Mechanistic studies of displacer-protein binding in chemically selective displacement systems using NMR and MD simulations.

A parallel batch screening technique was employed to identify chemically selective displacers which exhibited exclusive separation behavior for the protein pair alpha-chymotrypsin/ribonuclease A on a strong cation exchange resin. Two selective displacers, 1-(4-chlorobenzyl)piperidin-3-aminesulfate and N'1'-(4-methyl-quinolin-2-yl)-ethane-1,2-diamine dinitrate, and one non-selective displacer, spermidine, were selected as model systems to investigate the mechanism of chemically selective displacement chromatography. Saturation transfer difference (STD) NMR was used to directly evaluate displacer-protein binding. The results indicated that while binding occurred between the two chemically selective displacers and the more hydrophobic protein, alpha-chymotrypsin, no binding was observed with ribonuclease A. Further, the non-selective displacer, spermidine, was not observed to bind to either protein. Importantly, the binding event was observed to occur primarily on the aromatic portion of the selective displacers. Extensive molecular dynamic simulations of protein-displacer-water solution were also carried out. The MD results corroborated the NMR findings demonstrating that the binding of selective displacers occurred primarily on hydrophobic surface patches of alpha-chymotrypsin, while no significant long term binding to ribonuclease A was observed. The non-selective displacer did not show significant binding to either of the proteins. MD simulations also indicated that the charged amine group of the selective displacers in the bound state was primarily oriented towards the solvent, potentially facilitating their interaction with a resin surface. These results directly confirm that selective binding between a protein and displacer is the mechanism by which chemically selective displacement occurs. This opens up many possibilities for future molecular design of selective displacers for a range of applications.