Structure Determination From Single Molecule X-Ray Scattering Experiments using Photon Correlations
by Benjamin von Ardenne
Date of Examination:2017-10-18
Date of issue:2018-06-18
Advisor:Prof. Dr. Helmut Grubmüller
Referee:Prof. Dr. Marcus Müller
Referee:Prof. Dr. Holger Stark
Persistent Address:
http://dx.doi.org/10.53846/goediss-6928
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Description:Thesis Benjamin von Ardenne - Structure Determination From Single Molecule X-Ray Scattering Experiments using Photon Correlations
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Abstract
English
Scattering experiments with femtosecond high-intensity free-electron laser pulses provide a new route to macromolecular structure determination without the need for crystallization at low material usage. In these experiments, the X-ray pulses are scattered with high repitition on a stream of identical single biomolecules and the scattered photons are recorded on a pixelized detector. The main challenges in these experiments are the unknown random orientation of the molecule in each shot and the extremely low signal to noise ratio due to the very low expected photon count per scattering image, typically well below the number of over 100 photons required by available analysis methods. The latter currently limits the scattering experiments to nano-crystals or larger virus particles, but the ultimate goal remains to retrieve the atomic structure of single biomolecules. In light of that goal, here I present a correlation-based approach that can determine the molecular structure de novo from as few as three coherently scattered photons per image. I derive for the first time an analytic expression of the full three-photon correlation as a function of the molecules Fourier intensity using a spherical harmonics expansion and propose a Monte Carlo simulated annealing approach to solve the inverse problem of finding an intensity that fits the experimentally observed triple correlations. The size of the search space is reduced by using information from the analytic inversion of the two-photon correlation and the electron density is retrieved by applying an iterative phase retrieval method to the determined intensity. Using synthetic scattering data of a small protein (46 residues) at realistic average photons counts of 10 photons per image, I demonstrate that near-atomic resolution of 3.3 Å can be achieved using 3.3 · 109 images, which is within experimental reach. Remarkably, the data acquisition time required to achieve the same resolution decreases to minutes if the average number of photons per image is increased to only 100 photons (equivalent to a decrease in the number of images by a factor 1000). The noise levels in the experiment are expected to be quite high which is a challenge for all structure determination methods. To address this issue, I demonstrate that my three-photon correlation approach is robust to isotropic noise from incoherent scattering, and that the number of disordered solvent molecules attached to the macromolecular surface should be kept at a minimum.
Keywords: structure determination xfel single molecule photon correlations
