Jannette Kreusser, Hans Hasse, Fabian Jirasek

• The separation and purification of proteins is often carried out by chromatography. Mixed-mode adsorbents are interesting as they may combine favorable features of different chromatographic methods, such as ion-exchange chromatography (IEC) and hydrophobic interaction chromatography (HIC). Systematic experimental studies and models of adsorption isotherms of proteins on mixed-mode resins covering a wide range of parameters are still rare, which hampers both the scientific analysis of the complex processes that occur upon the adsorption of proteins on these resins as well as the practical separation design. Therefore, such studies were carried out in the present work for a model system: the adsorption of bovine serum albumin (BSA) on the mixed-mode resin Toyopearl MX-Trp-650M, which combines features of IEC and HIC resins. Adsorption isotherms were measured using sodium chloride, sodium sulfate, ammonium chloride, and ammonium sulfate at ionic strengths up to 3000 mM and for pH 4.0, 4.7, and 7.0 at 25 C. In the studied pH ranges, BSA exhibits strongly varying net charges and undergoes a conformational change. At pH 4.0 and 4.7, an exponential decay of the BSA adsorption with increasing ionic strength was found at ionic strengths up to approximately 1000 mM, while a rather linear increase was observed at higher ionic strengths for all studied salts; for all salts and ionic strengths, decreasing adsorption with increasing pH value was found. Moreover, a mathematical model was developed, which enables the prediction of equilibrium adsorption isotherms of BSA on Toyopearl MX-Trp-650M for any ionic strength of the studied salts.