Obtaining diffuse scattering signal from X-ray diffraction experiments on lysozyme

Abstract

For decades, X-ray diffraction has been used to determine protein structures in crystals to atomic level. Nevertheless, while performing X-ray diffraction experiments, the information regarding the dynamics of the protein molecule has been largely ignored. This information is contained in the so called diffuse scattering signal. However, obtaining this diffuse scattering signal remains challenging because of its low intensity and strong overlap with the signal originating from the crystal solvent. In this research we describe how to make suitable protein crystals and how to perform X-ray diffraction experiments to maximize the diffuse scattering signal. Furthermore, we describe methods to mask intense features such as Bragg reflections and ice scattering. Then we demonstrate the mapping and scaling of experimental images to the 3- dimensional reciprocal space. These experimental 3-dimensional reciprocal space maps can be compared to reciprocal space maps of models that represent the diffuse scattering signal. We observe that using method III, sharp features such as Bragg reflections can be removed as good as performing a modefilter. Next, if Any is not able to calculate scalefactors without fluctuations, the mean can be used after strong intensities that do not occur at least 1% as the most frequent occurring intensity have been removed. Using this approach the experimental reciprocal space maps show similar features compared to the model maps concluding that this newly described method can be used to isolate the diffuse scattering signal. Nevertheless, many features in the model map cannot be observed in the experimental map. We suggest that this is rather due to the low photoncount obtained during X-ray diffraction experiments than experimental data processing and asks for a different approach in performing X-ray diffraction experiments