The binding of soluble cytochrome c to an insoluble monolayer of dioleoylphosphatidylcholine was studied. Surface pressure, surface potential and ellipsometric isotherms show that (i) cytochrome c is not adsorbed on a phosphatidylcholine monolayer compressed at 20 mN m-1, (ii) gradual incorporation of cytochrome c takes place as surface pressure decreases, and (iii) on recompression, the adsorbed protein is desorbed from the monolayer at 14 <Π (mN m-1) < 22. In order to determine if the results can be explained by intrinsic interfacial properties of phosphatidylcholine and cytochrome c taken separately, we measured surface pressure, surface potential and ellipsometric-area isotherms of pure cytochrome c which had been (i) deposited from crystals, (ii) deposited from ethanol:H2O Solution (2:1, v/v), and (iii) adsorbed at the air-water interface after injection into the subphase. Cytochrome c behaves like an insoluble monolayer at surface pressures lower than 8-10 mN m-1 and is gradually solubilized at higher surface pressures. Then, surface pressure regulates the organization of the protein at the air-water interface and the spreading solvent does not affect its conformation. Surface potential isotherms of mixed phosphatidylcholine (18:1) -cytochrome c films compressed at different surface pressures can be explained by the simple additivity rule. We show that continuous adsorption of cytochrome c occurs when the PC (18:1) film is compressed at surface pressures lower than 8-10 mN m-1. Beyond 14 mN m-1, both surface pressure-area and ellipsometric-area isotherms demonstrate complete desorption of soluble cytochrome c from the phosphatidylcholine monolayer. The applicability of such a model system to biomembranes is discussed.
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