Nikolaus M Loening

Publications (2)3.58 Total impact

• Article: Broadband excitation pulses for high-field solid-state nuclear magnetic resonance spectroscopy.

  Nikolaus M Loening, Barth-Jan van Rossum, Hartmut Oschkinat
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  ABSTRACT: In nuclear magnetic resonance spectroscopy, experimental limits due to the radiofrequency transmitter and/or coil means that conventional radiofrequency pulses ("hard pulses") are sometimes not sufficiently powerful to excite magnetization uniformly over a desired range of frequencies. Effects due to nonuniform excitation are most frequently encountered at high magnetic fields for nuclei with a large range of chemical shifts. Using optimal control theory, we have designed broadband excitation pulses that are suitable for solid-state samples under magic-angle-spinning conditions. These pulses are easy to implement, robust to spinning frequency variations, and radiofrequency inhomogeneities, and only four times as long as a corresponding hard pulse. The utility of these pulses for uniformly exciting (13) C nuclei is demonstrated on a 900 MHz (21.1 T) spectrometer.
  Magnetic Resonance in Chemistry 04/2012; 50(4):284-8. · 1.44 Impact Factor
• Article: The effect of biradical concentration on the performance of DNP-MAS-NMR.

  Sascha Lange, Arne H Linden, Umit Akbey, W Trent Franks, Nikolaus M Loening, Barth-Jan van Rossum, Hartmut Oschkinat
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  ABSTRACT: With the technique of dynamic nuclear polarization (DNP) signal intensity in solid-state MAS-NMR experiments can be enhanced by 2-3 orders of magnitude. DNP relies on the transfer of electron spin polarization from unpaired electrons to nuclear spins. For this reason, stable organic biradicals such as TOTAPOL are commonly added to samples used in DNP experiments. We investigated the effects of biradical concentration on the relaxation, enhancement, and intensity of NMR signals, employing a series of samples with various TOTAPOL concentrations and uniformly (13)C, (15)N labeled proline. A considerable decrease of the NMR relaxation times (T(1), T(2)(∗), and T(1)(ρ)) is observed with increasing amounts of biradical due to paramagnetic relaxation enhancement (PRE). For nuclei in close proximity to the radical, decreasing T(1)(ρ) reduces cross-polarization efficiency and decreases in T(2)(∗) broaden the signal. Additionally, paramagnetic shifts of (1)H signals can cause further line broadening by impairing decoupling. On average, the combination of these paramagnetic effects (PE; relaxation enhancement, paramagnetic shifts) quenches NMR-signals from nuclei closer than 10Å to the biradical centers. On the other hand, shorter T(1) times allow the repetition rate of the experiment to be increased, which can partially compensate for intensity loss. Therefore, it is desirable to optimize the radical concentration to prevent additional line broadening and to maximize the signal-to-noise observed per unit time for the signals of interest.
  Journal of Magnetic Resonance 01/2012; 216:209-12. · 2.14 Impact Factor