Theory of electronic processes in Molecules Subject to Intense X-ray Radiation: Towards Single-Molecule X-ray Diffraction Spectroscopy

Understanding of mechanisms guiding the wide variety of biochemical processes in living organisms requires determination of structure of the biomolecules (for example, proteins) taking part in these processes. Indeed, vast scientific resources have been put into trying to unveil protein structure. The spectacular success that has been achieved along this path has contributed immensely to our ability to fight diseases by designing new drugs. Throughout the last century, this success has been marked by numerous Nobel Prizes in Physics and Chemistry. In spite of the remarkable achievements of the present-day methods, the structures of a vast bulk of biomolecules still remain a mystery. The reason is that the prerequisite for application of the main existing technique, called X-ray diffraction, is crystallization of the biomolecules under study. It turns out that obtaining a crystal, that is a geometrically ordered solid structure, out of protein solution is often an extremely difficult task. To overcome this difficulty, some researchers suggested to analyse proteins by X-ray diffraction in gas phase rather than in a crystal. This would certainly require application of much more intense X-ray radiation than the one available today since this way one would try to obtain a picture of a single biomolecule. Fortunately though, new powerful X-ray sources called X-ray free electron lasers are now being built at a number of facilities throughout the world. It has been proposed that the X-ray radiation generated by these new sources will be strong enough to give a picture of a single protein molecule in a single shot. The principal problem with such an approach is that the very same radiation that creates an image of a target molecule can also destroy the target. The fate of the new method of the single-molecule X-ray diffraction hangs on the delicate balance between these two basic consequences of X-ray-molecule interaction: diffraction of the radiation versus decomposition of the molecule. The central problem is to design such a radiation pulse that is strong enough and long enough to provide the diffraction picture of sufficient quality, but still short enough not to cause the molecule to disintegrate during the action of the pulse. Design of such pulses is possible only if one is able to understand in detail all possible mechanisms of molecular decomposition and eventually to model it in a computer simulation. The central goal of the present proposal is exactly this: theoretical study of the electronic processes that lead to disintegration of a molecule under the action of intense X-ray and computer simulation of this process based on the detailed understanding of the underlying physical mechanisms. These mechanisms include ionisation of molecules by the X-ray radiation and various types of rearrangements of the remaining electrons following the ionisation events. Often, such rearrangements lead to a delayed emission of more electrons and the magnitudes of these delays determine eventually the way in which the molecule decomposes and the time scale of the decomposition process. Thus, I plan to invest a considerable effort in trying to predict the time scales of the various electronic rearrangement processes as well as to determine the main factors that affect these time scales. The results of the proposed theoretical work will guide the application of the novel X-ray radiation sources towards towards determination of molecular structure by single-molecule X-ray diffraction.