Continuous-wave EPR at 275 GHz: Application to high-spin Fe³⁺ systems

The 275 GHz electron-paramagnetic-resonance spectrometer we reported on in 2004 has been equipped with a new probe head, which contains a cavity especially designed for operation in continuous-wave mode. The sensitivity and signal stability that is achieved with this new probe head is illustrated with 275 GHz continuous-wave spectra of a 1 mM frozen solution of the complex Fe(III)-ethylenediamine tetra-acetic acid and of 10 mM frozen solutions of the protein rubredoxin, which contains Fe³⁺ in its active site, from three different organisms. The high quality of the spectra of the rubredoxins allows the determination of the zero-field-splitting parameters with an accuracy of 0.5 GHz. The success of our approach results partially from the enhanced absolute sensitivity, which can be reached using a single-mode cavity. At least as important is the signal stability that we were able to achieve with the new probe head.