Abstract
Macromolecular crystallography at cryogenic temperatures has so far provided the majority of the experimental evidence that underpins the determination of the atomic structures of proteins and other biomolecular assemblies by means of single crystal X-ray diffraction experiments. One of the core limitations of the current methods is that crystal samples degrade as they are subject to X-rays, and two broad groups of effects are observed: global and specific damage. While the currently successful approach is to operate outside the range where global damage is observed, specific damage is not well understood and may lead to poor interpretation of the chemistry and biology of the system under study. In this work, we present a phenomenological model in which specific damage is understood as the result of a single process, the steady excitation of crystal electrons caused by X-ray absorption, which acts as a trigger for the bulk effects that manifest themselves in the form of global damage and obscure the interpretation of chemical information from XFEL and synchrotron structural research.
Highlights
Radiation damage in X-ray crystallography has been for a long time an active area of research amongst several groups, but a detailed and general understanding of the physical and chemical mechanisms responsible for the appearance of global or specific damage at a microscopic level is still largely incomplete, even in the case of experiments carried out at cryogenic temperatures.Global damage is characterized by loss of sample diffraction power, indicating reduced crystalline order
The model that we propose is based on the wealth of evidence available in the literature [1,7,9,15,16,17,18,19,20] complemented by our recent study of X-ray radiation damage in n-eicosane crystals with quantum-mechanical calculations [14], in which we have demonstrated that the appearance of structural defects following
We have described a new model of X-ray induced damage in macromolecular crystals
Summary
Radiation damage in X-ray crystallography has been for a long time an active area of research amongst several groups, but a detailed and general understanding of the physical and chemical mechanisms responsible for the appearance of global or specific damage at a microscopic level is still largely incomplete, even in the case of experiments carried out at cryogenic temperatures. The relaxation of the lattice, and the consequent formation of a localised defect in the crystal (Figure 1d), are a consequence of the exciton screening by the crystal phonons and they are promoted by the long lifetimes of excitonic states This model is consistent with the results of reference [14], in which hybrid time-dependent density-functional theory (TD-DFT) calculations were used to study the time evolution of the crystal structure of n-eicosane (C20 H42 ) after X-ray irradiation. [14] indicate how these electronic phenomena can be interpreted, and how they subsequently lead to defect formation, crystal symmetry reduction and, potentially, changes in the chemical composition of the system (e.g., bond breaking/formation, evolution of H2 , etc.) [1,6,7,8,15,16,17,18,19,20] This model can be used to interpret specific radiation damage effects in a variety of systems.
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