Low frequency anomaly in the hydration water of Copper Azurin

Abstract

A molecular-dynamics (MD) simulation of hydrated Copper Azurin has been performed at different temperatures to study the single-particle dynamics of water surrounding the protein. The self-intermediate scattering function I(q, t) of the water hydrogen atoms, is characterized by a two step relaxation behaviour at low temperature, where the fast relaxation is distinctly separated from the slower one by a plateau. Such a feature is in qualitative agreement with that recently obtained by MD studies on supercooled bulk water. The dynamical structure factor of hydration water, derived by a time Fourier transform of I(q, t) , shows a broad peak in the inelastic low-frequency region (∼1.3 meV); such a peak being particularly evident at low temperatures. This inelastic peak, whose intensity shows a q 2 dependence, is reminiscent of the so-called boson peak, experimentally observed in several amorphous disordered materials. An additional evidence of this boson peak is provided by the presence of an excess of modes, with respect to the Debye model, in the hydration water vibrational density of states g( ν), which has been calculated from the spectral density of the hydrogen velocity autocorrelation function. The reliability of the present MD simulations is further emphasized by the good agreement of the calculated dynamical susceptibility with that recently obtained by scattering experiments in similar systems. Possible implications of the hydration water low-frequency inelastic anomalies in the protein–solvent dynamical coupling mechanism are briefly discussed.