Protein-protein interactions and reduced excluded volume increase dynamic binding capacity of dual salt systems in...

Deploying two salts in hydrophobic interaction chromatography can significantly increase dynamic binding capacities. Nevertheless, the mechanistic understanding of this phenomenon is lacking. Here, we investigate whether surface tension or ionic strength govern dynamic binding capacities of the chromatographic resin Toyopearl Butyl-650 M in dual salt systems. Small-angle X-ray scattering was employed to analyze the model proteins and the protein-resin adduct in the respective dual salt systems. The dual salt systems incorporate sodium citrate and a secondary sodium salt (acetate, sulfate, or phosphate). As model proteins, we used lysozyme, GFP, and a monoclonal antibody (adalimumab). Moreover, for the protein-resin adduct, we determined the model parameters of a self-avoiding random walk model fitted into the pair density distribution function of the SAXS data. Ionic strength is more predictive for dynamic binding capacities in HIC dual salt systems than surface tension. However, dynamic binding capacities still differ by up to 30 % between the investigated dual salt systems. The proteins exhibit extensive protein-protein interactions in the studied dual salt HIC buffers. We found a correlation of protein-protein interactions with the well-known Hofmeister series. For systems with elevated protein-protein interactions, adsorption isotherms deviate from Langmuirian behavior. This highlights the importance of lateral protein-protein interactions in protein adsorption, where monomolecular protein layers are usually assumed. SAXS analysis of the protein-resin adduct indicates an inverse correlation of the binding capacity and the excluded volume parameter. This is indicative of the deposition of proteins in the cavities of the stationary phase. We hypothesize that increasing protein-protein interactions allow the formation of attractive clusters and multilayers in the cavities, respectively.