Molecular Simulation Study of Zwitterion-Ion Association

Materials with highly protein resistance are critical to many relevant applications such as drug delivery, biosensor and marine coating. Zwitterionic materials have received more and more attentions because they present excellent nonfouling performance even in the complicated biological environment. Ion plays an important role in industrial and biological fields. To apply the zwitterionic materials rationally, it is critical to understand their interactions with ions. In this work, we carried out a series of molecular dynamics simulation to study the associations between two zwitterionic molecules (carboxybetaine and sulfobetaine) and four cations (Li⁺, Na⁺, K⁺ and Cs⁺) in the Hofmeister series. We analyzed the association number and lifetime for each type of zwitterion-ion association and rank them in the order. We observe that both the carboxybetaine and sulfobetaine have the same order of association number and lifetime: Li⁺>Na⁺>K⁺>Cs⁺. The effect of anion on the order was investigated with varying the type of anions from Cl^- to Br^- and F^-. We also performed the simulations to investigate the cases when two cations are competing to associate with the same type of zwitterion, and that two zwitterions are competing to associate with the same type of cation. The observed order was evaluated directly from the association competition. The relative association preference of zwitterion to different types of cations was investigated, and we found that strong cations and weak cations have different association preference to the carboxybetaine and sulfobetaine. The results here offer the fundamental information to the selection of zwitterion when applying them in the ion-existing environment.