MIXED MONOLAYERS OF PHOSPHATIDYLINOSITOL AND DIPALMITOYLPHOSPHATIDYLCHOLINE AND THEIR INTERACTION WITH LIPOSOMES

Surface pressure-area isotherms for phosphatidylinositol (PI) and its mixtures with dipalmitoylphosphatidylcholine (DPPC) at the aqueous-air interface have been obtained on substrates of two ionic strengths (0.013 M and 0.117 M) at pH 6.6 and 25°C. Excess Gibbs energies of mixing in the monolayers have been calculated as a function of composition and surface pressure. At the lower ionic strength at low PI content, the monolayers show negative deviations from ideality whereas at the higher ionic strength deviations are positive and the existence of the liquid to close-packed phase transition of DPPC indicates lateral phase separation. The interaction of PI, DPPC and PI/DPPC sonicated liposomes with monolayers of the same corresponding composition has been studied at constant surface area. It is found that the transfer of lipid from liposome to monolayer at a given surface pressure is greater for the PI and PI/DPPC liposome-monolayer combinations where the lipids are in the liquid-crystalline state than for the gel-state DPPC system. Only a very small fraction of liposomal lipid is available for transfer to the monolayer (< 0.2%), consistent with the transfer of lipid from the outer layer of the liposomal bilayer with retention of the integrity of the liposomes. Addition of calcium ions does not markedly change the penetration of lipid into the monolayer but relatively high calcium concentrations (25 mM) increase the rate at which the surface pressure increases on interaction with liposomes. © 1990.