Protein hydration water: Focus on low density and high density local structures upon cooling

We report results about the structure of protein hydration water upon cooling compared to bulk water. We focus on the analysis of the local structure through order parameters: the tetrahedral order parameter q and a recently introduced parameter . Both parameters show that, similarly to bulk water, protein hydration water is a mixture of two liquids already at ambient conditions. These liquids are a low density liquid (LDL) characterized by a more tetrahedral-ordered local structure and a high density liquid (HDL) that is more distorted. Protein hydration water has a higher concentration of HDL with respect to LDL if compared to the bulk case upon cooling. The two water populations of hydration water are also less sensitive to temperature variations with respect to bulk water. The presence of the protein affects the structure of hydration water that is locally more distorted than bulk water. Both the parameters display a two-liquid picture of hydration water. Correlations between the two parameters are also investigated. In spite of the differences found in hydration water with respect to the bulk the bimodal structure is importantly preserved.