Development of Analysis- and Simulation-Routines for ENDOR Spectroscopy
Doctoral thesis
Date of Examination:2024-06-19
Date of issue:2024-10-04
Advisor:Prof. Dr. Marina Bennati
Referee:Prof. Dr. Marina Bennati
Referee:Prof. Dr. Helmut Grubmüller
Files in this item
Name:PhD_Thesis_240927_AKehl_noCV.pdf
Size:39.7Mb
Format:PDF
Abstract
English
Electron paramagnetic resonance (EPR) spectroscopy allows obtaining structural information on for example biomolecules by investigating the magnetic environment of paramagnetic centers on the atomic to nanoscale. This includes the interaction of the paramagnetic center with other electron and nuclear spins in close vicinity. The hyperfine interactions between electron and nuclear spins can be detected by electron nuclear double resonance (ENDOR) spectroscopy. Usually, structural information is derived from magnetic parameters extracted from experimental spectra by comparison with simulated spectra. Thus, the derived information is limited to the representation of the experiment by the simulation. Advances in experimental set-up constantly increase the resolution of spectra, e.g. by enabling measurements at higher magnetic fields, with higher pulse power or with shaped pulses. This increases the information content of the spectra and more sophisticated and detailed simulations are required, especially when the effect of pulse sequences can no longer be approximated by a line shape function. A suited method to directly quantify these effects are simulations of the spin dynamics. In this thesis the development of a spin dynamics simulation routine for ENDOR spectra is described. While earlier examples of spin dynamic simulations for ENDOR spectra were limited to specific applications, the simulation routine introduced here is more general. It covers simulations of powder patterns and single crystals, couplings to multiple nuclei and different nuclear isotopes. It is also possible to include the pseudo-secular hyperfine interaction and relaxation effects in the simulation. Additionally, effects of the nuclear chemical shielding and inter-nuclear dipole-dipole couplings are considered. These latter effects are usually detected by nuclear magnetic resonance (NMR) spectroscopy, but can now also be identified in ENDOR spectra. An application is presented for the analysis of 19F-ENDOR spectra, where simulations are used to extract the hyperfine interaction of a 19F nuclear spin with an electron spin as a direct measure for the dipolar inter-spin distance. For 19F-ENDOR spectra at high magnetic fields (9.4 T/263 GHz) the chemical shielding has to be taken into account for the spectral analysis, since its anisotropy induces an asymmetry in the ENDOR spectra. Hitherto, this effect has been neglected, but has now been successfully implemented in the simulations. Inclusion of the chemical shielding increases the number of simulation parameters for each 19F spin. Manual optimization of all spin parameters has been possible using prior information from density functional theory (DFT) calculations. However, automated optimization based on Bayesian optimization yielded better results. Both approaches will be demonstrated and discussed. In reality, the conformational flexibility of the investigated molecule induces a distribution of distances observed in the experiment. Extracting this distribution will provide further information on the sample. For ENDOR spectra a distance distribution contributes to the observed line width as an additional broadening. It has to be separated from other, intrinsic line broadening contributions such as power broadening and relaxation effects. Here, it is demonstrated that the latter contributions can be identified using spin dynamics simulations, allowing a subsequent identification of distance distributions.
Keywords: EPR; Hyperfine Spectroscopy; Spectral Simulation; 19F-ENDOR