Protein dynamics: determination of anisotropic vibrations at the haem iron of myoglobin

The phonon assisted Mössbauer effect is used to determine the anisotropic harmonic vibrations labelled by the iron in the active centre of myoglobin at room temperature. A single crystal of metmyoglobin is investigated in five different orientations. Several modes are assigned by the projection of the vibrational amplitude onto the beam directions. The density of phonons below 1 meV shows a quadratic increase with the energy like that in a Debye solid. An anisotropic velocity of sound in the protein crystal is extracted with a mean sound velocity of 1657 m s⁻¹. Modes between 4 and 5 meV are identified as haem sliding motions. Vibrations between 30.2 and 36.5 meV are mainly within the haem plane; those between 19 and 25.6 meV are perpendicular to the plane. These results together with phonon assisted Mössbauer effect measurements on hydrated myoglobin powder and polycrystal samples in the temperature range between 50 K and room temperature are compared with the results of Mössbauer absorption experiments. While the mean square displacements at the iron obtained by the phonon assisted Mössbauer effect increase linearly with temperature up to room temperature, Mössbauer absorption reveals a dynamical transition temperature. Above this temperature a much stronger increase of the mean square displacements occurs, indicating protein specific and functionally important dynamics. Since these displacements are not registered by the phonon assisted Mössbauer effect the involved energy transfer is beyond the energy resolution of the method i.e. smaller than 1 meV. Actually the energy transfer is in the nanoelectronvolt energy regime as seen from the energy profile of the Mössbauer absorption spectrum. Protein specific dynamics can be explained as diffusive motion of molecular segments in limited space.