Multipattern Rietveld refinement of protein powder data: an approach to higher resolution

By using combinations of multiple protein powder diffraction patterns obtained from an image plate to a d -spacing of 2 Å, which differ by solvent-induced and radiation-damage-induced lattice strains, the powder overlap problem is partially resolved in a stereochemically restrained Rietveld refinement. The results for hen egg white lysozyme (HEWL) include, for the first time with powder data, placement of a substantial number of water molecules and structural results that approach the quality normally obtained by single-crystal methods. This study explores the lattice strains induced by changes in salt concentration, changes in solvent pH and the effect of low-dose radiation damage. For HEWL, lattice strains are not monotonic, so that with increasing NaCl concentration (0.25–1.25 M ) the a axis increases by ∼0.5%, while the c axis decreases by ∼1.5%, and this variation is pH dependent. Low-dose radiation damage similarly induces non-monotonic lattice strains, similar but smaller than those arising from increased salt concentration. The effect of using these powder data in a combined Rietveld analysis is effectively to deconvolute the overlapping reflections by differing shifts in their relative positions.