Double resonance calibration of g factor standards: Carbon fibers as a high precision standard

The g factor of paramagnetic defects in commercial high performance carbon fibers was determined by a double resonance experiment based on the Overhauser shift due to hyperfine coupled protons. Our carbon fibers exhibit a single, narrow and perfectly Lorentzian shaped ESR line and a g factor slightly higher than gfree with g=2.002644=gfree·(1+162ppm) with a relative uncertainty of 15ppm. This precisely known g factor and their inertness qualify them as a high precision g factor standard for general purposes. The double resonance experiment for calibration is applicable to other potential standards with a hyperfine interaction averaged by a process with very short correlation time.ISSN:1090-780

High sensitivity and versatility of the DEER experiment on nitroxide radical pairs at Q-band frequencies

Polyhach Y, Bordignon E, Tschaggelar R, Gandra S, Godt A, Jeschke G. High sensitivity and versatility of the DEER experiment on nitroxide radical pairs at Q-band frequencies. Physical Chemistry Chemical Physics. 2012;14(30):10762-10773.Measurement of distances with the Double Electron-Electron Resonance (DEER) experiment at X-band frequencies using a pair of nitroxides as spin labels is a popular biophysical tool for studying function-related conformational dynamics of proteins. The technique is intrinsically highly precise and can potentially access the range from 1.5 to 6-10 nm. However, DEER performance drops strongly when relaxation rates of the nitroxide spin labels are high and available material quantities are low, which is usually the case for membrane proteins reconstituted into liposomes. This leads to elevated noise levels, very long measurement times, reduced precision, and a decrease of the longest accessible distances. Here we quantify the performance improvement that can be achieved at Q-band frequencies (34.5 GHz) using a high-power spectrometer. More than an order of magnitude gain in sensitivity is obtained with a homebuilt setup equipped with a 150 W TWT amplifier by using oversized samples. The broadband excitation enabled by the high power ensures that orientation selection can be suppressed in most cases, which facilitates extraction of distance distributions. By varying pulse lengths, Q-band DEER can be switched between orientationally non-selective and selective regimes. Because of suppression of nuclear modulations from matrix protons and deuterons, analysis of the Q-band data is greatly simplified, particularly in cases of very small DEER modulation depth due to low binding affinity between proteins forming a complex or low labelling efficiency. Finally, we demonstrate that a commercial Q-band spectrometer can be readily adjusted to the high-power operation

High-Bandwidth Q-Band EPR Resonators

Tschaggelar R, Breitgoff FD, Oberhänsli O, Qi M, Godt A, Jeschke G. High-Bandwidth Q-Band EPR Resonators. Applied Magnetic Resonance. 2017;48(11-12):1273-1300

Cryogenic 35 GHz pulse ENDOR probehead accommodating large sample sizes: performance and applications

The construction and performance of a cryogenic 35 GHz Pulse electron nuclear double resonance (ENDOR) probehead for large samples is presented. The resonator is based on a rectangular TE102 Cavity in which the radio frequency (rf) B-2-field is generated by a two turn saddle ENDOR coil crossing the resonator along the sample axis with minimal distance to the sample tube. An rf power efficiency factor is used to define the B-2-field strength per square-root of the transmitted rf power over the frequency range 2-180 MHz. The distributions of the microwave B-1- and E-1-field, and the rf B-2-field are investigated by electromagnetic field calculations. All dielectrics, the sample tube, and coupling elements are included in the Calculations. The application range of the probehead and the advantages of using large sample sizes are demonstrated and discussed on a number of paramagnetic samples containing transition metal ions