Investigation of X-ray induced radiation damage in proteins, nucleic acids and their complexes
Keywords: X-ray radiation
Electron density
Nucleic acids
Macromolecular crystallography

Macromolecular X-ray crystallography (MX) is currently the dominant technique for the structural eluci- dation of macromolecules at near atomic resolution. However, the progression and deleterious effects of radiation damage remains a major limiting factor in the success of diffraction data collection and subsequent structural solution at modern third generation synchrotron facilities. For experiments conducted at 100 K, protein specific damage to particular amino acids has been widely reported at doses of just several MGy, before any observable decay in average diffraction intensities. When undetected, such artefacts of X-ray irradiation can lead to significant modelling errors in protein structures, and ultimately the failure to derive the correct biological function from a model. It is thus vital to develop tools to help MX experimenters to detect and correct for such damage events. This thesis presents the development of an automated program, RIDL, which is designed to objectively quantify radiation-induced changes to electron density at individual atoms, based on Fobs,n − Fobs,1 Fourier difference maps between different dose states for a single crystal. The high-throughput RIDL program developed in this work provides the ability to systematically investigate a wide range of macromolecular systems.
To date, damage to the broad class of nucleic acids and nucleoprotein complexes has remained largely uncharacterised, and it is unclear how radiation damage will disrupt the validity of such models derived from MX experiments. This thesis presents the first systematic investigations on a range of nucleic acid, protein-RNA and protein-DNA complex case studies. In general, it is concluded that nucleic acids are highly robust to radiation damage effects at 100K, relative to control protein counterparts across the tested systems.
For protein crystals at 100K, cleavage of the phenolic C–O bond in tyrosine has disseminated through the MX radiation damage literature as a dominant specific damage event at 100K, despite the absence of any energetically favourable cleavage mechanism. To clarify the radiation susceptibility of tyrosine, this thesis presents a systematic investigation on radiation damage to tyrosine in a wide range of MX protein radiation damage series retrieved from the Protein Data Bank. It is concluded that the tyrosine C – O bond remains intact following X-ray irradiation, however the aromatic side-group can undergo radiation-induced displacement.
This thesis also presents further applications of the RIDL program. A protocol is introduced to calculate explicit half-dose values for the electron density at individual atoms to decay to half of their initial value at zero absorbed dose. In addition, a methodology is developed to detect radiation-induced changes to electron density occurring over the course of the collection of a single MX dataset of diffraction images, all of which are required for structural solution. These protocols aim to advise experimenters of when previously-undetected site-specific damage effects may have corrupted the quality of their macromolecular model.
Overall, the work in this thesis is highly applicable to both the future understanding of radiation damage in macromolecular structures, as well as of interest to the wider crystallographic community.