Origin of the low-frequency modes of globular proteins

The incoherent structure factor associated with the nonexchangeable hydrogen atoms of carbomonoxy myoglobin at 80 and 200 K in a solvated sample has been computed from molecular dynamics simulations. The evaluated mean square displacements are almost three times larger than the experimental ones. However, the low-frequency band centered at 3 meV, observed in neutron scattering experiments, is reproduced. Evaluation of the contribution to the spectrum from different protein shells and backbone and side chain atoms allows us to propose that the low-frequency mode around 3 meV, which is a constant feature of all globular proteins, is mainly due to the hydrogen atoms belonging to the inner shell, suggesting that this mode is intrinsic to the protein itself.