  Atomic displacements of hydrated proteins are dominated by phonon vibrations
at low temperatures and by dissipative large-amplitude motions at high
temperatures. A crossover between the two regimes is known as a dynamical
transition. Recent experiments indicate a connection between the dynamical
transition and the dielectric response of the hydrated protein. We analyze two
mechanisms of the coupling between the protein atomic motions and the
protein-water interface. The first mechanism considers viscoelastic changes in
the global shape of the protein plasticized by its coupling to the hydration
shell. The second mechanism involves modulations of the motions of partial
charges inside the protein by electrostatic fluctuations. The model is used to
analyze mean square displacements of iron of metmyoglobin reported by
Moessbauer spectroscopy. We show that high flexibility of heme iron at
physiological temperatures is dominated by electrostatic fluctuations. Two
onsets, one arising from the viscoelastic response and the second from
electrostatic fluctuations, are seen in the temperature dependence of the mean
square displacements when the corresponding relaxation times enter the
instrumental resolution window.
