(261a) Molecular Understanding of the Hydration of Zwitterionic Moieties: Effect of Charged Groups

Strong hydration of a zwitterionic moiety largely determines the ultralow fouling property of zwitterionic materials important for applications including drug delivery, protein detection and marine coating. One of the fundamental questions is how the hydration depends on the types of charged groups within the zwitterionic moiety. In order to shed light on this issue, we studied the hydration of zwitterionic moieties containing different charged groups systematically using quantum mechanical calculations and molecular simulations. We selected three anionic groups (carboxylic group, sulfonate group and sulfate group) and four cationic groups (quaternary amine, tertiary amine, secondary amine and primary amine). The carbon spacer length between the cationic and anionic groups was fixed to one. Combining these seven charged groups generates twelve zwitterionic moieties including glycine betaine, a naturally occurring protein protectant. We analyzed the charge distributions of these zwitterionic molecules in order to investigate the effect of diffeent charged group combinations on the interactions between charged groups. We developed force field parameters for these zwitterionic moieties and studied their hydration using molecular dynamics simulation and metadynamics simulation. The analyzed hydration properties involve the hydration number, residence time of water molecules in the first hydration shell, hydration free energy and dehydration free energies of anionic and cationic groups. We found that varying the cationic group causes variations of the hydration in the order of carboxylic >>sulfonate and sulfate groups.  We also investigated how the hydration of cationic group varies with the type of anionic group and found that the tertiary amine, secondary amine and primary amine groups show the same tendency. The simulation results provide a fundamental understanding of how the hydration of a zwitterionic moiety depends on its charged groups.