Structure modifications of AOT reverse micelles due to protein incorporation

Structural modifications of AOT/water/isooctane reverse micelles due to incorporation of proteins were studied at various water contents and protein concentrations, using small-angle X-ray scattering (SAXS) experiments under static conditions, rheometry analysis, and SR-SAXS experiments under induced shear flow. Two proteins, lysozyme (pI 11.1, Mw 14,300 Da) and BSA (pI 4.3, Mw 66,700 Da), were chosen as models. SAXS analysis of protein-containing reverse micelles at low water content detected minima in the average micelle size versus protein concentration curve, for both proteins, below and above their isoelectric point. This minimum was attributed to changes in the size distribution of the reverse micelles. SAXS measurements of reverse micelles at high water content have shown them to have a cylindrical form. Incorporation of lysozyme at pH 7 into the cylindrical micelles induced a shape transition to spherical micelles, which was associated with a decreased viscosity. SR-SAXS measurements under induced shear flow and dynamic conditions revealed alignment of the cylindrical micelles in the flow direction. The anisotropy parameter, a measure of the degree of the spatial order, was found to increase with increasing shear rate and to decrease with increasing lysozyme concentration.