Structure and dynamics of the protein hydration water at the protein dynamical transition

Nakagawa, Hiroshi   ; Jochi, Yasumasa*; Kitao, Akio*; Kataoka, Mikio

Most globular proteins work in an aqueous milieu and water molecules located at the protein surface strongly affects the protein stability and function. Structure and dynamics of hydration water on the protein, staphylococcal nuclease, were examined at various hydration levels to study the hydration effect on dynamics. We found that the hydration dependent protein dynamical transition around 240 K shows the threshold hydration level between 0.30 and 0.42 (g water/g protein). Below the threshold, hydration water is localized to form several independent clusters on the surface, while above the threshold level the hydration water encircles the protein via hydrogen bond, suggesting the percolation transition. The MD simulations at various hydration indicate that the size of the cluster of hydrogen bonded hydration water dramatically increased above the threshold level of the hydration level. We concluded that the dynamics of hydration water is coupled with the protein dynamics via hydrogen-bond network on the protein-water interface. Such a dynamical coupling drives the hydration dependent protein dynamical transition.