Role of electrophoretic mobility of protein on its retention by an ultrafiltration membrane - Comparison to chromatography mechanisms

Lysozyme and lactoferrin, two globular proteins, were first studied separat ely in order to elaborate a strategy for the improvement of their separatio n by ultrafiltration (UF) with zirconia-based membranes of different charge sign and pore radius. The electrophoretic mobility (mu) at fixed pH and va riable ionic strength was used for the characterisation of both proteins an d zirconia particles, similar to the active layer of the membrane during th e UF run. Specific adsorption of phosphate ions was shown for both proteins resulting in new isoelectric points. The occurrence of electrostatic exclu sion mechanism in addition to the molecular sieving in UF of charged solute s was shown for: Low molecular weight solute: multivalent citrate at pH 6 was specifically a dsorbed on zirconia and its transmission through the membrane (defined as t he ratio of the concentration in the permeate to that of the feed solution) was reduced in the range 0.001-0.01 mol l(-1) of citrate concentration Proteins: their transmissions increase when the ionic strength increases (i on-exchange is not the relevant mechanism because transmission is irrespect ive of the initial charge of the membrane compared with the protein charge) . A model based on convection, diffusion and electrophoretic migration mechan isms (CDE model) was proposed to take into account this behaviour. The CDE model predicts the possible existence of a depleted sub-layer of the charge d protein in the concentration polarisation layer, located in the close vic inity of the membrane surface. A strategy for the separation of two protein s in mixed solution was proposed by varying both the physico-chemical envir onment in the feed solution (pH, ionic strength, chemical nature of the ele ctrolyte) and the membrane pore radius. Maximum selectivity was obtained wh en the target protein (to be transmitted in the permeate side) is close to being uncharged due to specific adsorption of electrolyte ions. Ultrafiltra tion selectivity is enhanced with membrane of large pore radius, which prov ides high transmission of the target protein and efficient electrostatic ex clusion of the solute to be retained in the retentate side. This UF approac h corresponds roughly to the separation of one uncharged and one charged pr otein from a mixed solution by size exclusion chromatography of the uncharg ed protein combined with electrostatic exclusion of the charged protein due to packing of similar charge. (C) 2001 Elsevier Science BN. All rights res erved.