PROTEIN PARTITIONING IN 2-PHASE AQUEOUS NONIONIC MICELLAR SOLUTIONS

A theoretical formulation to describe and predict the partitioning of hydrophilic proteins in phase-separated aqueous nonionic micellar solutions which can exhibit significant one-dimensional micellar growth is presented. The theoretically predicted protein partitioning is compared with experimental measurements of the partitioning of the hydrophilic protein ovalbumin in two-phase aqueous micellar systems of the nonionic surfactant n-decyl tetra(ethylene oxide), C10E4, and is found to be in very good agreement. We propose that excluded-volume interactions between the hydrophilic proteins and the elongated, cylindrical nonionic micelles play the dominant role in determining the experimentally observed partitioning trends. The excluded-volume formulation incorporates (i) the self-assembling character of micelles, which enables them to grow into long, cylindrical microstructures with increasing temperature and/or surfactant concentration, and (ii) a broad polydisperse distribution of micellar sizes. We also present a detailed comparison of the similarities and differences in the partitioning of proteins in two-phase aqueous nonionic micellar systems and two-phases aqueous nonionic polymer systems. In particular, it appears that polymer solutions are more effective than micellar solutions containing long, cylindrical micelles in separating hydrophilic proteins on the basis of their sizes (molecular weights). This reflects the marked differences in the microscopic characteristics of micellar and polymer solutions, as "probed" by a typical hydrophilic protein molecule.