Interaction between polyene antifungal drug and saturated phospholipid monolayer regulated by calcium ions at the air-water interface

Amphotericin B (AmB) is a polyene antifungal drug, which could directly form pores on the sterol-free phospholipid monolayers. The interaction between AmB and phospholipid can be affected by calcium ions, but the mechanism is still unclear. DPPC is a saturated phospholipid with-PC group, which is often used to simulate the outer cell membrane leaflet. And DPPC is also the main constituent of pulmonary surfactants. In this work, the DPPC monolayer was used as a model membrane to study the effect of calcium ions on the interaction between AmB and phospholipid. The influence of different concentration of calcium ions on the elastic modulus, mean molecular area increment, excess Gibbs free energy and stability of the AmB/DPPC mixed monolayer has been researched at the surface pressure of 7.5 mN/m, 12.5 mN/m and 22.5 mN/m. The AmB/DPPC monolayers at the air-water interface have been observed in real-time by Brewster angle microscope and the microstructure of the Langmuir-Blodgett monolayer films transferred onto the mica have been researched by scanning electron microscope and atomic force microscope. The results showed that calcium ions had a significant influence on the elastic modulus, mean molecular area increment, excess Gibbs free energy, stability and microstructure of the AmB/DPPC monolayer. It has been indicated that the influence of calcium ions on the interaction between AmB and DPPC molecules mainly depended on the effect of calcium ions on the orientation of AmB molecules. The calcium ions could regulate the effect of AmB to the stability of the DPPC monolayer. This regulatory role changed with the different concentrations of calcium ions and the different phase states of the monolayer. This work provides useful information to further understand the influence mechanism of calcium ions on the interaction between AmB and saturated phospholipid with-PC group, which is helpful to find out the effect mechanism of calcium ion on the interaction between AmB and the outer layer of cell membrane or pulmonary surfactants in different phase states and to understand the toxicity mechanism of AmB on the cell membrane or lungs.