Structural Changes of AOT Reverse Micelles by the Presence of Proteins: Percolation Process Induced by Cytochrome C

Abstract

The formation of reverse micelles in non polar solvents have been investigated for many years [1,5]. The presence of water is necessary to form a large surfactant aggregate: close packing of the surfactant polar heads leaves an empty volume in the center of the micellar core which can only be filled with hydration (or bound) water. The reverse micelles considered here are then also microemulsions. We will not make differences between the terms swollen micelles and microemulsions, although certain authors use the first term for systems containing only bound water. For AOT, the maximum amount of bound water in the micelles correspond to a water-surfactant molar ratio w = [H₂O]/[AOT] of about 10. Above this amount, part of the water is “free”. Because AOT is soluble in many non polar solvents over a wide range of concentrations, a large number of studies of the structure of reverse micelles have been made with this surfactant.

Several models have been proposed to describe the solubilization of protein in reverse micelles. The models proposed take into account the size of the protein. In the case in which the protein size is smaller than the water pool two differents models are proposed: some groups claim that the presence of the protein gives rise to larger surfactant aggreggates than those formed in the absence of protein. Other groups suggest that the protein has no effect whatsoever on the size of the aggregate. In the case in which the protein size is larger than the water pool, very few models have been proposed.

In the present paper we will try to show that at relatively low water content, the solubilisation of protein [5] such as cytochrome c induces a very small perturbation whereas by increasing the water content a percolation process occurs with the appearance of a two phase transition.