A Low-Resolution Low-Temperature Neutron Diffraction Study of Myoglobin

Diffraction data to 5 Å resolution were collected on a myoglobin crystal at 80, 130, 180 and 240 K. The linear coefficient of thermal expansion for myoglobin was determined to be 45 × 10⁻⁶ K⁻¹, based on the measured expansion of the unit-cell parameters. The nature of the hydration layers surrounding the protein in the crystal is described in terms of a shell solvent model, which was used to calculate the coefficient of thermal expansion in reasonable agreement with the measured value. Wilson statistics were calculated and discussed in terms of an averaged disorder model. [F(T₂) − F(80 K) exp(−iφ)] Fourier maps were calculated where T₂ was taken as 130, 180 and 240 K, respectively. None of these difference maps showed any features above 2.0σ in the protein region. The 130 and 240 K difference maps showed many small and widely distributed negative difference features and showed very few positive difference features above 2.5σ in the solvent region. However, the 180 K difference map showed an extensive negative difference feature at the interface between symmetry-related molecules, occurring in the vicinity of residues 40-50 on one molecule and 76-80 on a symmetry-related molecule. These difference neutron Fourier maps indicate a concerted effect at 180 K, which is interpreted in terms of an onset of extended lattice disorder.