Temperature dependence of the structure of protein hydration water and the liquid-liquid transition

S. R. ACCORDINO; D. C. MALASPINA; J. A. RODRÍGUEZ FRIS; L. M. ALARCÓN; G. A. APPIGNANESI

Buenos Aires

Workshop; Workshop on "Structure and Dynamics of Glassy, Supercooled and Nanoconfined Fluids"; 2012

We study the temperature dependence of the structure and orientation of the first hydration layers of the protein lysozyme and compare it with the situation for a model homogeneous hydrophobic surface, a graphene sheet. We show that in both cases these layers are significantly better structured than bulk water. The geometrical constraint of the interface makes the water molecules adjacent to the surface to lose one water-water hydrogen bond and to expel the fourth neighbors away from the surface, lowering local density. We show that a decrease in temperature improves the ordering of the hydration water molecules but preserving such geometrical effect. For the case of graphene, this favors an ice Ih-like local structuring, similarly to the water/air interface but in the opposite way along the c-axis of the basal plane (while the vicinal water molecules of the air interface orient a hydrogen atom towards the surface, the oxygens of the water molecules close to the graphene plane orient a lone pair in such direction). In turn, the case of the first hydration layers of the lysozyme molecule is shown to be more complicated, but still displaying signs of both kinds of behavior, together with a tendency of the proximal water molecules to hydrogen bond to the protein both as donors and as acceptors. Additionally, we make evident the existence of signatures of a liquid-liquid transition (Widom line crossing) in different structural parameters at the temperature corresponding to the dynamic transition incorrectly referred to as “the protein glass transition”.