Balancing dipolar and exchange coupling in biradicals to maximize cross effect dynamic nuclear polarization

Equbal, Asif; Tagami, Kan; Han, Songi

doi:10.1039/d0cp02051f

Cited by 28 publications

(23 citation statements)

References 43 publications

(83 reference statements)

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“…Measuring the J values and their distributions through a multifield EPR study is out of the scope here and will be the subject of further work. The small predicted changes in the J couplings for 2 and 3 could nevertheless partially explain its relatively poorer MAS DNP results at 9.4 T. 86 Here, in order to exclude that the difference in the MAS DNP enhancement between O-and C-conformers is exclusively J-related, we have investigated the monoradicals 9−14, where intramolecular J interactions are absent. The open monoradical O-MbPyTol (11) at 20 mM, 9.4 T, and 100 K yields an exceptional ε C CP = 119 ± 25, which is a factor of ∼3 higher than that of the closed analogue C-MbPyTol (12), ε C CP = 41 ± 1 in DMSO/water mixture.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance

et al. 2020

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The development of magic-angle spinning dynamic nuclear polarization (MAS DNP) has allowed atomic-level characterization of materials for which conventional solid-state NMR is impractical due to the lack of sensitivity. The rapid progress of MAS DNP has been largely enabled through the understanding of rational design concepts for more efficient polarizing agents (PAs). Here, we identify a new design principle which has so far been overlooked. We find that the local geometry around the unpaired electron can change the DNP enhancement by an order of magnitude for two otherwise identical conformers. We present a set of 13 new stable mono-and dinitroxide PAs for MAS DNP NMR where this principle is demonstrated. The radicals are divided into two groups of isomers, named open (O-) and closed (C-), based on the ring conformations in the vicinity of the N−O bond. In all cases, the open conformers exhibit dramatically improved DNP performance as compared to the closed counterparts. In particular, a new urea-based biradical named HydrOPol and a mononitroxide O-MbPyTol yield enhancements of 330 ± 60 and 119 ± 25, respectively, at 9.4 T and 100 K, which are the highest enhancements reported so far in the aqueous solvents used here. We find that while the conformational changes do not significantly affect electron spin−spin distances, they do affect the distribution of the exchange couplings in these biradicals. Electron spin echo envelope modulation (ESEEM) experiments suggest that the improved performance of the open conformers is correlated with higher solvent accessibility.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance

et al. 2020

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“…The remarkable efficiency of HyTEK2 at a very high field was attributed to the conjunction of an optimal magnitude for the electron–electron interactions and of sufficiently long electron relaxation times. A recent computational study by Equbal and co-workers pointed out an optimal ratio between the electronic Heisenberg exchange and dipolar coupling interactions in HyTEK2 that could also explain its performance at high magnetic fields …”

Section: Introductionmentioning

confidence: 99%

Efficient Dynamic Nuclear Polarization up to 230 K with Hybrid BDPA-Nitroxide Radicals at a High Magnetic Field

et al. 2021

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Pairing the spectral resolution provided by high magnetic fields at ambient temperature with the enhanced sensitivity offered by dynamic nuclear polarization (DNP) is a major goal of modern solid-state NMR spectroscopy, which will allow one to unlock ever-challenging applications. This study demonstrates that, by combining HyTEK2, a hybrid BDPA-nitroxide biradical polarizing agent, with ortho-terphenyl (OTP), a rigid DNP matrix, enhancement factors as high as 65 can be obtained at 230 K, 40 kHz magic angle spinning (MAS), and 18.8 T. The temperature dependence of the DNP enhancement and its behavior around the glass transition temperature (T g ) of the matrix is investigated by variable-temperature EPR measurements of the electron relaxation properties and numerical simulations. A correlation is suggested between the decrease in enhancement at the passage of the T g and the concomitant drop of both transverse electron relaxation times in the biradical.

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“…Unlike the examples described earlier [ 27 , 39 , 40 ], for asymmetrically substituted 1,3-diazetidines, two possible reaction pathways can be proposed for the reaction with amines ( Figure 10 ). Given that the EPR spectra of radical 4 stayed virtually unchanged during the reaction, it can be assumed that in the spiro-substituted biradical, either the ring opening does not occur, or the reaction proceeds along pathway 1 (the resultant distance between two nitroxyl moieties is less in pathway 1 than in pathway 2).…”

Section: Resultsmentioning

confidence: 99%

“…Ring opening under the action of a nucleophile can cause a change in the backbone of a molecule, an increase in the distance between nitroxide moieties, and consequently, an alteration of the spin–spin interaction. It is worth noting that recent DNP studies highlighted the critical role of J and D couplings (in particular the J /D ratio) in achieving significant enhancements at magnetic fields >9.4 T [ 27 ]. Therefore, it is important to comprehensively analyze the influence of various structural factors of the biradical on the DNP enhancement.…”

Section: Introductionmentioning

confidence: 99%

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization

et al. 2022

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Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.

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Balancing dipolar and exchange coupling in biradicals to maximize cross effect dynamic nuclear polarization

Abstract: Balancing dipolar and exchange coupling is essential for efficient Cross Effect DNP. This explains the complex performance of standard radicals (AMUPOL and HyTek) at high magnetic field and fast spinning.

Cited by 28 publications

References 43 publications

Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance

Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance

Efficient Dynamic Nuclear Polarization up to 230 K with Hybrid BDPA-Nitroxide Radicals at a High Magnetic Field

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization

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