Effect of solution pH on protein transport through ultrafiltration membranes

Although a number of previous studies have demonstrated that solution pH ca n have a dramatic effect on protein transport through ultrafiltration membr anes, the exact origin of this behavior has been unclear. Experimental data were obtained for the transport of a broad range of proteins with differen t surface charge and molecular weight. The effective hydrodynamic size of t he proteins was evaluated using size-exclusion chromatography. The membrane charge, both before and after exposure to a given protein, was evaluated u sing streaming potential measurements. In most cases, the electrostatic int eractions were dominated by the distortion of the electrical double layer s urrounding the protein, leading to a distinct maximum in protein transmissi on at the protein isoelectric point. Attractive electrostatic interactions did occur when the protein and membrane had a large opposite charge, causin g a second maximum in transmission at a pH between the isoelectric points o f the protein and membrane. The sieving data were in good agreement with th eoretical calculations based on available models for the partitioning of ch arged solutes in cylindrical pores. (C) 1999 John Wiley & Sons, Inc.