Internal mobility of ferrocytochrome c.

In the refinement of the X-ray diffraction structures of molecules, it is conventional to introduce atomic 'temperature factors' of the Debye-Waller form to characterize the widths of the electron density peaks corresponding to the atoms. Although these factors are known to include a variety of contributions other than thermal fluctuations of the atomic positions, recent progress in the refinement of protein structures has led to inferences concerning atomic mobilities from the temperature factor data for several proteins. Atomic position fluctuations can be calculated independently by the molecular dynamics method, in which the classical equations of motion for the atoms of an equilibrated protein are solved on a computer. We now show that the X-ray diffraction and dynamical simulation methods yield similar pictures of the atomic mobility in tuna ferrocytochrome c.