Time-resolved fluorescence and circular dichroism of porphyrin cytochrome c and Zn-porphyrin cytochrome c incorporated in reversed micelles.

Interactions between fluorescent horse heart cytochrome c derivatives (e. g. porphyrin cytochrome c and Zn-porphyrin cytochrome c) with surfactant interfaces in reversed micellar solutions have been studied, using different spectroscopic techniques. Anionic [sodium bis(2-ethylhexyl)sulfosuccinate, AOT] and cationic (cetyltrime-thylammonium bromide, CTAB) surfactant solutions have been used in order to investigate the effects of charge interactions between proteins and interfaces. Circular dichroism reveals that much of the protein secondary structure is lost in AOT-reversed micelles, especially when the molar water/surfactant ratio, wo, is high (wo = 40), whereas in CTAB-reversed micelles secondary structure seems to be preserved. Time-resolved fluorescence measurements of the porphyrin in the cytochrome c molecule yields information about the changes in structure and the dynamics of the protein upon interaction with surfactant assemblies both in aqueous and in hydrocarbon solutions. With AOT as surfactant a strong interaction between protein and interface can be observed. The effects found in aqueous AOT solution are of the same kind as in hydrocarbon solution. In the CTAB systems the interactions between protein and surfactant are much less pronounced. The measured effects on the fluorescence properties of the proteins are different in aqueous and hydrocarbon solutions. In general, the observations can be explained by an electrostatic attraction between the overall positively charged protein molecules and the anionic AOT interface. Electrostatic attraction can also occur between the cytochrome c derivatives and CTAB because there is a negatively charged zone on the surface of the proteins. From the fluorescence anisotropy decays it can be concluded that in the CTAB-reversed micellar system these interactions are not important, whereas in an aqueous CTAB solution the proteins interact with surfactant molecules.