Low frequency vibrational anomalies in hydrated copper azurin: a neutron scattering and MD simulation study

Azurin, a P-sheet copper containing protein, has been studied by inelastic neutron scattering at different temperatures in a dry and a deuterium hydrated state. At low temperature, an excess of vibrational modes, centered at about 3 meV, has been registered in both the dynamical structure factor and density of states of the hydrated sample; a similar bump, but slightly shifted to a lower frequency, having been observed in the dry sample. The inelastic peak is found to be very similar to the so-called "boson peak", a common feature of amorphous, glassy, systems. The most relevant features of the spectra have been well-reproduced by molecular dynamics simulations; in particular, the inelastic bump, although peaking at a lower frequency, has been detected in both the simulated dynamical structure factor and density of states, a detailed analysis, exploiting the atomic resolution of molecular dynamics simulation, has been performed to single out the contribution, to both the quasielastic and the inelastic spectral components, arising from the different mobility classes of protein protons. Such an approach has revealed that the main contribution to the inelastic peak arises from protein protons with low mobility; suggesting a possible connection between the inelastic bump and the collective vibrational motions involving elements of the secondary structure of the macromolecule. (C) 2000 Elsevier Science B.V. All rights reserved.