Low-temperature and high-pressure induced swelling of a hydrophobic polymer-chain in aqueous solution

We report molecular dynamics simulations of a hydrophobic polymer-chain in aqueous solution between 260 K and 420 K at pressures of 1 bar, 3000 bar, and 4500 bar. The simulations reveal a hydrophobically collapsed structure at low pressures and high temperatures. At 3000 bar and about 260 K and at 4500 bar and about 260 K, however, an abrupt transition to a swelled state is observed. The transition is driven by a smaller volume and a remarkably strong lower enthalpy of the swelled state, indicating a steep positive slope of the corresponding transition line. The swelling is strongly stabilized by the energetically favorable state of water in the polymer's hydrophobic first hydration shell at low temperatures. This finding is consistent with the observation of a positive heat capacity of hydrophobic solvation. Moreover, the slope and location of the estimated swelling transition line for the collapsed hydrophobic chain coincides remarkably well with the cold denaturation transition of proteins.