X-ray diffuse scattering from protein crystals caused by the lattice defects

Abstract

Introduction Recent high resolution (better than 1A) X-ray protein crystallographic analysis requires high quality protein crystals. High quality crystals in turn produce a highresolution diffraction data set. In protein crystallography the quality of the crystal is usually defined as follows: an often-used alternative criterion is whether the crystal produces a high resolution Bragg reflection data set or not. One possible explanation of the quality of protein crystals is whether the protein molecules are orientationally ordered within the unit cell. In that case, the overall B-factor obtained by the Wilson plot technique [1] becomes a good measure of the quality of the protein crystals. As a matter of the fact, we have already made such a proposal and we have proved that the overall B-factors obtained from Wilson plots correlate much better with the quality of the crystals experimentally [1]. In other words, the quality of the protein crystals is discussed by the degree of the disorder of the protein molecules in the unit cell. Generally in X-ray diffraction phenomena it is well known that the disorder of molecules in unit cells provides diffuse scattering under a Bragg reflection profile. In the past such a diffuse scattering from a protein crystal has been often observed and this phenomenon has been discussed from the view point of molecular dynamics. We do not deny that one part of the diffuse scattering is the contribution of molecular dynamics, but we think that the main part is one of the disorder of molecules, incompleteness of crystals. The diffraction pattern on the imaging plate observed at 77K shows very distinctive diffuse scattering under Bragg reflections, which do not depend on temperature. We tried to measure the accurate profile of diffuse scattering in order to analyze the relation between the crystal quality and the molecular disorder.