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The objective of this work is to study water interactions with zwitterionic polymers and ions. We have been tackling this problem from three angles – (a) studying the three transitions of zwitterionic polymer networks in water; (b) investigating the hydration capabilities of different zwitterions; (c) exploring the effect of the chain length between two charged groups on the hydration of a zwitterion.  

Phase Transitions in a Polymer Gel (see the figure in Nugget.pdf): In this work, three transition states were found from a series of molecular dynamics (MD) simulations on carboxybetaine (CB) hydrogels at different swelling states. These transitions have distinctive physical meanings. The first transition indicates that there are two types of water molecules inside hydrogels: bounded and non-bounded water. The second transition shows that the equilibrium water content of CB hydrogel can be determined by the change in the lifetime of side-chain pairs. The CB polymer network in this work can hold 62% of water at its equilibrium swelling state. Through the examination of the lifetime of physical crosslinker in the hydrogel, the current approach can also be extended to identify the equilibrium water content of other hydrogels from simulation. Finally, the third transition shows that the CB hydrogel in this study can have water content up to 81%. Results also indicate that the topology of the chemical bonds of the polymer network of the hydrogel will affect the upper limit of its equilibrium water content [Y. He, Q. Shao, H.K. Tsao, S. Chen, and S. Jiang, Understanding Three Transition States of Zwitterionic Carboxybetaine Methacrylate Hydrogel from Molecular Dynamic Simulations, Submitted to Journal of Physical Chemistry B (2011)].

Hydration of Different Zwitterions (see the figure in Nugget.pdf): We investigated the hydration structures, dynamics and free energy of CB and sulfobetaine (SB) using quantum mechanics, MD simulations and free energy perturbation calculations. The main hydration difference was observed around the negatively charged groups of the molecules, while the hydration of the positively charged groups of these two betaine molecules was observed to be very similar. The negatively charged group of the SB molecule has more water molecules in its first coordination shell than that of CB molecule. However, the water molecules around the negatively charged group of CB molecule have sharper spatial distributions, more preferential dipole orientation and longer residence time. These simulation results show that the water molecules around the negatively charged group of CB interact with the solute stronger than those around the negatively charged group of SB, whereas the latter will have more water molecules around it. For either CB or SB, the coordination number of the positively charged group is larger than that of the negatively charged group. However, the water molecules around the negatively charged group are found to have higher structure order and lower mobility. Both CB and SB have hydration a free energy considerably lower than that of oligo(ethylene glycol) (OEG). [Q. Shao, Y. He, A. White and S. Jiang, Difference in Hydration between Carboxybetaine and Sulfobetaine, Journal of Physical Chemistry B, 114, 16625 (2010).]

Chain-length Dependence of a Zwitterion on Hydration: In this work, quantum chemical calculations and molecular dynamics simulations were performed to investigate the conformation and solute-solvent interactions of CB molecules with one to five methylene spacer groups between the charged quaternary amine and carboxylate groups in water and a sodium chloride solution of physiological ionic strength. As the number of methylene spacer groups in the CB molecules increases, the number of possible molecular conformations also increases.  The conformational potential energy was calculated to determine the most stable conformation for each CB molecule studied.  The optimized CB structure and the resulting force field parameters for each conformation were then used to calculate the solvation free energy.  The calculated solvation free energy was used to evaluate hydration interactions. Results are compared with those from experiments. Results show that the chemical structure of CB (or the number of methylene spacer groups in CB) can significantly impact its molecular conformation.  These conformational differences lead to differences in the CB solute salvation capacity.  A paper has been prepared and is under review. [J.C. Hower, L. Yao, and S. Jiang, Physical and Chemical Properties of Carboxybetaine Zwitterions from Quantum Chemistry and Molecular Dynamics Simulations, To be submitted to Journal of Chemical Physics (2011).]