(710d) Molecular Dynamics Simulations of Thermoresponsive Poly (N-isopropylacrylamide) and Its Copolymer

Salt ions play a critical role in many biological processes including protein solubility, stability, denaturation and aggregation. Understanding ion-protein interaction is crucial for elucidating the fundamental underlying mechanisms. The Hofmeister series refers to the ability for salt ions to precipitate proteins from an aqueous solution based on empirical observations. However, to date, the fundamental mechanisms remain elusive. Previously it was thought that ions in solution can affect the bulk water structure thus affecting the hydrophobic-hydrophilic transition of proteins in aqueous solution. Ions are considered chaotropic (structure breaking) or kosmotropic (structure making). Recently spectroscopic and calorimetry studies show that ions only affect the closest hydration shells and the bulk water structure is not altered by the presence of the salt ions. In addition, attempts have been made to correlate the Hofmeister series with surface tension, the solvation entropy of the ions and other physical and thermodynamics properties of the ions. However, all these correlations have failed to explain the entire series and the inverse Hofmeister series sometimes observed. A fundamental molecular level understanding is also lacking.

PNIPAM has been widely studied as a prototype temperature responsive polymer which exhibits hydrophobic-hydrophilic phase transition at its lower critical solution temperature (LCST). It has been found that salt ions have a significant impact on the LCST. Most salts are found to reduce the transition temperature and the effect of ions on LCST follows the Hofmeister series. Classical molecular dynamics simulations were carried out to investigate the ion-PNIPAM interactions in aqueous solutions. The salt investigated include MCl_n (M=Li, Na, K, Rb, Cs, Mg, and Ca) and NaX (X=Cl, Br and I). The effects of salt on LCST transition as well as ion specific interactions with PNIPAM will be elucidated.