(466a) Development of Surface Modified Membranes with Zwitterionic Ligands to Control Protein Transport and Fouling During Ultrafiltration

Several recent studies have demonstrated that protein transport through semipermeable ultrafiltration membranes is strongly affected by electrostatic interactions between the charged membrane and the charged protein.  These studies have typically used common cationic or anionic ligands to produce electrically-charged membranes that provide very high retention of species with like-polarity.  The objective of this study was to examine the behavior of novel charged ultrafiltration membranes generated by covalent attachment of a series of zwitterionic ligands having different physical properties to a base cellulose membrane.  

Commercially available 30 kDa composite regenerated cellulose (Ultracel^TM) membranes were first actived with glycidyl allyl ether, followed by bromination using N-bromosuccinimide and then reaction with a specific zwitterionic ligand containing both an amine and a sulfonic acid or carboxylic acid functionality, e.g., taurine and glycine.  Negatively charged versions of the Ultracel^TM membrane were generated by chemical attachment of a small ligand containing a sulfonic acid group to the membrane surface using a base-activated chemistry.  The membrane surface charge density was evaluated as a function of pH using streaming potential measurements.  Ultrafiltration experiments were performed over a range of solution pH using cytochrome c, myoglobin and a-lactalbumin as model proteins.  These proteins have similar size, but very different charge characteristics with isoelectric points varying from pH 4.6 – 10.4. 

The apparent zeta potential of the membrane modified with the zwitterionic ligand was a strong function of pH, in sharp contrast to the nearly constant negative zeta potential for the membrane modified with the sulfonic acid functionality.  Transmission of cytochrome c through the charge modified membranes was very high (sieving coefficient greater than 0.9) at pH values below the protein isoelectric point.  The sieving coefficient for cytochrome c decreased by more than an order of magnitude at pH 11, reflecting the strong electrostatic repulsion between the negatively-charged protein and negatively charged membrane under these conditions.  Similar behavior was observed for myoglobin, but with slightly lower sieving coefficients, consistent with the slightly larger size of myoglobin compared to cytochrome c.  Transmission of a-lactalbumin was maximum near its isoelectric point (pI 4.6) when using the membrane with the sulfonic acid functionality, but was a monotonically decreasing function of increasing pH for the zwitterionic membranes.  The observed sieving behavior for the model proteins could be explained in terms of the differences in electrostatic interactions arising from the different charge-pH profiles for the membranes and proteins.  Results for the zwitterionic membranes were highly correlated with the apparent zeta potential of the membrane, irrespective of the presence of carboxylic (weak) versus sulfonic acid (strong) functional groups.  There was no evidence of significant membrane fouling, even under conditions where the protein and membrane had opposite polarity, possibly due to the very hydrophilic nature of the charged ligands.  The results clearly demonstrate the potential of using zwitterionic ligands to generate high performance ultrafiltration membranes for bioprocessing applications.