Pairwise Hydrophobicity at Low Temperature: Appearance of a Stable Second Solvent-Separated Minimum with Possible Implication in Cold Denaturation.

The hydrophobic effect appears to be a key driving force for many chemical and biological processes, such as protein folding, protein-protein interactions, membrane bilayer self-assembly, and so forth. In this study, we calculated the potential of mean force (PMF) using umbrella sampling technique between different model hydrophobes (methane-methane, cyclobutane-cyclobutane, and between two rodlike hydrophobes) at lower than ambient temperatures (300, 260, and 240 K). We find the appearance of a second solvent-separated minimum at ∼1.0 nm apart from the usual contact and first solvent-separated minimum in the PMF profile of the methane pair at low temperature. In the PMF between both cyclobutane and the rodlike hydrophobe pairs, the second solvent-separated pair (SSSP) becomes even more stable than the first solvent-separated pair (FSSP) at 240 K. Analysis of the water structure shows that, at 240 K, the core water of SSSP for the rodlike hydrophobe pair is more strongly hydrogen bonded and more tetrahedrally oriented than that of the FSSP. Strongly hydrogen-bonded ordered water molecules implicate strong water-water interactions, which are responsible for stabilization of SSSP at low temperature. This weakening of hydrophobic interactions through stabilization of SSSP may play a key role in the cold denaturation of protein.