Competitive cation-pi interactions between small cations and polycations with phenyl groups in poly(cation-pi) hydrogels

Although cation-pi interactions play a critical role in biological systems, experimental studies in aqueous environments are limited due to a lack of suitable systems and methods. The recent success in synthesizing water-soluble poly(cation-pi) bearing adjacent cationic/aromatic residues provides excellent systems for studying cation-pi interactions in aqueous environments. In this study, we investigated competitive cation-pi interactions between small cations and macrocations with phenyl groups in hydrogels consisting of various poly(cation-pi)s. By measuring the Donnan potential of the hydrogels equilibrated in NaCl solutions of different ionic strengths using the microelectrode technique, the binding ratios of small cations (Na+) to the phenyl residues of polymer networks were determined. It was found that when NaCl concentration was lower than the macrocation concentration, the activity coefficient of the counterions (Cl-) in the hydrogels was very high (similar to 2), indicating that a large amount of excess Cl- was trapped in the hydrogels due to Na+ adsorption on the phenyl groups of the polymer chains via cation-pi interactions. Further calculations showed that each phenyl group can adsorb a maximum of 1.5 Na+ despite the presence of adjacent macrocations. Additionally, we discuss the mechanism by which small cations compete with adjacent macrocations in poly(cation-pi) hydrogels.