Synchrotron X-ray powder diffraction study of hexagonal turkey egg-white lysozyme

The structure of turkey egg-white lysozyme (TEWL) has been refined from high-resolution X-ray powder diffraction data. The sample was rapidly obtained as a polycrystalline precipitate at high protein concentration using 0.5 M NaCl solvent pH 6 and was deposited in the PDB with code 1xft. The diffraction data were collected at room temperature. Molecular replacement was shown to give a suitable starting point for refinement, illustrating that powder data can be sufficient for this approach. Crystallographic models were then refined by combined Rietveld and stereochemical restraint analysis of the powder data (d_min = 3.35 Å), resulting in the extraction of reliable lattice parameters and the refinement of the molecular conformation at room temperature. The structure is hexagonal [space group P6₁22, unit-cell parameters a = 71.0862 (3), c = 85.0276 (5) Å] with 12 symmetry-related molecules in the unit cell, in agreement with previous studies. The results of our analysis are indicative of specific amino acids being disordered at this temperature. Upon cooling, a sudden drop in the lattice parameters at ∼250 K is observed concurrently with the freezing of the mother liquor. The observation of severe peak broadening below this temperature indicates strain effects accompanying the freezing transition, which are found to be reversible. Finally, a correlation between the unit-cell parameters and the pH of the buffer solution is evident, in a similar manner to earlier observations on HEWL.