Electron Decoupling with Dynamic Nuclear Polarization in Rotating Solids

Abstract

Dynamic nuclear polarization (DNP) can enhance NMR sensitivity by orders of magnitude by transferring spin polarization from electron paramagnetic resonance (EPR) to NMR. However, paramagnetic DNP polarizing agents can have deleterious effects on NMR signals. Electron spin decoupling can mitigate these paramagnetic relaxation effects. We demonstrate electron decoupling experiments in conjunction with DNP and magic-angle-spinning NMR spectroscopy. Following a DNP and spin diffusion period, the microwave irradiation frequency is quickly tuned on-resonance with electrons on the DNP polarizing agent. The electron decoupling performance shows a strong dependence on the microwave frequency and DNP polarization time. Microwave frequency sweeps through the EPR line shape are shown as a time domain strategy to significantly improve electron decoupling. For C spins on biomolecules frozen in a glassy matrix, electron decoupling reduces the line widths by 11% (47 Hz) and increases the intensity by 14%.

Electron Decoupling with Chirped Microwave Pulses for Rapid Signal Acquisition and Electron Saturation Recovery

“…In the case of dynamic nuclear polarization (DNP), hyperfine interactions are required to transfer spin polarization from electron to nuclear spins. [30,31] DNP experiments typically require seconds to hours to spread enhanced polarization via nuclear spin diffusion to chemical sites of interest. [26] Here, we use as table organic monoradical, trityl (Finland radical), which has ar elatively narrow EPR lineshape.…”

“…[26] Here, we use as table organic monoradical, trityl (Finland radical), which has ar elatively narrow EPR lineshape. [31,35] Theo ptimal repetition time for direct DNP is limited by the electron longitudinal relaxation (T 1e )a nd DNP transfer, and not by nuclear spin diffusion. [28,29] We have shown that chirped microwave pulses can partially decouple the electron-nuclear spin interaction after DNP in am ethod known as electron decoupling,w ith amore pronounced effect on observed nuclear spins in direct proximity to electron spins.…”

Pulsed Electron Decoupling and Strategies for Time Domain Dynamic Nuclear Polarization with Magic Angle Spinning

“…44 We have recently successfully attenuated these detrimental effects by employing pulsed eDEC with frequency-chirped microwave irradiation. 45 Although pulsed eDEC has been demonstrated in conjunction with MAS, this has yet to be accomplished with time domain DNP. Challenges include generating sufficiently intense microwave fields within MAS rotors and shaping microwave pulses in the time domain.…”

“…This figure was reproduced with modification with permission from the Journal of the American Chemical Society. 45 (b) Possible frequency-swept ISE/off-resonance NOVEL pulse sequence. (c) eNCP.…”

“…Figure reproduced with modifications by permission of the Journal of the American Chemical Society. 45 …”