The X-ray absorption spectroscopy Debye-Waller factors of an iron compound and of met-myoglobin as a function of temperature

Protein dynamics can be characterized by the mean square displacements of the individual atoms of a molecule. This concept is extended to X-ray absorption spectroscopy (XAS) of proteins where the physical information in the Debye-Waller factor is in general neglected. In a first step, a procedure for the investigation of the temperature dependence of XAS spectra has been developed for a small iron compound. Subsequently, experiments have been performed on met-myoglobin. It is shown that the mean square displacements of XAS are smaller than those obtained by Mössbauer spectroscopy and far smaller than crystallographic mean square displacements. This behavior is explained by the different sensitivity of the methods. XAS measures a relative mean square displacement between the absorbing and backscattering atoms only. A comparison with mean square displacements calculated from normal modes shows that static displacements contribute significantly. It becomes obvious that the atoms of the active center show a high correlation of their motions.