Cytochrome-c in reverse micelles: Small angle x-ray scattering measurements, percolation process, and critical behavior: An interpretation with an association model

It has been shown that solubilization of cytochrome-e in water in oil reverse micellar systems induces a change in the small angle x-ray scattering (SAXS) spectrum which suggests an increase in the attractive part of the intermicellar potential. In addition, from conductivity measurements a percolation threshold appears for a micellar concentration which is smaller than that observed without protein. Finally, for the liquid-liquid phase transition, a decrease in the critical temperature and concentration is observed. To explain these results, we introduce a pairing sticky hard sphere model. In this approach, the empty micelles are described by the sticky hard sphere model as is usually done. The presence of cytochrome-e is represented by an additional attractive potential characterized by an association parameter that leads to the pair formation of micelles. The association parameter was determined by fitting the experimental structure factors and from this parameter the decrease in the percolation threshold has been estimated. The change in the critical parameters induced by cytochrome-e can be interpreted in terms of formation of micelle-pairs and not by a van der Waals-type theory. These results are also supported by dynamic light scattering experiments. (C) 1997 American Institute of Physics.