Role of the physico-chemical environment on ultrafiltration of lysozyme with modified inorganic membrane

On the impact of ethanol on the rejection and transfer mechanism during ultrafiltration of a charged macromolecule in water/ethanol

Ultrafiltration (UF) is a sustainable membrane separation technique. It could be useful for the concentration/purification of bio-sourced molecules that are extracted either by pure ethanol or by water/ethanol mixtures. Nevertheless, the process optimization requires an in-depth understanding of the transfer mechanisms of solute through membranes, especially for charged solutes, that are nowadays not sufficiently documented. Previous studies achieved in aqueous media have shown that the rejection of charged solutes by an UF membrane involves at least three mechanisms: convection, diffusion and electrostatic interactions. The present study aims at a systematic analysis of the transfer mechanisms of a model protein (lysozyme) in water/ethanol mixtures (100/0-70/30 v/v) during UF by a zirconia inorganic membrane. The influence of the pH varying in the 4-9 range and of the ionic strength (I) is also discussed. The ionic strength I can be adjusted by addition of an indifferent electrolyte (NaCl) only aiming at the screening of the electrostatic interactions or by addition of a selectively adsorbed electrolyte(KH₂PO₄) that is able to change the isoelectric pH of the protein and thus to modulate the electrostatic interactions in a different way when compared to NaCl. Of course, both salts have an impact on the protein rejection in UF. The results are analysed using the CDE model previously developed in our group to explain the behaviour of a single protein during UF in water and accounting for convection, diffusion and electrophoretic migration. The applicability of the CDE model in water/ethanol mixtures up to 70/30 v/v is finally shown.

Small molecular ion adsorption on proteins and DNAs revealed by separation techniques

Ion binding is a term that assumes that the ion is included in the solvation sphere characterising the biomolecule. The binding forces are not clearly stated except for electrostatic attraction; weak forces (hydrogen bonds and Van der Waals forces) are likely involved. Many publications have dealt with ion binding to proteins and the consequences over the past 10 years, but only a few studies were performed using high-performance liquid chromatography (HPLC: ion exchange, reversed phase without the well-identified immobilised metal affinity chromatography) and capillary zone electrophoresis (CZE). This review focuses on the binding of proteins and DNAs mainly to the oxyanions (phosphate, borate, citrate) and amines used as buffers for both the HPLC eluent and the background electrolyte of CZE. Such specific ion adsorption on biomolecules is evidenced by physico-chemical characteristics such as the mobility or retention volume, closely associated with the net charge, which differ from the expected or experimental data obtained under the conditions of an indifferent electrolyte. It is shown that ion binding to proteins is a key parameter in the electrostatic repulsion between the free protein and a fouled membrane in the ultrafiltration separation of a protein mixture.

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 separately in order to elaborate a strategy for the improvement of their separation by ultrafiltration (UF) with zirconia-based membranes of different charge sign and pore radius. The electrophoretic mobility (mu) at fixed pH and variable ionic strength was used for the characterisation of both proteins and zirconia particles, similar to the active layer of the membrane during the UF run. Specific adsorption of phosphate ions was shown for both proteins resulting in new isoelectric points. The occurrence of electrostatic exclusion mechanism in addition to the molecular sieving in UF of charged solutes was shown for: * Low molecular weight solute: multivalent citrate at pH 6 was specifically adsorbed on zirconia and its transmission through the membrane (defined as the 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 (ion-exchange is not the relevant mechanism because transmission is irrespective of the initial charge of the membrane compared with the protein charge). A model based on convection, diffusion and electrophoretic migration mechanisms (CDE model) was proposed to take into account this behaviour. The CDE model predicts the possible existence of a depleted sub-layer of the charged protein in the concentration polarisation layer, located in the close vicinity of the membrane surface. A strategy for the separation of two proteins in mixed solution was proposed by varying both the physico-chemical environment in the feed solution (pH, ionic strength, chemical nature of the electrolyte) and the membrane pore radius. Maximum selectivity was obtained when the target protein (to be transmitted in the permeate side) is close to being uncharged due to specific adsorption of electrolyte ions. Ultrafiltration selectivity is enhanced with membrane of large pore radius, which provides high transmission of the target protein and efficient electrostatic exclusion of the solute to be retained in the retentate side. This UF approach corresponds roughly to the separation of one uncharged and one charged protein from a mixed solution by size exclusion chromatography of the uncharged protein combined with electrostatic exclusion of the charged protein due to packing of similar charge.

Specific adsorption of phosphate ions on proteins evidenced by capillary electrophoresis and reversed-phase high-performance liquid chromatography

Specific adsorption of phosphate ions at pH=7.0 was studied on different proteins, either counter-ions of phosphate (lysozyme, lactoferrin) or co-ion of phosphate (alpha-lactalbumin). The theoretical electrophoretic mobility of globular proteins lysozyme and alpha-lactalbumin (apo and holo (+1 calcium per molecule) forms) was compared with those measured by capillary electrophoresis in phosphate at pH 7.0, versus the ionic strength (I) in the range 0-0.775 mol L(-1). The specific adsorption of phosphate ions was evidenced by difference. From the experimental charge number (Z(eff)) of protein in phosphate medium, a phosphate content per protein molecule was determined at pH=7.0. * For lactoferrin (pI=8-9), the electrophoretic mobility (mu) was constant and negative, highlighting a charge reversal due to phosphate adsorption. * For alpha-lactalbumin (holo form) experimental mu was roughly constant and more negative than predicted. Z(eff) increased continuously from -4 to -11 in the ionic strength range from 0.005 to 0.775 mol l(-1), respectively. Accordingly, one to six phosphates were bound per molecule, respectively. * For lysozyme, experimental electrophoretic mobility was positive but lower than predicted. Z(eff) was only discrete values +5 for I in the range 0.001-0.020 mol l(-1) and about +3 in the range 0.050-0.500 mol l(-1), whereas the theoretical Z value was +7 at pH = 7.0. Lysozyme bounds one phosphate at low ionic strength and about two-three at higher ionic strength. Reversed-phase HPLC confirms that adsorption of phosphate is different for the three proteins.

Ultrafiltration to fractionate wheat polypeptides

An ultrafiltration process allowing the fractionation of two kinds of polypeptides issued from limited chymotryptic hydrolysis of wheat gliadins was applied to wheat gluten hydrolysates. Hydrophilic and poorly charged polypeptides were well transmitted through an inorganic ZrO2-based membrane at acidic pH, whereas hydrophobic and positively charged polypeptides were highly retained. By combining reversed-phase and cation-exchange chromatography (CEC), it was proved that the fractionation of the polypeptides was based on electrostatic repulsion of the charged polypeptides by the positively charged membrane. After a continuous diafiltration process, retentates containing 75 to 88% of hydrophobic polypeptide and permeates containing 84 to 90% of hydrophilic polypeptides were recovered, depending on the size of membrane used. Even if the ultrafiltration fractions were less purified than fractions issued from CEC, it was shown that they exhibited very different foaming properties: permeate did not produce nor stabilize foams, whereas retentate was more efficient than the whole hydrolysates and BSA.

Protein Adsorption onto Zirconia Modified with Terminally Grafted Polyvinylpyrrolidone

The potential effectiveness of poly(vinyl pyrrolidone) (PVP) as a zirconia surface modifier for protein adsorption reduction was investigated using lysozyme (LYS). The relatively small size of LYS (45 x 30 x 30 Å) allowed for testing the adequacy of the graft polymerization method for producing a dense surface chain coverage to exclude LYS from direct interaction with the zirconia surface. The study demonstrated that a PVP brush layer is capable of reducing lysozyme adsorption. Overall, the maximum adsorption capacity decreased (by up to about 76%) due to surface modification with increasing polymer/silane surface coverage ratio (mol/mol). Adsorption reduction, due to protein exclusion from the surface by the tethered polymer layer, increased significantly when the distance between surface chains was less than the large axis of LYS (i.e., 45 Å). The present results are encouraging and suggest further consideration of polymer-modified ceramic surfaces for reducing fouling of ceramic membranes during protein ultrafiltration and producing ceramic biocompatible surfaces for biomedical applications. Copyright 2001 Academic Press.