[Thermal movements of proteins: small-scale fluctuation and conformation substates]

A comparison is made of the published data on different physical methods concerning small-scale (small-amplitude) thermal motions, conformers and conformational substates of myoglobin and some other proteins in crystal and solution. Because of the underestimation of the statical and dynamical rotational disorder of the crystal lattice, the X-ray diffraction method may somewhat overestimate the amplitude values of the intramolecular thermal motions, which leads to the underestimation of the extent of conformational rigidity of the protein molecule. In solution the rigid-body rotation of the macromolecules makes a significant contribution to the apparent amplitude of atomic displacement. The damping of these motions upon the increase of viscosity or freezing of solution, crystallization or intramolecular crosslinking can partially simulate the depression of intramolecular thermal motions. According to infrared spectroscopy data the crystallization of carboxymyoglobin as well as temperature decrease down to 5K are not accompanied by preferential stabilization of one of the several discrete conformers of the active site or by diminished distribution breadth of conformational substates within individual conformers. The structural differences between these discrete conformers and the more so between conformational substates are not sizable and are confined within a limit of the amplitude of thermal motion in the polymeric monocrystal including their paracrystalline regions. The dehydration of the hemoglobin and myoglobin films leads to the loss of conformational rigidity of the active site and to the distortion of its native conformation, i.e. to the inactivation of protein.