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Report No.

Hydration dependent protein dynamics by incoherent neutron scattering

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

Protein dynamics in a solvated sample are strongly coupled to their environment. The dynamical transition and boson peak of a protein were examined in relation to hydration using staphylococcal nuclease. A dynamical transition of protein around 230K is observed only for the hydrated protein. It is demonstrated that the functions of some proteins are suppressed with the loss of anharmonic dynamics as the proteins are cooled down below the dynamical transition temperature. Hydration level dependence of the dynamical transition was examined. The dynamical transition was observed at higher hydration level, 0.26gwater/gprotein. The previous work reported that about 0.2 gwater/gprotein hydration is necessary for the protein function. This suggests that dynamical transition is important for protein function. On the other hand, below the 150K, even low hydration affects the harmonic vibration of protein. At low temperature the protein boson peak was observed. Although the boson peak is commonly observed in synthetic polymers, glassy materials and amorphous materials, the origin of the boson peak has not been fully understood. The motions that contribute to the peak are harmonic vibrations. Upon hydration the peak frequency shifts to a higher frequency and the effective force constant of the vibration increases at low temperatures, suggesting that the protein energy surface is modified. Hydration of the protein leads to a more rugged potential surface and the vibrational motions are trapped within a local minimum at cryogenic temperatures. The origin of the protein boson peak is related to this rugged energy surface and the distribution of low-energy vibrations.



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