Molecular-dynamics simulation evidences of a boson peak in protein hydration water

Molecular-dynamics (MD) simulations of a hydrated protein system, performed at different temperatures, allowed us to point out anomalies in the low-frequency spectral features of hydration water. The dynamical structure factor calculated from the water MD trajectories shows, below 180 K, a broad inelastic peak in the low-frequency region $\left(\sim 1.3\mathrm{meV}\right)$ reminiscent of the so-called boson peak observed in amorphous disordered materials. Additional evidence of this boson peak is provided by the calculated vibrational density of states. The behavior of the simulated dynamical susceptibility at various temperatures was found to be very similar to that recently obtained by scattering experiments in similar systems. Possible implications of these anomalies in the protein-solvent coupling mechanisms are briefly discussed.