Although protein retention in ion exchange is clearly dominated by electrostatic effects, quantitative prediction of relative retention and hence of selectivity has remained elusive.  This is due mainly to the complexity of capturing protein properties, salt and solvent effects, and the local details of adsorbent architecture; the latter factor is especially severe for polymer-functionalized ion exchangers.  This work has made use of a thermodynamically rigorous molecular theory that accounts for key structural details of both the protein and the stationary phase, including sulfonated dextran in the case of a polymer-functionalized adsorbent.  The method has been applied to the competitive adsorption of two small globular proteins, lysozyme and cytochrome c, as a function of salt concentration and pH, and the predictions compared to supporting experimental data.  The modelling predicts lysozyme to be preferentially adsorbed except under highly acidic conditions (pH &lt; 4.5), so that preferential adsorption of lysozyme is overtaken by that of cytochrome c as the pH is reduced.  The critical pH at which this transition occurs, pH<sub>c</sub>, has a non-trivial dependence on the salt concentration. These results are complemented by further molecular details of binary adsorption, such as local pH cariations and the chemical potential as a function of distance to the charged surface, which in turn modulate the net charge of the proteins. This non-trivial coupling between surface charge distribution and chemical equilibrium gives rise to the complex behavior of protein adsorption on charged adsorbents.

Mechanistic basis for adsorptive selectivity in conventional and dextran-functionalized ion exchangers: Model and experiment

251st National Meeting (2016)

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