| Abstract
| - Changes in structure of protein molecules and water-in-oil (w/o) microemulsion aggregates were investigatedusing the large protein immunoglobulin G (IgG, MW 155,000) and an equivolume oil/water mixture composedof brine, sulfosuccinic acid bis[2-ethylhexyl]ester (sodium salt) (AOT), and isooctane. The protein solutionin the microemulsion phase was metastable: over time this solution changed, as protein and w/o dropletsaggregated and precipitated to the interface between aqueous and organic phases. Such factors as AOTconcentration, temperature, and salt concentration were found to influence the protein and surfactant structuresin the microemulsion. Protein conformation was probed using circular dichroism spectroscopy whereas themicroemulsion structure was determined from dynamic light scattering measurements. Protein conformationand microemulsion structure were found to have significant effects on protein stability in the microemulsion.The stabilizing effects of clusters formed at higher salt and/or AOT concentrations are discussed. IgG adoptsan intermediate denatured state in the microemulsion phase close to the alternatively folded state known asthe A state, with well-defined contacts in the tertiary structure immediately after phase equilibration. Thechange in protein conformation with time accompanied by the cluster growth eventually leads to the proteinand surfactant transfer into a third, solid middle phase from the organic solution.
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