Collapse-Swelling Transitions of a Thermoresponsive, Single Poly(N-isopropylacrylamide) Chain in Water.

We present molecular dynamics (MD) simulations of a single poly(N-isopropylacrylamide) (PNIPAM) chain in explicit water at temperatures between 270 and 320 K near the lower critical solution temperature (LCST). The force-fields of OPLS-AA and TIP4P/2005 are used for a PNIPAM chain and water molecules, respectively. Three independent simulations with durations of 1 μs are performed at each temperature for a 30-mer PNIPAM chain starting with three distinct conformations: extended, loosely collapsed, and tightly collapsed states. The simulation trajectories exhibit reversible conformational transitions between swollen- and collapsed-chain conformations, which has rarely been reported in previous simulation studies, with the overall transition occurring at different temperatures depending on the initial conformation. The inconsistency of the transition temperatures depending on the initial conformation implies that, in spite of the simulation duration of 1 μs distinctly longer than that in previous simulation studies, the conformational sampling from the MD simulations is not enough to draw conclusions on equilibrium properties. Instead of evaluating average properties, therefore, the focus is on dynamic changes in the chain conformation during reversible collapse-swelling transitions at each temperature. The simulation trajectories are analyzed in terms of the radius of gyration, intrachain distances, hydrophobic contacts, and chain-water and intrachain hydrogen bonding. In particular, the formation of stable intrachain hydrogen bonds is a signature of the tightly collapsed-chain conformations that persist, once formed, for the entire simulation duration.