Tan, Kong Ooi, Ralph T. Weber, Thach V. Can, and Robert G. Griffin. “Adiabatic Solid Effect.” The Journal of Physical Chemistry Letters, April 20, 2020, 3416–21.
The solid effect (SE) is a two spin dynamic nuclear polarization (DNP) mechanism that enhances the sensitivity in NMR experiments by irradiation of the electron-nuclear spin transitions with continuous wave (CW) microwaves at 𝜔0S ± 𝜔0I, where 𝜔0S and 𝜔0I are electron and nuclear Larmor frequencies, respectively. Using trityl (OX063), dispersed in a 60/40 glycerol/water mixture at 80 K, as a polarizing agent, we show here that application of a chirped microwave pulse, with a bandwidth comparable to the EPR linewidth applied at the SE matching condition, improves the enhancement by a factor of 2.4 over the CW method. Furthermore, the chirped pulse yields an enhancement that is ~20 % larger than obtained with the ramped-amplitude NOVEL (RA-NOVEL), which to date has achieved the largest enhancements in time domain DNP experiments. Numerical simulations suggest that the spins follow an adiabatic trajectory during the polarization transfer; hence, we denote this sequence as an adiabatic solid effect (ASE). We foresee that ASE will be a practical pulsed DNP experiment to be implemented at higher static magnetic fields due to moderate power requirement. In particular, the ASE uses only 13 % of the maximum microwave power required for RA-NOVEL.
Tan, Kong Ooi, Sudheer Jawla, Richard J Temkin, and Robert G Griffin. “Pulsed Dynamic Nuclear Polarization,” 8:14, 2019.
In the last two decades continuous-wave (CW) microwave irradiation obtained from gyrotron microwave sources has been utilized extensively in the development and applications of new experimental approaches to high frequency dynamic nuclear polarization (DNP). Despite the abundant successes of this approach, it is well established experimentally and understood theoretically that at higher magnetic fields, where the resolution of the NMR spectra is optimal, the enhancement factors in CW DNP experiments decrease. Potentially this issue can be mitigated by using time domain or pulsed DNP techniques, which theoretically have field-independent enhancement factors. In this contribution, we discuss the pulsed DNP experiments that have been developed to date, along with the theory and the applicability of the sequences. As we will see pulsed techniques are fundamentally different from the CW-DNP methodology and require a different array of instrumentation, spin physics, and radicals. Hence, in addition to the underlying theory, we discuss the specifications of the microwave sources, DNP probes, and optimal radicals for pulsed DNP. The review ends with a summary of the current and future applications of pulsed DNP and conjectures as to the development of the pulsed methods for experiments at increasingly higher magnetic fields.
Judge, Patrick T., Erika L. Sesti, Nicholas Alaniva, Edward P. Saliba, Lauren E. Price, Chukun Gao, Thomas Halbritter, Snorri Th. Sigurdsson, George B. Kyei, and Alexander B. Barnes. “Characterization of Frequency-Chirped Dynamic Nuclear Polarization in Rotating Solids.” Journal of Magnetic Resonance 313 (April 2020): 106702.
Continuous wave (CW) dynamic nuclear polarization (DNP) is used with magic angle spinning (MAS) to enhance the typically poor sensitivity of nuclear magnetic resonance (NMR) by orders of magnitude. In a recent publication we show that further enhancement is obtained by using a frequency-agile gyrotron to chirp incident microwave frequency through the electron resonance frequency during DNP transfer. Here we characterize the effect of chirped MAS DNP by investigating the sweep time, sweep width, center-frequency, and electron Rabi frequency of the chirps. We show the advantages of chirped DNP with a tritylnitroxide biradical, and a lack of improvement with chirped DNP using AMUPol, a nitroxide biradical. Frequency-chirped DNP on a model system of urea in a cryoprotecting matrix yields an enhancement of 142, 21% greater than that obtained with CW DNP. We then go beyond this model system and apply chirped DNP to intact human cells. In human Jurkat cells, frequency-chirped DNP improves enhancement by 24% over CW DNP. The characterization of the chirped DNP effect reveals instrument limitations on sweep time and sweep width, promising even greater increases in sensitivity with further technology development. These improvements in gyrotron technology, frequency-agile methods, and incell applications are expected to play a significant role in the advancement of MAS DNP.
Jain, Sheetal K., Guinevere Mathies, and Robert G. Griffin. “Off-Resonance NOVEL.” The Journal of Chemical Physics 147, no. 16 (October 28, 2017): 164201.
Dynamic nuclear polarization (DNP) is theoretically able to enhance the signal in nuclear magnetic resonance (NMR) experiments by a factor gamma_e/gamma_n, where gamma’s are the gyromagnetic ratios of an electron and a nuclear spin. However, DNP enhancements currently achieved in high-field, high-resolution biomolecular magic-angle spinningNMRare well below this limit because the continuous-wave DNP mechanisms employed in these experiments scale as w0^(-n) where n ~ 1–2. In pulsed DNP methods, such as nuclear orientation via electron spin-locking (NOVEL), the DNP efficiency is independent of the strength of the main magnetic field. Hence, these methods represent a viable alternative approach for enhancing nuclear signals. At 0.35 T, the NOVEL scheme was demonstrated to be efficient in samples doped with stable radicals, generating 1H NMR enhancements of 430. However, an impediment in the implementation of NOVEL at high fields is the requirement of sufficient microwave power to fulfill the on-resonance matching condition, omega_0I = omega_1S, where omega_0I and omega_1S are the nuclear Larmor and electron Rabi frequencies, respectively. Here, we exploit a generalized matching condition, which states that the effective Rabi frequency, omega_1Seff, matches omega_0I . By using this generalized off-resonance matching condition, we generate 1H NMR signal enhancement factors of 266 (70% of the onresonanceNOVEL
enhancement) with omega_1S/2pi = 5 MHz.We investigate experimentally the conditions for optimal transfer of polarization from electrons to 1H both for the NOVEL mechanism and the solid-effect mechanism and provide a unified theoretical description for these two historically distinct forms of DNP.
We are delighted to announce the 11th user meeting of the French infrastructure for high-field NMR spectroscopy (IR-RMN THC, https://www.ir-rmn.fr/), which will be held at the University of Lille (France) on November 19, 2019.
The invited talks for this meeting are:
“NMR Crystallography of Photoresponsive Materials” by John Griffin (Lancaster University, UK);
“Electron Decoupling and DNP with Chirped Microwave Pulses and MAS Spheres” by Alexander Barnes (Washington University, USA).
Furthermore, some of the users of the IR-RMN THC will present their recent achievements using high-field NMR spectroscopy in research areas, such as structural biology and materials science. IR-RMN THC staff will also present recent developments related to high-field NMR. A round table will also be chaired by the committee of IR-RMN THC users in order to discuss the needs of the users, the operation of the infrastructure and the possible improvements.
This meeting aims at bringing together users and staff of the IR-RMN THC, fostering exchanges between them and the submission of new proposals to the IR-RMN THC.
You can find the full program and register for the workshop at the website:
The workshop is sponsored by IR-RMN THC and the registration is free of charge. The recent users of IR-RMN THC, who have been granted access to its high-field NMR spectrometers, are cordially invited to participate to this meeting.
We look forward to welcoming you at the University of Lille in November.
The organization committee:
Christian Bonhomme, François-Xavier Cantrelle, Olivier Lafon, Charlotte Martineau, Jean-Pierre Simorre, Davy Sinnaeve, Julien Trébosc, Xavier Trivelli, Nicolas Wolff
This is the AMPERE MAGNETIC RESONANCE mailing list:
NMR web database:
Pulsed DNP experiments have been discussed in the literature for quite a while already. While several experiments have been proposed and conducted at low magnetic fields, where instrumentation is less demanding, this is the first example of pulsed DNP experiments performed at high magnetic fields.
Tan, Kong Ooi, Chen Yang, Ralph T Weber, Guinevere Mathies, and Robert G Griffin. “Time-Optimized Pulsed Dynamic Nuclear Polarization.” SCIENCE ADVANCES 5 (2019): 8. https://doi.org/10.1126/sciadv.aav6909.
Pulsed dynamic nuclear polarization (DNP) techniques can accomplish electron-nuclear polarization transfer efficiently with an enhancement factor that is independent of the Zeeman field. However, they often require large Rabi frequencies and, therefore, high-power microwave irradiation. Here, we propose a new low-power DNP sequence for static samples that is composed of a train of microwave pulses of length τp spaced with delays d. A particularly robust DNP condition using a period τm = τp + d set to ~1.25 times the Larmor period τLarmor is investigated which is a time-optimized pulsed DNP sequence (TOP-DNP). At 0.35 T, we obtained an enhancement of ~200 using TOP-DNP compared to ~172 with nuclear spin orientation via electron spin locking (NOVEL), a commonly used pulsed DNP sequence, while using only ~7% microwave power required for NOVEL. Experimental data and simulations at higher fields suggest a field-independent enhancement factor, as predicted by the effective Hamiltonian.
Sesti, Erika L., Edward P. Saliba, Nicholas Alaniva, and Alexander B. Barnes. “Electron Decoupling with Cross Polarization and Dynamic Nuclear Polarization below 6 K.” Journal of Magnetic Resonance 295 (October 2018): 1–5.
Dynamic nuclear polarization (DNP) can improve nuclear magnetic resonance (NMR) sensitivity by orders of magnitude. Polarizing agents containing unpaired electrons required for DNP can broaden nuclear resonances in the presence of appreciable hyperfine couplings. Here we present the first cross polarization experiments implemented with electron decoupling, which attenuates detrimental hyperfine couplings. We also demonstrate magic angle spinning (MAS) DNP experiments below 6 K, producing unprecedented nuclear spin polarization in rotating solids. 13C correlation spectra were collected with MAS DNP below 6 K for the first time. Longitudinal magnetization recovery times with MAS DNP (T1DNP, 1H) of urea in a frozen glassy matrix below 6 K were measured for both the solid effect and the cross effect. Trityl radicals exhibit a T1DNP (1H) of 18.7 s and the T1DNP (1H) of samples doped with 20 mM AMUPol is only 1.3 s. MAS below 6 K with DNP and electron decoupling is an effective strategy to increase NMR signal-to-noise ratios per transient while retaining short recovery periods.
Saliba, Edward P., Erika L. Sesti, Nicholas Alaniva, and Alexander B. Barnes. “Pulsed Electron Decoupling and Strategies for Time Domain Dynamic Nuclear Polarization with Magic Angle Spinning.” The Journal of Physical Chemistry Letters 9, no. 18 (September 20, 2018): 5539–47.
Magic angle spinning (MAS) dynamic nuclear polarization (DNP) is widely used to increase nuclear magnetic resonance (NMR) signal intensity. Frequency-chirped microwaves yield superior control of electron spins, and are expected to play a central role in the development of DNP MAS experiments. Time domain electron control with MAS has considerable promise to improve DNP performance at higher fields and temperatures. We have recently demonstrated that pulsed electron decoupling using frequency-chirped microwaves improves MAS DNP experiments by partially attenuating detrimental hyperfine interactions. The continued development of pulsed electron decoupling will enable a new suite of MAS DNP experiments which transfer polarization directly to observed spins. Time domain DNP transfers to nuclear spins in conjunction with pulsed electron decoupling is described as a viable avenue toward DNP-enhanced, high-resolution NMR spectroscopy over a range of temperatures from <6K to 320 K.
Annabestani, Razieh, Maryam Mirkamali, and Raffi Budakian. “Adiabatic-NOVEL for Nano-Scale Magnetic Resonance Imaging.” ArXiv:1712.09128 [Quant-Ph], December 25, 2017.
We propose a highly efficient dynamic nuclear polarization technique that is robust against field in-homogeneity. This technique is designed to enhance the detection sensitivity in nano-MRI, where large Rabi field gradients are required. The proposed technique consists of an adiabatic half passage pulse followed by an adiabatic linear sweep of the electron Rabi frequency and can be considered as an adiabatic version of nuclear orientation via electron spin locking (adiabatic-NOVEL). We analyze the spin dynamics of an electron-nuclear system that is under microwave irradiation at high static magnetic field and at cryogenic temperature. The result shows that an amplitude modulation of the microwave field makes adiabatic-NOVEL highly efficient and robust against both the static and microwave field in-homogeneity.
Schöps, P., E. Spindler Philipp, and F. Prisner Thomas, Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla, in Z. Phys. Chem. 2017. p. 561.
Dynamic nuclear polarization (DNP) is a methodology to increase the sensitivity of nuclear magnetic resonance (NMR) spectroscopy. It relies on the transfer of the electron spin polarization from a radical to coupled nuclear spins, driven by microwave excitation resonant with the electron spin transitions. In this work we explore the potential of pulsed multi-frequency microwave excitation in liquids. Here, the relevant DNP mechanism is the Overhauser effect. The experiments were performed with TEMPOL radicals in aqueous solution at room temperature using a Q-band frequency (1.2 T) electron paramagnetic resonance (EPR) spectrometer combined with a Minispec NMR spectrometer. A fast arbitrary waveform generator (AWG) enabled the generation of multi-frequency pulses used to either sequentially or simultaneously excite all three 14N-hyperfine lines of the nitroxide radical. The multi-frequency excitation resulted in a doubling of the observed DNP enhancements compared to single-frequency microwave excitation. Q-band free induction decay (FID) signals of TEMPOL were measured as a function of the excitation pulse length allowing the efficiency of the electron spin manipulation by the microwave pulses to be extracted. Based on this knowledge we could quantitatively model our pulsed DNP enhancements at 1.2 T by numerical solution of the Bloch equations, including electron spin relaxation and experimental parameters. Our results are in good agreement with theoretical predictions. Whereas for a narrow and homogeneous single EPR line continuous wave excitation leads to more efficient DNP enhancements compared to pulsed excitation for the same amount of averaged microwave power. The situation is different for radicals with several hyperfine lines or in the presence of inhomogeneous line broadening. In such cases pulsed single/multi-frequency excitation can lead to larger DNP enhancements.