Electron Paramagnetic Resonance (EPR) spectroscopy and related techniques are powerful tools for the characterization of paramagnetic compounds, providing detailed insights into their electronic, magnetic, and geometric properties. In the present thesis, such techniques are applied to investigate electron-nuclear interactions in both transition metal complexes and main group radicals.

Chapter 2.3 revolves around lytic polysaccharide monooxygenases (LPMOs), which are a class of enzymes that oxidatively degrade polysaccharides. Their active sites are composed of a single copper ion coordinated by the so-called histidine brace, a motif that has attracted considerable attention for its ability to stabilize the highly oxidizing intermediates that are generated during catalysis.
In the present work, the bacterial LPMO SmAA10A was characterized between pH 4.0 and pH 12.5 by a range of pulsed EPR techniques, namely Electron Nuclear Double Resonance (ENDOR), Electron Spin Echo Envelope Modulation (ESEEM) and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy, which allowed to fully map out the LPMO’s active site. It also enabled the identification of changes in the solvent coordination and deprotonation events in the histidine brace, in dependence of the pH. The findings shed light on the properties of the individual features of the histidine brace (e.g. pKa values of the respective amino acid residues) and provided insights into potential stabilization mechanisms for reaction intermediates. As the first full characterization of an LPMO’s active site with advanced EPR techniques, it also set a foundation for future mechanistic studies.
To investigate copper-oxygen intermediates emerging during the catalysis of LPMOs or other copper enzymes involved in O₂ conversions, the identification of H_xO ligands is crucial.

In chapter 2.4, ENDOR and Electron-Electron Double Resonance (ELDOR) detected NMR (EDNMR) spectroscopies were employed to characterize a series of small copper complexes with H₂O and OH^– ligands in various positions. Through employment of ²H and ¹⁷O enriched water, general and systematic trends
were established that can help to differentiate between 1) equatorial and axial H_xO ligands and 2) equatorial H₂O and OH^– ligands. Additionally, Density Functional Theory (DFT) calculations revealed potential influences of trans-positioned ligands on the ¹⁷O hyperfine couplings of H_xO ligands, setting a foundation for the understanding of other copper oxygen systems.

While hyperfine techniques, such as the above described ENDOR, ESEEM, HYSCORE and EDNMR help resolve small nuclear interactions, other techniques are required to probe systems with large couplings. This can be studied in heavy main group radicals, such as As, Sb and Bi centered radicals, of which only few stable molecular compounds are generally known. Chapter 3.3 describes the EPR characterization of a variety of As and Sb centered radicals, for which multi-frequency approaches allowed the determination of their extremely large hyperfine interactions, and thereby draw conclusions about spin distributions within the respective radicals. This chapter also discusses challenges in the acquisition and interpretation of EPR spectra of Bi centered radicals. For many of such compounds a multi-frequency approach fails.

Instead, the applicability of parallel-mode EPR, a technique that is traditionally almost exclusively employed for S>1/2 systems, is explored for paramagnetic Bi compounds in chapter 3.4. Additional information was retrieved by this technique for bismuth-doped silicon (Bi:Si) and two Bi radicals, which in combination with multi-frequency data, resolved their full g- and A(²⁰⁹Bi)-tensor. This approach also opens new possibilities for other S=1/2 systems with large hyperfine couplings.