The electronic structure of transition-metal sites can be probed by electron-paramagnetic-resonance (EPR) spectroscopy. The study of high-spin transition-metal sites benefits from EPR spectroscopy... Show moreThe electronic structure of transition-metal sites can be probed by electron-paramagnetic-resonance (EPR) spectroscopy. The study of high-spin transition-metal sites benefits from EPR spectroscopy at frequencies higher than the standard 9.5 GHz. However, high-frequency EPR is a developing field. In particular it is still difficult to achieve the sensitivity needed to study high-spin transition-metal sites in proteins and enzymes. In this thesis we show that high-quality EPR spectra of mM frozen solutions of high-spin Fe3+ proteins can be obtained at 275.7 GHz in continuous-wave mode using a single-mode cavity. This new possibility is exploited in the study of the high-spin transition-metal sites in the proteins rubredoxin, desulforedoxin and human serum transferrin. Moreover, a multi-frequency EPR study of a high-spin Fe2+ imidodiphosphinate complex and a pulsed ENDOR study at 94.9 GHz of a high-spin Co2+ imidodiphosphinate complex are described. Finally, another spectroscopic technique, namely resonance-Raman spectroscopy, is employed to establish the configuration of spheroidene in the photosynthetic reaction center of Rhodobacter sphaeroides. Show less
A spectrometer for Electron Paramagnetic Resonance (EPR), operating at the very high microwave frequency of 275 GHz in both continuous wave (cw) and pulsed mode, is described and its capabilities... Show moreA spectrometer for Electron Paramagnetic Resonance (EPR), operating at the very high microwave frequency of 275 GHz in both continuous wave (cw) and pulsed mode, is described and its capabilities are illustrated. A superconducting magnet provides the magnetic field up to 12 Tesla and experiments are possible between room temperature and 5 K. The system utilizes pseudo-optical techniques to minimize microwave losses and a single-mode resonator is used to maximize the microwave field at the sample position. The material under study can also be subjected to a radiofrequency radiation field to enable Electron Nuclear Double Resonance (ENDOR) experiments. Technical details of the design and the operation are presented. First results are discussed which demonstrate the increased sensitivity and resolution compared to experiments at lower microwave frequencies. These concern various materials, ranging from semi-conductors to proteins, including an aqueous solution of a proxyl radical at room temperature. ENDOR performance is illustrated with data from a sample of zinc-germanium-phosphide (ZnGeP2) doped with Manganese (Mn). Separate chapters cover experiments on the organic semiconductor P3HT and the observation of Dynamic Nuclear Polarization (DNP) in zinc oxide (ZnO). Show less
