An in-depth knowledge of protein dynamics is essential for a thorough understanding of their functionality. Among the wide range of motions - from the femtoseconds to seconds time scale- that proteins sustain, the term “protein collective dynamics” refers to the intricate patterns of coordinated motions of a large fraction of protein atoms in the sub-picosecond time scale, which are hypothesized to be involved in functional dynamical mechanisms.

Using the theoretical framework of hydrodynamics, the collective dynamics of proteins had previously been described in a manner akin to that of simple liquids, i.e. in terms of a single acoustic-like excitation, related to intra-protein (secondary structure) vibrational motions. We investigate herein the intra-protein collective dynamics of a model globular protein by analysing -in terms of an interacting phonon model- its longitudinal and transverse modes, calculated from molecular dynamics simulations. This approach allows us to reveal a complex low-frequency vibrational landscape, populated by multiple acoustic-like and low-frequency optic-like modes, with mixed symmetry and interfering with each other.

We propose an interpretation of the observed collective dynamical behavior, by means of a correlation to the structure of the investigated protein and an evaluation of the origin of the excitations. Finally, we discuss the coupling between the collective dynamics of proteins and their hydration water, which is particularly effective in the terahertz frequency range. The present findings, likely to be encountered for all globular proteins, provide a molecular-level perspective for describing energy-transfer mechanisms in proteins and their hydration 
environment.