Phase diagrams for the mixture of globular proteins and non-ionic polymers
Abstract
In this study, phase diagrams of globular proteins and non-ionic polymers were measured. Four proteins, namely, bovine serum albumin (BSA), human serum albumin (HSA), ovalbumin and lysozyme and two polymers, namely, polyethylene glycol (PEG) of molecular weights 3350 and 8000 and dextran of molecular weight 67900 were investigated. Protein concentration in the solid phase was found to be much higher than that in the liquid phase and exhibited a shallow maximum at an intermediate polymer concentration. The polymer was found to partition preferable to the liquid phase, the difference between the protein free polymer concentrations in the liquid and the solid phases being more pronounced at higher polymers concentrations. Phase diagrams of BSA/lysozyme/PEG-8000 mixture were measured at pH 4.8 for initial BSA/lysozyme concentration ratio of 1:1, 1:2 and 2:1. Similar to single protein systems, maximum weight fractions of BSA and lysozyme were observed at intermediate polymer concentrations for all three initial compositions. Preferential precipitation of BSA was found to occur from BSA/lysozyme mixtures and PEG partitioned more in the liquid phase except at high polymer concentrations. The proposed statistical mechanical model accounted for the effects of polymer and solvent on the Gibbs free energy of protein through the interaction potential between protein molecules. The Gibbs free energy of protein in the solid and liquid phases, evaluated using a second order perturbation theory around hard sphere as the reference, were equated to predict the phase diagram. Experimental measurements of solubility of BSA, HSA, and ovalbumin for different molecular weights of polyethylene glycol agreed well with the model predictions. Because of the simplifying assumption of vertical tie lines, the model over predicted the solid phase protein concentration. A hybrid model combining the above model with virial model was proposed to calculate the partitioning of polymer between two phases. The model predictions of the liquid and solid phase compositions as well as the tie lines agreed well with the experimental data for BSA/PEG-8000 at pH 4.8.
Degree
Ph.D.
Advisors
Narsimhan, Purdue University.
Subject Area
Agricultural engineering
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