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Monolayers at the air/water interface are used as 2D models of biological membrane to study the interaction of a new Gemini catanionic surfactant, Gem16,12, with the phospholipid DPPC. Gem16,12 is a synthetic analog of galactosylceramide (a cell receptor for HIV-1) and shows both good anti-HIV activity and low toxicity. Adsorption kinetics measurements are performed at Gemini concentrations below the critical aggregation concentration (CAC) and show a decrease of surface tension with increasing concentration. If the surface is covered with a DPPC monolayer at molecular areas with still zero surface pressure the adsorption of Gem16,12 is faster and leads to a higher surface pressure compared with the pure air/water interface. Film balance, fluorescence microscopy and grazing incidence X-ray diffraction (GIXD) experiments were performed after reaching the equilibrium surface tension. The experiments show an incorporation of Gem16,12 molecules into the fluid phase as well as into the condensed domains of a DPPC monolayer. GIXD experiments confirmed these results showing a smaller tilt angle in a DPPC monolayer on a Gemini subphase compared to DPPC on pure water. Compressing such a mixed monolayer leads first to the squeezing out of Gem16,12 molecules from the fluid part of the monolayer and above 30 mN/m to the squeezing out from the condensed part. |
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