  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 state at low pressures and high temperatures. At $3000 {bar}$ and
about $260 {K}$ and at $4500 {bar}$ and about $260 {K}$, however, a 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 stabilized
almost completely by the energetically favorable state of water in the polymers
hydrophobic first hydration shell at low temperatures. Although surprising,
this finding is consistent with the observation of a positive heat capacity of
hydrophobic solvation. Moreover, the slope and location of the observed
swelling transition for the collapsed hydrophobic chain coincides remarkably
well with the cold denaturation transition of proteins.
