Protein transport in selective membrane filtration.

摘要

There is considerable interest in using membrane filtration for the fractionation of protein mixtures, but most previous studies have found insufficient selectivity for actual commercial applications. This poor selectivity has been attributed to the presence of a pore size distribution in available membranes, bulk mass transfer limitations, protein adsorption within the porous structure of the membrane, protein deposition on the surface of the membrane, and protein-protein interactions. The overall goals of this thesis were: (1) to develop an improved understanding of the underlying physical processes governing the performance of selective protein filtration systems, and (2) to develop appropriate strategies for significantly improving the performance of these membrane devices for fractionating actual protein mixtures.;Experimental data were obtained for the transport of bovine serum albumin (BSA), immunoglobulin G (IgG), and hemoglobin (Hb) through 100,000 molecular weight cut-off polyethersulfone membranes in a stirred ultrafiltration device over a range of filtrate flux at different solution pH and ionic strengths. Under physiological conditions (pH 7.0 and 0.15 M NaCl), the maximum BSA-IgG selectivity was only about 2 due to the combined effects of protein adsorption and bulk mass transfer limitations. However, by changing the solution environment to pH 4.7 and 0.0015 M NaCl, the selectivity was increased to as much as 50, indicating that significant BSA-IgG separation could be obtained in just a single-stage membrane device. This large selectivity was a direct result of the negligible amount of IgG transport through the membrane under these conditions due to the very strong electrostatic repulsion between the positively charged IgG molecules and the membrane pores. BSA transport under these conditions was still significant due to the minimal electrostatic interactions for the uncharged BSA at its isoelectric pH. Experimental studies of the BSA-Hb system confirmed and extended these results, with very high selectivities (on the order of 70) obtained at pH 7.0 and at low salt concentrations due to the strong electrostatic exclusion of the negatively charged BSA molecules from the membrane pores. Hb transmission remained relatively high due to the absence of any significant net charge on the hemoglobin at this pH. Detailed theoretical calculations were also performed to study the effects of these electrostatic and electrokinetic interactions on both solute and solvent transport through charged ultrafiltration membranes with different pore size distributions.;Another phenomenon that can significantly affect the performance of many protein filtration processes is the presence of protein-protein interactions. A theoretical model was developed which explicitly accounted for the concentration dependence of the diffusive protein flux in multicomponent systems. Model calculations were in good agreement with experimental data for the filtrate flux and sieving coefficients for BSA and IgG, both alone and in protein mixtures. The albumin-immunoglobulin interactions in the protein mixture cause a significant increase in the back-transport of albumin, which can have a dramatic effect on the performance of membrane devices designed for BSA-IgG separation.