Hydration shell of the TS-Kappa protein: Higher density than bulk water

• We studied the density and H-bonds behavior of water in a TS-Kappa protein system.
• The density of water near to protein is higher than the bulk water.
• The H-bonds between water molecules increase with the decrease of temperature.
• The water around the hydrophobic sites are highly bonded between them.
• The water around hydrophilic sites are more connected with the protein surface.

The density of the water molecules in the presence of hydrophobic and hydrophilic amino acids was studied. Molecular dynamic simulation were employed to analyze the behavior of hydrated TS Kappa protein in SPC/E and TIP4P-2005 water models. The simulations were performed in the NPTNPT ensemble with the Nosé–Hoover thermostat and the Parrinello–Rahman barostat. The density profile of these systems were obtained for different temperatures at constant pressure. Two complementary phenomena were observed. The protein–water system exhibits a temperature of maximum density lower than the temperature observed in the pure water system. The densities of the water in vicinity of the hydrophobic and hydrophilic sites are higher than the density of the water in the bulk. Our results suggest that interactions between protein and water and the water–water Hydrogen bonds are essential to the understanding of these phenomena.