The Effects of Secondary Stationary-Phase Polymers on Adsorption and Transport in Ion-Exchange Chromatography
Petroff, Matthew G.
University of Delaware
Recent attempts to improve the adsorption and transport properties of ion-exchange adsorbents have included the modification of the adsorbent base matrix with a secondary polymer layer. This work attempts to characterize the relevant adsorption and transport properties of two related strong cation exchange adsorbents, Toyopearl GigaCap and Toyopearl SP-650M, which differ in that the GigaCap adsorbent consists of the SP-650M base matrix functionalized with a secondary-polymer layer. The studies performed utilized a combination of equilibrium adsorption isotherm determination, batch uptake, and isocratic pulse response experiments. Results were obtained for two model proteins, lysozyme (14.7 kDa) and lactoferrin (78 kDa), and were compared between the adsorbents to allow for elucidation of the effects of protein charge, protein size, and GigaCap’s secondary-polymer layer on the transport and adsorption behavior. The batch uptake results indicate higher effective pore diffusivities for lysozyme than for lactoferrin, but similar effective pore diffusivities for the respective proteins in the two adsorbents. The adsorption isotherms indicate that both proteins display much higher static capacities for the GigaCap S-650M particles than for the SP 650M particles. While the capacity differences are significant at low ionic strengths, they are drastically reduced at high ionic strengths, a trend that is more significant in lactoferrin than lysozyme. Retention experiments indicate decreasing retention between the protein-adsorbent pairs of lactoferrin on SP-650M, lysozyme on GigaCap, and lysozyme on SP-650M, as well as evidence of lactoferrin exclusion from GigaCap’s secondary polymer layer. Taken together, the adsorption and retention experiments indicate that lactoferrin is excluded from GigaCap’s secondary polymer layer high ionic strengths, and is partially excluded at lower ionic strengths. This exclusion appears to be a combination of steric effects, due to conformational changes in the polymer layer that reduce the accessibility for protein binding due to polymer shrinkage at high ionic strengths, and a decrease in the electrostatic forces that drive solute partitioning into the polymer layer. These effects are most significant at high ionic strengths, where lactoferrin is excluded from the polymer layer, but also appear at lower ionic strengths, where large differences in lactoferrin capacity are observed between ionic strengths of 20 mM and 50 mM. This exclusion behavior is observed for ionic strengths that are relevant for chromatographic process operations. It is thus important to be aware of these effects during process design, as relatively small ionic strength deviations may result in drastic changes in process performance. It is also relevant to future resin design, as it may be desirable to either reduce or exploit these effects in future adsorbents. Specifically, it presents an opportunity to design ion-exchange resins that have high affinity for ions of a specific molecular size range. It also demonstrates that changes must be made to the nature of the polymer layer before these adsorbents can display robust, high capacities for larger macromolecules.