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Bacterioferritin isolated from Escherichia coli is of two kinds: a protein containing a polynuclear iron compound, the bacterioferritin proper and a protein free of the polynuclear iron compound, the apo-bacterioferritin. Bacterioferritin of both kinds is characterized by absorption maxima at 417,530 and 560 nm, contributed by protohaem IX. Single crystals of bacterioferritin of the space group I432 suggest that the molecule is made up of 24 identical subunits related by a cubic point symmetry. The molecular weight of the protein subunit, as determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, is 15000. In the electron microscope the bacterioferritin molecule appears to be a sphere of 9.5 nm (95 A) diameter composed of a negatively staining outer shell and an inner electron-dense core of 6 nm (60 A) diameter.

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The 57Fe v-ray resonance absorption spectra have been measured in crystals of metmyoglobin and deoxymyoglobin over a wide range of temperatures. Above a critical temperature common to both proteins (220 K), the dynamics of heme iron display a dramatic change, in that two kinds of thermal fluctuations come into play-a fast fluctuation associated with a steep decrease of the total resonance absorption with increasing temperature, and a slower fluctuation of characteristic time 10-8 sec, associated with bounded diffusive motion. By using both discrete jump and continuous diffusion models, the latter based on the Brownian motion of an overdamped harmonic oscillator, the essential parameters of the iron motion (mean square displacement andjump frequency or diffusion constant) can be derived as a function oftemperature. Thus, for deoxy Mb at 288 K, the mean square displacement for the fast fluctuation is about 6 x 10-2 A and for the diffusive motion is 1.6 x 10-2 A2; the diffusion constant is 4 X 10-10 cm2/sec. The diffusive process is associated with an activation energy ofabout 0.75 kcal/mol. Although the same general kinds of phenomena are observed in crystals of MetMb and deoxy Mb, significant differences in behavior are found, which suggest that the main dynamical phenomenon observed reflects internal large-scale motions of the protein.There has been a rapidly growing interest in recent years in the study of the dynamics of structural fluctuations and conformational changes in proteins (1). These studies, which extend our knowledge beyond the time-averaged models of protein structure obtained by x-ray diffraction from protein crystals, are important in understanding aspects of the biological function of proteins. A variety of techniques, appropriate to different time scales, have been used in experimental investigations of the dynamics of proteins (1). The analysis of the x-ray diffraction temperature factors from crystals of some globular proteins (2, 3) has provided valuable information concerning the mean square displacement (msd), (x2), of atoms in different parts of the protein.The dynamics of active sites in proteins are of particular interest. In this respect, Mossbauer resonance absorption spectroscopy has been especially useful in the study ofthe dynamics of iron atoms in iron proteins. From the recoilless fraction f, estimates of the msd of the iron atoms can be obtained. Information on the dependence of (x2) on temperature has been obtained in this way for oxy Mb in frozen solution (4) and for crystals of MetMb (5, 6) and deoxy Mb (7). Rayleigh-Mossbauer scattering also has been used to investigate the dynamics of Mb (8,9). Recent investigations of the 57Fe y-ray absorption spectrum over an extended range of scanning velocities in crystals of the proteins ferritin (10-12), MetMb (11,12), and HbCO (13) have revealed striking new phenomena highly pertinent to the dynamics of iron in these proteins.In this paper we present the results ofa detailed investigation and comparison of these phenomena i...

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