Dynamical transition of myoglobin in a crystal: comparative studies of X-ray crystallography and Mossbauer spectroscopy

The crystallographic normal mode refinements of myoglobin at a wide range o f temperature from 40 K to 300 K were carried out to study the temperature dependence of the internal atomic fluctuations. The refinement method decom poses the mean square displacement from the average position, (Deltar(2)), into the contributions from the internal degrees of freedom and those from the external degrees of freedom. The internal displacements show linear tem perature dependence as (Deltar(2)) = alphaT + beta, throughout the temperat ure range measured here, and exhibit no obvious change in the slope a at th e dynamical transition temperature (T-c = ca. 180 K). The slope alpha is pr actically the same as the value predicted theoretically by normal mode anal ysis. Such linear dependence is considered to be due to the following reaso n. The crystallographic Debye-Waller factor represents the static distribut ion caused by convolution of temperature-dependent normal mode motions and a temperature-independent set of the conformational substates. In contrast, Mossbauer absorption spectroscopy shows a clear increase in the gradient a lpha at T-c. This difference from X-ray diffraction originates from the inc oherent nature of the Mossbauer effect together with its high-energy resolu tion, which yields the self-correlation, and the temporal behavior of indiv idual Fe atoms in the myoglobin crystal.