Role of the solvent in the dynamical transitions of proteins:The case of the lysozyme-water system

We study the dynamics of hydration water in the protein lysozyme in the temperature range 180 K < T < 360 K using Fourier-transform-infrared and nuclear magnetic resonance (NMR) spectroscopies. By analyzing the thermal evolution of spectra of the OH-stretching vibration modes and the NMR self-diffusion ( D S ) and spin-lattice relaxation time ( T 1 ) , we demonstrate the existence of two dynamical transitions in the protein hydration water. Below the first transition, at about 220 K , the hydration water displays an unambiguous fragile-to-strong dynamic crossover, resulting in the loss of the protein conformational flexibility. Above the second transition, at about 346 K , where the protein unfolds, the dynamics of the hydration water appears to be dominated by the non-hydrogen-bonded fraction of water molecules.