Hydration related protein dynamics studied by incoherent neutron inelastic scattering

Nakagawa, Hiroshi   ; Jochi, Yasumasa*; Kitao, Akio*; Shibata, Kaoru  ; Go, Nobuhiro; Kataoka, Mikio

Pico-second dynamics of protein and its hydration water were examined by incoherent neutron inelastic scattering with protein sample at various hydration levels. Hydration affects the protein low frequency modes significantly and brings the shift of the boson peak toward higher energy. At cryogenic temperature, the force constant of the harmonic potential for the protein low frequency mode increases in proportion to the hydration level, which is correlated with the boson peak shift. Theoretical study indicated that the hydration water molecules hydrogen-bonded with a protein make the potential surface of protein low frequency collective mode more rugged. The observed boson peak shift is consistent with the theoretical conclusion. Hydration-dependent protein dynamical transition appears around 240K above the hydration level of h=0.20. At every hydration level examined, the mean square displacement of the hydration water is almost identical to that of the protein up to the dynamical transition temperature. While hydration water shows similar dynamical transition to the protein dynamical transition above the threshold hydration level, hydration water never shows the transition below the threshold hydration. The anomalous dynamical behavior of hydration water should be coupled with the protein collective mode via hydrogen-bond network. Such a dynamical coupling drives the hydration dependent protein dynamical transition.