Water-Soluble Narrow-Line Radicals for Dynamic Nuclear Polarization

Haze, Olesya; Corzilius, Björn; Smith, Albert A.; Griffin, Robert G.; Swager, Timothy M.

doi:10.1021/ja304918g

Cited by 98 publications

(143 citation statements)

References 35 publications

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“…Similar to the published data at 5 T, 30 the field profile at 9.4 T of SA-BDPA showed a DNP enhancement symmetrically disposed about ω 0S corresponding to μw irradiation on resonance with the EPR transition. We attribute this Note that the EPR spectrum of trityl exhibits no hyperfine structure due to 1 H couplings.…”

Section: Discussionsupporting

confidence: 88%

“…In contrast BDPA has a total of 21 1 H's coupled to the electron. 41,42 SA-BDPA has 5 fewer 1 H's as suggested by Haze et al 30 due to the addition of -SO 3 groups to the rings. The simulation for BPDA uses isotropic hyperfine coupling constants of −0.…”

Section: Discussionmentioning

confidence: 85%

“…30 In experiments at 5 T (211 MHz for 1 H) these radicals showed the expected SE enhancement at ω SE = ω 0S ± ω 0I and in addition a relatively weak, positive enhancement when microwaves were applied at ω 0S . We have continued investigations of SA-BDPA and at higher fields, 9.4 and 14.1 T (400 and 600 MHz for 1 H), and observed larger enhancements Can et al J. Chem.…”

Section: Introductionmentioning

confidence: 88%

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Overhauser effects in insulating solids

Can

Caporini

Mentink-Vigier

et al. 2014

The Journal of Chemical Physics

172

226

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We report magic angle spinning, dynamic nuclear polarization (DNP) experiments at magnetic fields of 9.4 T, 14.1 T, and 18.8 T using the narrow line polarizing agents 1,3-bisdiphenylene-2-phenylallyl (BDPA) dispersed in polystyrene, and sulfonated-BDPA (SA-BDPA) and trityl OX063 in glassy glycerol/water matrices. The 1 H DNP enhancement field profiles of the BDPA radicals exhibit a significant DNP Overhauser effect (OE) as well as a solid effect (SE) despite the fact that these samples are insulating solids. In contrast, trityl exhibits only a SE enhancement. Data suggest that the appearance of the OE is due to rather strong electron-nuclear hyperfine couplings present in BDPA and SA-BDPA, which are absent in trityl and perdeuterated BDPA (d 21 -BDPA). In addition, and in contrast to other DNP mechanisms such as the solid effect or cross effect, the experimental data suggest that the OE in non-conducting solids scales favorably with magnetic field, increasing in magnitude in going from 5 T, to 9.4 T, to 14.1 T, and to 18.8 T. Simulations using a model two spin system consisting of an electron hyperfine coupled to a 1 H reproduce the essential features of the field profiles and indicate that the OE in these samples originates from the zero and double quantum cross relaxation induced by fluctuating hyperfine interactions between the intramolecular delocalized unpaired electrons and their neighboring nuclei, and that the size of these hyperfine couplings is crucial to the magnitude of the enhancements. Microwave power dependent studies show that the OE saturates at considerably lower power levels than the solid effect in the same samples. Our results provide new insights into the mechanism of the Overhauser effect, and also provide a new approach to perform DNP experiments in chemical, biophysical, and physical systems at high magnetic fields. INTRODUCTIONThe last decade has witnessed a renaissance in the use of high frequency dynamic nuclear polarization (DNP) to enhance sensitivity in nuclear magnetic resonance (NMR) experiments. In particular, the development of gyrotron and other high frequency microwave sources permits DNP to be performed at magnetic fields used in contemporary NMR experiments (5-20 T). 1-8 To date these experiments, which have focused mostly on insulating solids formed from glassy, frozen solutions of proteins and other nonconducting materials, have relied primarily on narrow line monoradicals and the solid effect (SE) 1, 9-11 or nitroxide biradicals and the cross effect (CE) [12][13][14][15][16][17][18] to mediate the polarization process. These approaches have resulted in large signal enhancements and have enabled many experiments that would otherwise be impossible. [19][20][21][22][23] by Overhauser 25 and confirmed by Carver and Slichter, 26 has not been identified or utilized during the course of this renaissance. Although the possibility of an OE in insulator was discussed by Abragam, 27 the conventional wisdom is that Overhauser DNP is important only in systems with mobile electrons ...

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Section: Discussionsupporting

confidence: 88%

Section: Discussionmentioning

confidence: 85%

Section: Introductionmentioning

confidence: 88%

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Overhauser effects in insulating solids

Can

Caporini

Mentink-Vigier

et al. 2014

The Journal of Chemical Physics

172

226

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“…The mixing time was set to 150 ns, the same as what we used for BDPA/PS, because the sulfonation process retains all the protons that are strongly coupled to the electron in BDPA. 57 The repetition time was optimized at 8 ms due to longer T 1e at 80 K. We obtained an enhancement of 70. The long repetition time allows longer mixing time.…”

Section: Discussionmentioning

confidence: 90%

Time domain DNP with the NOVEL sequence

Can

Walish

Swager

et al. 2015

The Journal of Chemical Physics

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We present results of a pulsed dynamic nuclear polarization (DNP) study at 0.35 T (9.7 GHz/ 14.7 MHz for electron/ 1 H Larmor frequency) using a lab frame-rotating frame cross polarization experiment that employs electron spin locking fields that match the 1 H nuclear Larmor frequency, the so called NOVEL (nuclear orientation via electron spin locking) condition. We apply the method to a series of DNP samples including a single crystal of diphenyl nitroxide (DPNO) doped benzophenone (BzP), 1,3-bisdiphenylene-2-phenylallyl (BDPA) doped polystyrene (PS), and sulfonated-BDPA (SA-BDPA) doped glycerol/water glassy matrices. The optimal Hartman-Hahn matching condition is achieved when the nutation frequency of the electron matches the Larmor frequency of the proton, ω 1S = ω 0I , together with possible higher order matching conditions at lower efficiencies. The magnetization transfer from electron to protons occurs on the time scale of ∼100 ns, consistent with the electron-proton couplings on the order of 1-10 MHz in these samples. In a fully protonated single crystal DPNO/BzP, at 270 K, we obtained a maximum signal enhancement of ε = 165 and the corresponding gain in sensitivity of ε(T 1 /T B ) 1/2 = 230 due to the reduction in the buildup time under DNP. In a sample of partially deuterated PS doped with BDPA, we obtained an enhancement of 323 which is a factor of ∼3.2 higher compared to the protonated version of the same sample and accounts for 49% of the theoretical limit. For the SA-BDPA doped glycerol/water glassy matrix at 80 K, the sample condition used in most applications of DNP in nuclear magnetic resonance, we also observed a significant enhancement. Our findings demonstrate that pulsed DNP via the NOVEL sequence is highly efficient and can potentially surpass continuous wave DNP mechanisms such as the solid effect and cross effect which scale unfavorably with increasing magnetic field. Furthermore, pulsed DNP is also a promising avenue for DNP at high temperature. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4927087] INTRODUCTIONDynamic nuclear polarization (DNP) is a process whereby the large polarization present in an electron spin reservoir of a paramagnetic polarizing agent is transferred via microwave irradiation to nuclei, thereby enhancing the nuclear spin polarization. The initial mechanism supporting the DNP process, the Overhauser effect (OE), was proposed in 1953 1 and confirmed experimentally 2,3 in samples with mobile electrons (i.e., metals, solutions, and 1D conductors). In contrast, in insulating solids, such as glycerol/water glasses and biological samples, the OE was thought to be forbidden, but we have recently observed Overhauser enhancements using polarizing agents with narrow EPR spectra that exhibit strong 1 H−e − hyperfine couplings. 4 Furthermore, in these sorts of samples, DNP processes can also be mediated by three other mechanisms -the solid effect (SE), 5,6 the cross effect, 7-11 and/or thermal mixing. 12 Initially, the primary application of DNP was preparation of p...

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“…This is not the case for Solid Effect DNP, where at high microwave power, the build-up time becomes shorter than T 1n . In literature, there are very few systematic studies of the relevant time constants [88,89]. This is partly due to the fact that the most common microwave sources, such as gyrotron oscillators, are continuous wave instruments.…”

Section: Non-polar Solvents: Bdpamentioning

confidence: 99%