Conversion of a Single-Frequency X-Band EPR Spectrometer into a Broadband Multi-Frequency 0.1–18 GHz Instrument for Analysis of Complex Molecular Spin Hamiltonians

Abstract

A broadband EPR spectrometer is an instrument that can be tuned to many microwave frequencies over several octaves. Its purpose is the collection of multi-frequency data, whose global analysis affords interpretation of complex spectra by means of deconvolution of frequency-dependent and frequency-independent interaction terms. Such spectra are commonly encountered, for example, from transition-metal complexes and metalloproteins. In a series of previous papers, I have described the development of broadband EPR spectrometers around a vector network analyzer. The present study reports on my endeavor to start from an existing X-band spectrometer and to reversibly re-build it into a broadband machine, in a quest to drastically reduce design effort, building costs, and operational complexity, thus bringing broadband EPR within easy reach of a wide range of researchers.

A Comparative Multi-Frequency EPR Study of Dipolar Interaction in Tetra-Heme Cytochromes

“…The broadband EPR spectrometer, its cryogenics, and its dedicated software have been described in detail in [ 4 , 5 , 6 , 7 , 8 ]. In brief (cf.…”

“…Occasionally, data taken with spectrometers running at different frequencies have been scrutinized, for example, for a Mo 5+ -[2Fe-2S] 1+ system [ 3 ]. An analysis of heme-heme interactions in hemoproteins was never attempted until our recent work [ 4 ], which became possible by the construction of a novel type of EPR spectrometer that can be tuned to a large number of microwave frequencies over a very broad range of 0.1–18 GHz [ 4 , 5 , 6 , 7 , 8 ]. The present work is a systematic extension of our previous study [ 4 ], aiming at identifying what is now possible in terms of the analysis of dipolar interactions in multiheme proteins and what remaining shortcomings should be addressed in the future.…”

“…One of us has recently described such an instrument for frequencies below and around the standard X-band, approximately in the range of 0.1–18 GHz. The key novelty of this approach is to replace the single-mode cavity with a multi-mode resonator based on strip line technology [ 4 , 5 , 6 , 7 , 8 ]. Application of the spectrometer to the study of intermolecular dipolar broadening in a monoheme cytochrome and intramolecular dipolar broadening in a tetraheme cytochrome led to an unexpected result: in the tetraheme cytochrome, dipolar interaction was detectable up to X-band frequency [ 4 ].…”

Broadband EPR Spectroscopy of the Triplet State: Multi-Frequency Analysis of Copper Acetate Monohydrate

“…Home-build helium-flow cryogenics were as in [72,73]. Multifrequency data were obtained with the most recent version of the broadband EPR spectrometer, consisting of the conventional Bruker EMX-plus spectrometer extended with a multi-frequency conversion kit to replace the X-band bridge [13]. The source power is expressed in absolute units of dBm, where 0 dBm ≡ 1 mW.…”

“…In high-frequency EPR (ν circa ≥ 100 GHz) this may be implemented by switching between several microwave sources to step through a frequency range of over two octaves [5][6][7][8]. For more conventional frequencies, around the standard X-band, I have developed a broadband EPR spectrometer [9][10][11][12][13][14] as a convenient, affordable single station, using a conventional electromagnet, whose resonator circuit can be tuned to many frequencies over the approximate range of 0.1 to 18 GHz, that is, over some seven to eight octaves. Here, I describe application of the broadband methodology to the analysis of triplet-state EPR.…”