The interaction of cyclodextrins with phosphatidylethanolamine Langmuir monolayers: Influence of the spreading solvent and subphase conditions

This contribution presents the influence of cyclodextrins (β-CD and γ-CD), dissolved in the subphase, on the behavior of phosphatidylethanolamine (PE) Langmuir monolayers. The PE films were spread from chloroform, chloroform/ethanol (1/4: v/v) and chloroform/hexane (1/4: v/v) solutions on pure water/air and cyclodextrin solution/air interfaces. The experiments have been carried out at pH 3, 5.5 and 7. Plots of surface pressure (π) against mean area per molecule (A) were recorded at constant temperature (20 °C). The influence of the subphase pH on the behavior of PE monolayers was explained by the ionization of the polar head-group, whereas the effects of the spreading solvents were attributed to changes in the orientation of the polar head with respect to the interface. Isotherms of PE spread on subphases containing CDs were shifted to lower A values. This corroborates the interaction between the monolayer and CDs, which modifies the arrangement of the PE molecules at the interface. Static stability tests of the PE liquid condensed (LC) monolayers spread on pure water and on CD solutions have also been undertaken (π-time curves). The decay of surface pressure observed after 2 h in the tests carried on CD solutions proves that the film molecules are complexed by the macrocycles and, finally, desorbed into the bulk phase. It occurred that γ-CD includes the PE molecules less specifically, regardless the orientation of the lipid polar head-group at the water/air interface.