Category Archives: ENDOR

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

Tkach, Igor, Isabel Bejenke, Fabian Hecker, Annemarie Kehl, Müge Kasanmascheff, Igor Gromov, Ion Prisecaru, Peter Höfer, Markus Hiller, and Marina Bennati. “1H High Field Electron-Nuclear Double Resonance Spectroscopy at 263 GHz/9.4 T.” Journal of Magnetic Resonance 303 (June 2019): 17–27.

https://doi.org/10.1016/j.jmr.2019.04.001

We present and discuss the performance of 1H electron-nuclear double resonance (ENDOR) at 263 GHz/9.4 Tesla by employing a prototype, commercial quasi optical spectrometer. Basic instrumental features of the setup are described alongside a comprehensive characterization of the new ENDOR probe head design. The performance of three different ENDOR pulse sequences (Davies, Mims and CP-ENDOR) is evaluated using the 1H BDPA radical. A key feature of 263 GHz spectroscopy – the increase in orientiation selectivity in comparison with 94 GHz experiments – is discussed in detail. For this purpose, the resolution of 1H ENDOR spectra at 263 GHz is verified using a representative protein sample containing approximately 15 picomoles of a tyrosyl radical. Davies ENDOR spectra recorded at 5 K reveal previously obscured spectral features, which are interpreted by spectral simulations aided by DFT calculations. Our analysis shows that seven internal proton couplings are detectable for this specific radical if sufficient orientation selectivity is achieved. The results prove the fidelity of 263 GHz experiments in reporting orientation-selected 1H ENDOR spectra and demonstrate that new significant information can be uncovered in complex molecular systems, owing to the enhanced resolution combined with high absolute sensitivity and no compromise in acquisition time.

Population transfer for signal enhancement in pulsed EPR experiments on half integer high spin systems

Kaminker, I., et al., Population transfer for signal enhancement in pulsed EPR experiments on half integer high spin systems. Phys Chem Chem Phys, 2009. 11(31): p. 6799-806.

http://www.ncbi.nlm.nih.gov/pubmed/19639154

High resolution pulse EPR techniques applied to half integer high spin systems, such as Mn(2+) (S = 5/2), usually focus only on the central |-1/2–> |1/2 transition. The reason is that at high fields, where the zero field splitting is considerably smaller than the Zeeman interaction, the spectrum of this transition is intense and narrow. However, because the experiments are carried out at low temperatures, the low lying levels are heavily populated and the signal of the central transition is nevertheless diminished. This, in turn affects the sensitivity of the pulse EPR technique applied. A transfer of populations from the lower lying levels, which for Mn(2+) are the |-3/2 and |-5/2 levels, to the |-1/2 level will therefore increase the sensitivity. Here we describe such an experiment, where a rapid magnetic field sweep over the |-3/2–> |-1/2 sub-spectrum is carried out, concomitantly with a low power microwave (mw) irradiation, which results in population inversion. After this sweep any pulsed EPR sequence can be applied to the central transition that now has a population difference that deviates from the equilibrium value. The feasibility of the experiment is demonstrated at W-band (95 GHz) on Mn(2+) doped in MgO for echo-detected EPR measurements and the dependence of the signal enhancement on the rate and range of the magnetic field sweep and on the mw power is described. The results are then accounted for theoretically by considering a simple fictitious spin 1/2 system. In addition, preliminary enhanced (55)Mn pulse ENDOR electron nuclear double resonance (ENDOR) spectra are presented.

Population transfer for signal enhancement in pulsed EPR experiments on half integer high spin systems

Kaminker, I., et al., Population transfer for signal enhancement in pulsed EPR experiments on half integer high spin systems. Phys Chem Chem Phys, 2009. 11(31): p. 6799-806.

http://www.ncbi.nlm.nih.gov/pubmed/19639154

High resolution pulse EPR techniques applied to half integer high spin systems, such as Mn(2+) (S = 5/2), usually focus only on the central |-1/2–> |1/2 transition. The reason is that at high fields, where the zero field splitting is considerably smaller than the Zeeman interaction, the spectrum of this transition is intense and narrow. However, because the experiments are carried out at low temperatures, the low lying levels are heavily populated and the signal of the central transition is nevertheless diminished. This, in turn affects the sensitivity of the pulse EPR technique applied. A transfer of populations from the lower lying levels, which for Mn(2+) are the |-3/2 and |-5/2 levels, to the |-1/2 level will therefore increase the sensitivity. Here we describe such an experiment, where a rapid magnetic field sweep over the |-3/2–> |-1/2 sub-spectrum is carried out, concomitantly with a low power microwave (mw) irradiation, which results in population inversion. After this sweep any pulsed EPR sequence can be applied to the central transition that now has a population difference that deviates from the equilibrium value. The feasibility of the experiment is demonstrated at W-band (95 GHz) on Mn(2+) doped in MgO for echo-detected EPR measurements and the dependence of the signal enhancement on the rate and range of the magnetic field sweep and on the mw power is described. The results are then accounted for theoretically by considering a simple fictitious spin 1/2 system. In addition, preliminary enhanced (55)Mn pulse ENDOR electron nuclear double resonance (ENDOR) spectra are presented.

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