Water structure around hydrophobic amino acid side chain analogs using different water models

The water structure around hydrophobic groups governs various biochemical processes. There is an ongoing debate on whether water molecules near hydrophobic groups are more ordered with greater participation in water-water hydrogen bonding with respect to water in the pure bulk state. The water structure around six different hydrophobic amino acid side chain analog molecules has been studied in pure water using molecular dynamics simulations. The analysis of water tetrahedral order parameter and the number of hydrogen bonds formed by the individual water molecules in the first hydration shell of the hydrophobic analogs provide evidence that both ordering and hydrogen bonds involving water molecules are to some extent reduced in the hydrophobic hydration shell. It is revealed that the water tetrahedrality in the outer part of the first hydrophobic hydration shell is equivalent to bulk water for all the water models except for the TIP4P-2005 model which shows marginally higher tetrahedrality. However, irrespective of the model employed, water tetrahedrality has always been found to be reduced in the inner part of the first hydration shell, which eventually makes the overall water tetrahedrality in the first hydrophobic hydration shell marginally lower than that observed for pure bulk water. Importantly, it is noticed that the decrease in water structuring exhibits solute size dependencies. Around a small solute like methane, the water tetrahedral ordering or hydrogen bonding propensity is quite similar to that of the bulk state. The effect, reduction in water structuring, is however more pronounced for relatively larger solutes.