64.60.Cn