Solid State DNP at 200 GHz
Truncated Cross Effect Dynamic Nuclear Polarization: An Overhauser Effect Doppelgänger
A. Equbal, Y. Li, A. Leavesley, S. Huang, S. Rajca, A. Rajca, and S. Han, The Journal of Physical Chemistry Letters 9, 2175 (2018). DOI: 10.1021/acs.jpclett.8b00751
The discovery of a truncated cross-effect (CE) in dynamic nuclear polarization (DNP) NMR that has the features of an Overhauser-effect DNP (OE-DNP) is reported here. The apparent OE-DNP, where minimal μw power achieved optimum enhancement, was observed when doping Trityl-OX063 with a pyrroline nitroxide radical that possesses electron-withdrawing tetracarboxylate substituents (tetracarboxylate-ester-pyrroline or TCP) in vitrified water/glycerol at 6.9 T and at 3.3 to 85 K, in apparent contradiction to expectations. While the observations are fully consistent with OE-DNP, we discover that a truncated cross-effect (tCE) is the underlying mechanism, owing to TCP’s shortened T1e. We take this observation as a guideline and demonstrate that a crossover from CE to tCE can be replicated by simulating the CE of a narrow-line (Trityl-OX063) and a broad-line (TCP) radical pair, with a significantly shortened T1e of the broad-line radical.
Effect of Electron Spectral Diffusion on Static Dynamic Nuclear Polarization at 7 Tesla
A. Leavesley, D. Shimon, T.A. Siaw, A. Feintuch, D. Goldfarb, S. Vega, I. Kaminker, and S. Han, Physical Chemistry Chemical Physics 19, 3596 (2017). DOI:10.1039/C6CP06893F
Here, we present an integrated experimental and theoretical study of 1H dynamic nuclear polarization (DNP) of a frozen aqueous glass containing free radicals at 7 T, under static conditions and at temperatures ranging between 4 and 20 K. The DNP studies were performed with a home-built 200 GHz quasi-optics microwave bridge, powered by a tunable solid-state diode source. DNP using monochromatic and continuous wave (cw) irradiation applied to the electron paramagnetic resonance (EPR) spectrum of the radicals induces the transfer of polarization from the electron spins to the surrounding nuclei of the solvent and solutes in the frozen aqueous glass. In our systematic experimental study, the DNP enhanced 1H signals are monitored as a function of microwave frequency, microwave power, radical concentration, and temperature, and are interpreted with the help of electron spin–lattice relaxation times, experimental MW irradiation parameters, and the electron spectral diffusion (eSD) model introduced previously. This comprehensive experimental DNP study with mono-nitroxide radical spin probes was accompanied with theoretical calculations. Our results consistently demonstrate that eSD effects can be significant at 7 T under static DNP conditions, and can be systematically modulated by experimental conditions.
Reversal of Paramagnetic Effects by Electron Spin Saturation
S.K. Jain, T.A. Siaw, A. Equbal, C.B. Wilson, I. Kaminker, and S. Han, J. Phys. Chem. C 122, 5578 (2018). DOI:10.1021/acs.jpcc.8b00312
We present a study in which both significant dynamic nuclear polarization (DNP) enhancement of 7Li NMR and reversal of the paramagnetic effects (PEs) are achieved by microwave (μw) irradiation-induced electron spin saturation of nitroxide radicals at liquid-helium temperatures. The reversal of the PE was manifested in significant narrowing of the 7Li NMR line and reversal of the paramagnetic chemical shift under DNP conditions. The extent of the PE was found to decrease with increased saturation of the electron paramagnetic resonance line, modulated as a function of microwave (μw) power, frequency, duration of irradiation, and gating time between μw irradiation and NMR detection. The defining observation was the shortening of the electron phase memory time, Tm, of the excited observer spins with increasing μw irradiation and concurrent electron spin saturation of the electron spin bath. This and a series of corroborating studies reveal the origin of the NMR line narrowing to be the reversal of paramagnetic relaxation enhancement (PRE), leading us to debut the term REversal of PRE by electron Spin SaturatION (REPRESSION). The shortening of electron Tm of any paramagnetic system as a function of electron spin saturation has not been reported to date, making REPRESSION a discovery of this study. The reversal of the paramagnetic dipolar shift is due to the decrease in electron spin order, also facilitated by electron spin saturation. This study offers new fundamental insights into PE under DNP conditions and a method to detect and identify NMR signal proximal to paramagnetic sites with reduced or minimal line broadening.
Arbitrary Waveform Modulated Pulse EPR at 200 GHz
I. Kaminker, R. Barnes, and S. Han, J Magn Reson 279, 81 (2017). 10.1016/j.jmr.2017.04.016
We report here on the implementation of arbitrary waveform generation (AWG) capabilities at ∼200 GHz into an Electron Paramagnetic Resonance (EPR) and Dynamic Nuclear Polarization (DNP) instrument platform operating at 7 Tesla. This is achieved with the integration of a 1 GHz, 2 channel, digital to analogue converter (DAC) board that enables the generation of coherent arbitrary waveforms at Ku-band frequencies with 1ns resolution into an existing architecture of a solid state amplifier multiplier chain (AMC). This allows for the generation of arbitrary phase- and amplitude-modulated waveforms at 200 GHz with > 150 mW power. We find that the non-linearity of the AMC poses significant difficulties in generating amplitude-modulated pulses at 200 GHz. We demonstrate that in the power-limited regime of ω1 < 1MHz phase-modulated pulses were sufficient to achieve significant improvements in broadband (>10MHz) spin manipulation in incoherent (inversion), as well as coherent (echo formation) experiments. Highlights include the improvement by one order of magnitude in inversion bandwidth compared to that of conventional rectangular pulses, as well as a factor of two in improvement in the refocused echo intensity at 200GHz.
Pulsed EPR at > 200 GHz
Gd3+ - Gd3+ Distances Exceeding 3 nm Determined by Very High Frequency Continuous Wave Electron Paramagnetic Resonance
J.A. Clayton, M. Qi, A. Godt, D. Goldfarb, S. Han, and M.S. Sherwin, Phys Chem Chem Phys 19, 5127 (2017). DOI:10.1039/C6CP07119H
EPR spectroscopy in combination with site-directed spin-labeling is a very powerful tool for elucidating the structure and organization of biomolecules. Gd3+ complexes have recently emerged as a new class of spin labels for distance determination by pulsed EPR spectroscopy at Q- and W-band. We present CW EPR measurements at 240 GHz (8.6 Tesla) on a series of Gd-rulers of the type Gd-PyMTA—spacer—Gd-PyMTA, with Gd-Gd distances ranging from 1.2 nm to 4.3 nm. CW EPR measurements of these Gd-rulers show that significant dipolar broadening of the central |−1/2〉 → |1/2〉 transition occurs at 30 K for Gd-Gd distances up to ~ 3.4 nm with Gd-PyMTA as the spin label. This represents a significant extension for distances accessible by CW EPR, as nitroxide-based spin labels at X-band frequencies can typically only access distances up to ~ 2 nm. We show that this broadening persists at biologically relevant temperatures above 200 K, and that this method is further extendable up to room temperature by immobilizing the sample in glassy trehalose. We show that the peak-to-peak broadening of the central transition follows the expected 1/r 3 dependence for the electron-electron dipolar interaction, from cryogenic temperatures up to room temperature. A simple procedure for simulating the dependence of the lineshape on interspin distance is presented, in which the broadening of the central transition is modeled as an S = 1/2 spin whose CW EPR lineshape is broadened through electron-electron dipolar interactions with a neighboring S = 7/2 spin.
MAS DNP below 30K
Tuning Nuclear Depolarization under MAS by Electron T1e
A. Lund, A. Equbal, and S. Han, Physical Chemistry Chemical Physics 20, 23976 (2018). DOI: 10.1039/C8CP04167A
The Cross-Effect (CE) Dynamic Nuclear Polarization (DNP) mechanism under Magic Angle Spinning (MAS) induces depletion or ‘‘depolarization’’ of the NMR signal, in the absence of microwave irradiation. In this study, the role of T1e on nuclear depolarization under MAS was tested experimentally by systematically varying the local and global electron spin concentration using mono-, bi- and tri-radicals. These spin systems show different depolarization effects that systematically tracked with their different T1e rates, consistent with theoretical predictions. In order to test whether the effect of T1e is directly or indirectly convoluted with other spin parameters, the tri-radical system was doped with different concentrations of GdCl3, only tuning the T1e rates, while keeping other parameters unchanged. Gratifyingly, the changes in the depolarization factor tracked the changes in the T1e rates. The experimental results are corroborated by quantum mechanics based numerical simulations which recapitulated the critical role of T1e. Simulations showed that the relative orientation of the two g-tensors and e–e dipolar interaction tensors of the CE fulfilling spin pair also plays a major role in determining the extent of depolarization, besides the enhancement. This is expected as orientations influence the efficiency of the various level anti-crossings or the ‘‘rotor events’’ under MAS. However, experimental evaluation of the empirical spectral diffusion parameter at static condition showed that the local vs. global e–e dipolar interaction network is not a significant variable in the commonly used nitroxide radical system studied here, leaving T1e rates as the major modulator of depolarization.