Size and Polydispersity Determinations of AOT/Bile Salt Reversed Micelles Obtained by Small-Angle Neutron Scattering

Previous work has shown that trihydroxy bile salt cosurfactants significantly modify the interfacial properties and aggregate size of Aerosol-OT (AOT) reversed micelles and can also increase the activity of enzymes such as chymotrypsin and lipase that are solubilized in the AOT reversed micelles. In the present work, small-angle neutron scattering (SANS) was used to study the effects of one such bile salt, sodium taurocholate (NaTC), on the size and polydispersity of AOT reversed micelles. At low concentrations, NaTC decreases the overall size of the reversed micelles, the size of the interior water pools, and the apparent thickness of the detergent layer, while causing an increase in polydispersity. At higher NaTC concentrations, the overall aggregate size, the size of the water pools, the thickness of the detergent layer, and the polydispersity all are increased. It is proposed that, at low concentrations, NaTC monomers are randomly dispersed among the AOT molecules in a perpendicular orientation, thereby disrupting the organization of the detergent layer and increasing penetration of heptane and water into the detergent layer. When NaTC is increased, it may form hydrogen-bonded dimers that are aligned parallel to the AOT molecules. In this orientation, there is less disruption of the detergent layer and penetration of heptane and water molecules is reduced. Finally, it was found that reversed micelles containing chymotrypsin are smaller than unoccupied aggregates, suggesting structural reorganization in order to accommodate the protein. From these results, the effects of NaTC on chymotrypsin activity in AOT reversed micelles can be attributed to structural and chemical modification of the detergent layer rather than to changes in the overall dimensions of the reversed micelles.