Previous studies of electrostatic Interactions during protein ultrafiltration have generally assumed that the membrane surface charge density and pore size are the only membrane properties that determine the overall performance, without any consideration of the detailed molecular structure of the charged ligands This is in sharp contrast to the behavior seen in ion exchange chromatography where the detailed physical and chemical properties of the functional ligand can have a significant impact on the adsorption characteristics The objective of this study was to examine the behavior of several novel positively-charged ultrafiltration membranes generated by covalent attachment of a series of ligands having similar size but containing different numbers of primary, secondary, and quaternary amines Membrane surface charge was evaluated using both streaming potential and dye-binding measurements, with the number of amine nitrogens verified by X-ray photoelectron spectroscopy (XPS) Ultrafiltration experiments were performed over a range of solution ionic strength using cytochrome c as a model protein Protein transmission was well correlated with the apparent zeta potential of the membrane, with minimal contribution from the number of amine groups along the length of the membrane or the presence of a quaternary versus primary amine The results provide important insights Into the relationship between ligand properties and membrane performance, providing a framework for the development of novel charged ultrafiltranon membranes for high performance bioprocessing applications (C) 2010 Elsevier B.V All rights reserved