The binding affinity of uncharged aromatic solutes for negatively charged resins is enhanced by cations via cation-π interactions: The case of sodium ion and arginine.
Salt solutions are widely used as eluents for ion-exchange chromatography. In general, salts reduce the retention of applied solutes on ion-exchange columns via electrostatic screening effects. The reverse phenomenon, namely, salt-enhanced retention, has not been reported. Here, we report that cations, including arginine, guanidine and sodium ions, enhance the retention of uncharged aromatic solutes on a cation-exchange resin, i.e., a negatively charged resin, with carboxyl groups, where we used alkyl gallates as model uncharged aromatic solutes and a carboxymethyl agarose gel (CM Sepharose) as a model negatively charged resin. Enhancement of retention was observed at concentrations of tens of millimolar of the salts, in which arginine hydrochloride was more effective than guanidinium salts and NaCl. Similar trends were observed for other phenolic compounds, including phenol and 4-hydroxybenzyl alcohol. Molecular dynamics simulations showed that the binding free energy between the alkyl gallate molecule and the CM Sepharose resin ligand molecule increased with increasing salt concentration. The increase in binding free energy caused by the salts was accounted for by the binding of the salt cations to the aromatic moiety of the alkyl gallate via cation-π interactions, leading to attenuation of intrinsic repulsive interactions between the ligand carboxyl group and the alkyl gallate aromatic moiety. Therefore, the salt-enhanced retention of the uncharged aromatic solutes on the negatively charged resins was ascribable to the increase in binding free energy induced by the cation-π interactions. This unique reverse phenomenon of the effect of salts on solute retention indicates the importance of cation-π interactions in ion-exchange chromatography. This phenomenon can be used for selective chromatographic separation of aromatic solutes, including organic solutes, proteins and nucleic acids.