1H high field electron-nuclear double resonance spectroscopy at 263 GHz/9.4 T

High-Field Pulsed ENDOR with Intra-cavity Radiofrequency Coil

“…The conversion factor of the hairpin coil in the center of the resonator, |B rf |= 0.77 mT/A, is comparable with the value calculated for the 4-wire coil internal to the 35 GHz resonator investigated in [35], as well as with the value calculated for the Bruker ENDOR resonator operating at the same frequency [35]. For the Helmholtzlike coils, the obtained value |B rf |= 0.18 mT/A is comparable with the conversion factor of the coils external to the slotted cavity operating at 263 GHz in the Bruker spectrometer presented in [24].…”

“…The configurations with mw cavity housing an internal rf coil or having cylindrical wall acting as rf coil are the most commonly employed at standard frequencies [18]. At high fields, the necessary miniaturization of the probe head favored the solution with rf coil external to the cavity, implemented with the expedient of cutting several slits in the cylindrical body of the resonator [19][20][21][22][23][24], as proposed by Burghaus et al to facilitate the penetration of the rf field [25]. However, from the point of view of the B rf conversion factor, the configuration with the rf coil external to the mw cavity is doubly inefficient in comparison to the one with the intra-cavity coil.…”

Measurement of Angstrom to Nanometer Molecular Distances with 19F Nuclear Spins by EPR/ENDOR Spectroscopy

“…Thus, the t values have to be optimized for each sample to detect the canonical resonances of the dipolar powder pattern (Pake pattern), with principal axes frequencies at n HFC =AE T and AE T/2. Under consideration of the relaxation times T m ( Figure S11, Supporting Information), t values were chosen that optimize detection of the parallel component of the dipolar tensor at n HFC =AE T. All individual ENDOR spectra report orientation selection [9] due to the narrow excitation bandwidth of the microwave (mw) pulses with respect to the broad EPR line. In principle, the full Pake pattern might be reconstructed by summing over all orientations, which would correspond to measuring and summing spectra at a set of narrowly spaced resonance fields.…”

“…A possibility to circumvent this issue was presented by Zänker et al, who employed 31 P nuclei instead of 19 F. [20] Using Mims' ENDOR experiments, [21] couplings as small as % 33 kHz could be resolved corresponding to a distance of about 1 nm between a nitroxide and a 31 P nucleus. The availability of high-field/high-frequency EPR spectrometers (n EPR 0 94 GHz) meanwhile allows for a sufficient resolution of nuclear frequencies, as we reported in recent publications, [9,22] and for a general implementation of 19 F ENDOR for structural investigations. Here we demonstrate the ability of 94 GHz (3.4 T) 19 F ENDOR to detect distances up to % 15 with atomic resolution in orthogonally labelled 19 F/nitroxide model systems and RNA duplexes.…”

“…[7] This advantage gets even more pronounced at high magnetic fields and frequencies, [8] where also spectral resolution increases. [9] Moreover, EPR is not restricted by the size of the biomacromolecule, however, it requires the presence of at least one paramagnetic center. In the last decade, EPR-based pulsed dipolar spectroscopy [10] has emerged as an important tool for structural biology, as it allows for measuring dipolar coupling between two paramagnetic centers (typically nitroxide spin labels) separated by % 1.5 up to approximately 8 nm, where the distance range can be extended by using either sophisticatedly shaped pulses [11] or deuterated proteins.…”