Molecular dynamics simulation study of ion interactions with zwitterions

Using molecular dynamics simulations, we investigated the associations between two zwitterions (carboxybetaine and sulfobetaine) and four types of cations (Li⁺, Na⁺, K⁺, and Cs⁺) in aqueous solutions. We studied the number and lifetime of various zwitterion-cation associations and observed that both carboxybetaine and sulfobetaine have the same order of association number and lifetime: Li ⁺ > Na⁺ > K⁺ > Cs⁺. Simulation results showed that the association variation as a function of cation types for these two zwitterions is significantly different. The effect of anion type on the order was also investigated by varying the type of anions from Cl^- to Br^- and F^-. In order to further investigate zwitterion-cation association, we simulated the systems either with one type of zwitterion and two types of cations or with one type of cation and both carboxybetaine and sulfobetaine presented. This allowed direct competition between the solutes, and the observed association number and lifetime validated the order. Simulation results further demonstrated that, although CB associates stronger with Li⁺ and Na⁺ than SB, the latter is associated preferentially by K⁺ and Cs⁺.