Low-Temperature and High-Pressure Induced Swelling of a Hydrophobic Polymer-Chain in Aqueous Solution

TL;DR

This study uses molecular dynamics simulations to explore how a hydrophobic polymer in water swells under high pressure and low temperature, revealing a transition driven by water's energetic state and resembling protein cold denaturation.

Contribution

It demonstrates the pressure-temperature conditions causing swelling of hydrophobic polymers and links this behavior to protein cold denaturation phenomena.

Findings

Swelling occurs at high pressure and low temperature.

Swelling is stabilized by water's energetic state around the polymer.

Transition line has a steep positive slope, similar to protein cold denaturation.

Abstract

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.