Aqueous Two-Phase Extraction: Theory and Experiment

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When two aqueous solutions of water soluble polymer such as PEG and dextran are mixed together, above critical concentrations, a liquid-liquid phase separation occurs. A similar situation occurs when PEG and salt are mixed together. Proteins or enzymes added to either of the resulting two phase mixtures will tend to partition unequally between the two phases, thus allowing for the extraction of a particular protein. Over the past three years, in collaboration with Professor Kula of the Institute for Enzymetechnology in Julich, West Germany, and under the auspices of an NSF grant (CBT-872084), the PI has been developing a statistical mechanical theory for aqueous two phase partitioning, and has been undertaking complementary experiments on the phase behavior of the PEG/Dx system. The experimentation involved the careful assemblage of phase equilibria diagrams, and the identification of the interfacial tension between the two phases as a sensitive indicator of composition. Theoretically, simple models of polymer-polymer, polymer-protein, ion-ion, protein-protein, protein-ion and polymer-ion interactions have been developed which can be used to calculate potentials of mean force in water. The proposed research suggests additional theoretical work along the same lines. The mean force potentials which have been developed are to be used to obtain the radial distribution function via the closure postulation of the integral equation technique. From the radial distribution function, thermodynamic properties can be obtained, and phase separations behavior identified. These calculations will be supplemented by Gibbs Monte-Carlo computer simulations of the phase equilibria. These simulations will allow verification of the equilibria obtained by the integral equation route, and will provide a method for obtaining phase equilibria for which the integral equation approach proves inadequate. Simulations and integral phase equilibria calculations will be carried out to develop theories capable of predicting phase equilibria in polymer/polymer and polymer/salt mixtures, and the segregation g of proteins including multiple protein partitioning and possibly the effect of affinity ligands attached to the polymer molecule.. Complementary experiments on protein partitioning will also be undertaken.