Equilibrium penetration of pluronic F-68 in lipid monolayers

Interaction of Pluronic F-68 with monolayers composed of DPPC (1,2-Dipalmytoyl-sn-glycero-3-phosphocholine), POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and their mixture have been studied. A series of isotherms were obtained by compressing mixed PF-68/lipid monolayers over subphase containing varying concentrations of PF-68. Results obtained were analyzed by applying various thermodynamic equations derived from Gibbs adsorption equation. The mole fractions of PF-68 in the monolayer as computed by these equations (except Barnes' equation) agree quantitatively with each other. The computed extent of penetration of PF-68, based on thermodynamic analysis, is also consistent with the following experimental observations: (i) high rate of exclusion of PF-68 from monolayers upon compression of highly expanded monolayers, and (ii) complete expulsion of PF-68 from monolayers at surface pressures well below 30 mN/m. It was concluded from this study that Pluronic F-68 cannot penetrate into the lipid monolayers at surface pressures (∼30 mN/m) believed to correspond to the lateral pressure in plasma membranes. This conclusion is in agreement with the low toxicity of PF-68 towards cells. The present work demonstrates that thermodynamic analysis is a simple, but effective quantitative tool when applied to monolayer penetration experiments.