Molecular Dynamics Simulation Study of Hydration of Zwitterions

Carboxybetaine and sulfobetaine are two types of novel zwitterionic materials with excellent non-specific protein resistance. Different from the polyethylene glycol and other classical biomaterials, the zwitterionic materials have both highly negative and positive charged groups in a molecule. The hydration is recognized as an important factor to the ultralow fouling performance of the material. Therefore, it is urgent to investigate the hydration of these two betaines in the aim to understand their nonfouling mechanism. In this study, we studied the hydration of the carboxybetaine and sulfobetaine with the quantum mechanism and molecular dynamics simulations. We compared the spatial distributions, orientation distributions and the residence time of the water molecules around the carboxybetaine and sulfobetaine. The simulation results show that the hydration of sulfobetaine and carboxybetaine has different features and the the main difference of these two betaines comes from the hydration of different negative groups. We also investigated the hydration free energy of these two betaine molecules with free energy perturbation method and found they are quite similar. The relationship between the hydration free energy and the hydration structure and dynamic properties were also discussed.