Effect of surface charge distribution on protein transport through semipermeable ultrafiltration membranes

Although several previous studies have demonstrated the importance of electrostatic interactions on the transport of charged proteins through semipermeable ultrafiltration membranes, all of these analyses have implicitly assumed that the protein can be treated as a sphere with uniform surface charge. This is in contrast to the behavior seen in ion exchange chromatography where the presence of localized charge “patches” can significantly affect protein adsorption to the charged resin. The objective of this study was to examine the possible effects of surface charge distribution on the magnitude of the electrostatic interactions that occur during protein ultrafiltration. Data were obtained with cytochrome c and lysozyme, two proteins that have similar size and net charge but different amino acid composition and surface charge distribution. Ultrafiltration experiments were performed with cellulosic membranes, both as received and surface-modified by chemical attachment of a quaternary ammonium functionality to generate a positively charged membrane. Data were obtained over a range pH with the sieving results compared to the measured protein retention on a strong cation-exchange resin. Protein transmission during ultrafiltration through oppositely charged membranes was governed primarily by the net protein charge, with minimal contribution from the surface charge distribution.