High-frequency EPR on high-spin transition-metal sites

Abstract

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 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.