Crossovers in supercooled solvation water: Effects of hydrophilic and hydrophobic interactions

Systematic 8 ns ab initio molecular dynamics (AIMD) were performed to study the structure and dynamics of water in bulk and close to hydrophobic (CH₃) and hydrophilic (carbonyl) groups of tetramethylurea (TMU). Dynamical behaviour showed two crossovers: The first around the hydrophobic group at $T_X=256\pm 4\text{K}$, and the second at $265\pm 5\text{K}$ related to the relative strengths of water-water and water-carbonyl hydrogen bonds (HBs). For bulk water, relaxation times appear to diverge at $T_c=213\pm 10\text{K}$, rendering support to the liquid-liquid critical point hypothesis. To identify the effects due to the hydrophilic carbonyl group, systems of water with one methane molecule were used as references. Our findings are related to the structural and thermodynamic transitions reported for proteins in solution and may play a role in cold denaturation. (letter)