Equilibrium studies of phospholipid bilayer assembly. Coexistence of surface bilayers and unilamellar vesicles

Abstract

To understand how single lipid bilayers might form in cells, we have examined the equilibrium phase diagram of dilute dimyristoylphosphatidylglycerol (DMPG) dispersions in water. Using surface pressure measurements, quasielastic light scattering and phase-contrast microscopy, unilamellar vesicle formation has been observed at a critical temperature, T*, the identical temperature where single bilayers form in films of the lipid at the air–water surface. T* is ca. 8 °C higher than the gel–liquid-crystal transition temperature, T_m, for DMPG dispersions. At T < T*, DMPG exists as a jelly which appears to consist of a matrix of extended bilayer sheets. At T > T*, DMPG dispersions form multilamellar vesicles. The solubility of DMPG in water, X_s, was measured over the temperature interval of 8–40 °C. The function ∂lnX_s/∂T was found to be discontinuous at T_m, but was continuous in the temperature interval that encompasses T*.