(366b) Effects of Electrostatic and Intermolecular Interactions On Ultrafiltration of Pegylated Proteins

It is now well-established that protein transmission through ultrafiltration membranes is strongly effected by electrostatic interactions, with the magnitude of these interactions very effectively described using the partitioning model developed by Bill Deen and co-workers.  This presentation will examine the extension of this theoretical framework to describe the ultrafiltration behavior of pegylated proteins produced by covalent attachment of a polyethylene glycol chain to a native protein.  Experimental data were obtained for the transmission of pegylated α-lactalbumin through both unmodified and negatively charged composite regenerated cellulose membranes over a range of solution ionic strength and PEG concentrations.  The attachment of the PEG altered the protein charge, the effective size, and the electrostatic potential surrounding the protein. The pegylated α-lactalbumin was strongly retained by the negatively charged membrane due to the significant electrostatic interactions in this system.  The transmission of the pegylated protein also increased with increasing PEG concentration due to the increase in the solute partition coefficient arising from unfavorable intermolecular interactions in the bulk solution.  A simple model was developed to account for the effects of the electrostatic and intermolecular interactions on the ultrafiltration of the pegylated proteins based on an extension of the classical Smith and Deen analysis.  The pegylated protein was treated as an effective hard sphere with the surface charge shifted to the outer edge of the PEG by accounting for the alteration in the electrostatic potential associated with the reduced electrolyte concentration within the PEG layer.  The experimental results were in good agreement with model calculations, providing a framework for analyzing the behavior of pegylated proteins in batch ultrafiltration and diafiltration processes.