Dynamics of metmyoglobin crystals investigated by nuclear gamma resonance absorption

This paper demonstrates the applicability of the Mössbauer effect to the study of the dynamic behaviour of proteins. The Lamb-Mössbauer factor f′ of a polycrystalline sample of metmyoglobin was measured between 4·2 K and 293 K. The experiments revealed two distinct temperature regions with different motional behaviour. The measurements indicate in particular that above 210 K a new motional degree of freedom appears in addition to the usual vibrations. Such additional motions are specific for biomolecules in their active state. They are absent from dry myoglobin and may be described as fluctuations between slightly different conformational substates of the molecule. Simple models for a description of fluctuations between the conformational substates are discussed. The characteristic linear dimension involved in such fluctuations is at least 0·2 Å. The latter dimension defines a resolution limit for X-ray structure determination.