Behavior of insulin–sphingomyelin mixed Langmuir monolayers spread at the air–water interface

The behavior of the binary mixed Langmuir monolayers of bovine insulin (INS) and sphingomyelin (SP) spread at the air–water interface was investigated under various subphase conditions. Miscibility and interactions between the components were studied on the basis of the analysis of the surface pressure–average area per molecule (π–A) isotherms and surface compression modulus versus   surface pressure (CS−1−π) curves. Plots of mean molecular area (A) against mole fraction of INS (XINS) show deviations from the ideal behavior, which indicates that both components are miscible and interact at the interface. Our results show that intermolecular interactions between INS and SP depend both on the monolayer state and on the structural characteristics of INS at the interface, which are strongly influenced by subphase pH as well as by the presence of Zn2+ ions in the subphase. Brewster angle microscopy (BAM) was applied to prove the miscibility of both substances in monolayers and to evidence that INS does not aggregate at the air–water interface in the presence of the studied lipid at any experimental condition investigated.