| Surface pressure (II) and surface area isotherms as a function of surface area were measured for monolayers of amphotericin B (AmB) and cholesterol mixtures at the air/water interface at 10°C, 20°C, and 30°C. Two different spreading solvents were used to form the monolayers. They were (2:1) v/v chloroform/methanol and (3:1) v/v 2-propanol/water. The mean molecular area, free energy of mixing, and excess entropies and enthalpies of mixing were calculated as a function of the mole fraction of AmB. When chloroform/methanol was used as a spreading solvent, the II-A isotherms of the mixed monolayers exhibited characteristic transitions from the gas to liquid-expanded, then liquid-condensed, and finally the solid state. The mean molecular area of the mixed monolayers was significantly higher than the calculated sum of the molecular areas of the pure components at high II. This expanding effect was accompanied by a large II-A hysteresis and a positive excess of free energy of mixing at high II. At low II, a condensing effect was observed with the most significant deviation from ideality occurring at a mole fraction of AmB (XAmB) of 0.667. Free energy calculations revealed a condensing effect at low II and an expanding effect at high II except at 30°C, where a condensing effect was observed for XAmB around 0.334. In contrast, when 2-propanol/water was used as spreading solvent, the mixed monolayers at 20°C exhibited II-A isotherms with no visible transitions, low hysteresis, a condensing effect, and a negative free energy of mixing. The most stable monolayers were produced from mixtures of AmB and cholesterol with a 2:1 stoichiometry. At this ratio, amphotericin B and cholesterol form ion channels in lipid bilayers with conductance of 4--400 pS.;Monolayers of phage specific for the binding of the model "threat agent" streptavidin were prepared by a "phage-skinning" technique. This technique involves treating filamentous phage with chloroform to form spheroids, then spreading the spheroid suspension at the air/water interface using a glass rod. This monolayer was then compressed and transferred to quartz crystals where binding of streptavidin was measured gravimetrically and was found to be dose dependent and specific. Binding of the analyte was further confirmed visually using electron microscopy. |
