Pulsed 180-GHz EPR/ENDOR/PELDOR spectroscopy

Abstract

Within this review, we describe a home-built pulsed electron paramagnetic resonance (EPR) spectrometer operating at 180 GHz as well as the incorporation of two double resonance techniques, electron nuclear double resonance (ENDOR) and pulsed electron double resonance (PELDOR), along with first applications. Hahn-echo decays on a TEMPO/polystyrene sample are presented, demonstrating that the observation of anisotropic librational motions is possible in a very precise manner at high magnetic fields. Bisdiphenylene-phenyl-allyl is used as a model system to illustrate the performance of the setup for 1H-ENDOR using the Mims as well as the Davies sequence. Furthermore, first 1H-Mims and Davies ENDOR spectra on a biological sample, the wild-type Ras*Mn2+*GDP protein, are reported. The capability of the 180-GHz PELDOR setup is demonstrated using the three-pulse ELDOR sequence on the protein ribonucleotide reductase (RNR) subunit R2 from Escherichia coli, which contains two tyrosyl radicals at a 33 angstroms distance. At 180 GHz, orientation selectivity is observed and the modulation frequency is found to be in good agreement with theoretical predictions.

A multifrequency high-field pulsed electron paramagnetic resonance/electron-nuclear double resonance spectrometer

“…The sensitivity 9,14 was measured with a 0.1% (by mass) sample of the organic radical TEMPOL (C 9 H 18 NO 2 ) in a polystyrene film. It has been suggested that TEMPOL may have a role in treating cancer 25 .…”

“…Electron paramagnetic resonance (EPR) experiments have realized new opportunities as magnetic fields have been increased 1,2,3,4,5,6,7 and pulsed capabilities have been added 8,9,10,11,12,13,14 .…”

“…High field/high frequency EPR has the following advantages: (i) At higher fields an EPR experiment can resolve more resonances, such as those from multiple spin centres and anisotropic g-tensors 1,14 .…”

“…The microwave sources used for high frequency EPR include orotrons 12 , gyrotrons 2 , extended interaction klystrons 11 , far infra-red lasers 3,10 , Gunn diodes 1,13,14 and IMPATT phase locked oscillators 9 . The frequency and power available from amplified synthesizers with multipliers have increased in recent years, and such sources were used in the spectrometer described here, supplied by Virginia Diodes Inc. (VDI) 22 .…”

A field-sweep/field-lock system for superconducting magnets—Application to high-field EPR

“…Furthermore, In order to compensate for hysteresis effects, the sweep of the magnetic field has to start at much lower (or higher) field position than the first resonance appears. Many other experiments, such as ENDOR [11,12], PELDOR [11] or DNP [13] require fixed, reproducible and stable magnetic field values, which are difficult to achieve at high magnetic fields using only control of the magnet or sweep coil current without any feedback loops.…”