Hyperfine interactions of narrow-line trityl radical with solvent molecules

Trukhan, S. N.; Yudanov, V. F.; Tormyshev, Victor M.; Rogozhnikova, Olga Yu.; Trukhin, Dmitry V.; Bowman, Michael K.; Krzyaniak, Matthew D.; Chen, H.; Martyanov, Oleg N.

doi:10.1016/j.jmr.2013.04.017

Cited by 43 publications

(46 citation statements)

References 35 publications

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“…In contrast, trityl has a maximum proton hyperfine coupling of 0.05 MHz. 33 As we will see below this is the likely source of the Overhauser effect observed in the SA-BDPA and BDPA and absent in trityl OX063. The fact that there is a large enhancement at ω 0S could possibly be explained as electron decoupling and is discussed in Ref.…”

Section: Discussionmentioning

confidence: 93%

“…In particular, the lineshape is characteristic of an OE in which the sign of the DNP enhancement does not depend upon the offset of the microwave frequency. Interestingly, trityl OX063 radical, 31 which was designed to optimize Overhauser effects in solution by eliminating hyperfine couplings, 32,33 shows only SE enhancements at ω 0S ± ω 0I . Thus, the OE effect appears to require the presence of multiple hyperfine or dipolar couplings that permit zero quantum (ZQ) or double quantum (DQ) relaxation, respectively, and Overhauser enhancements.…”

Section: Introductionmentioning

confidence: 99%

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Overhauser effects in insulating solids

Can

Caporini

Mentink-Vigier

et al. 2014

The Journal of Chemical Physics

161

221

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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: Discussionmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

Overhauser effects in insulating solids

Can

Caporini

Mentink-Vigier

et al. 2014

The Journal of Chemical Physics

161

221

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“…For nitroxide radicals the unpaired spin density is localized mainly on the N-O bond [32] and in a bis-nitroxide the exchange interaction between the nitroxides is smaller than the dipolar coupling, which is about 35 MHz for AMUPol. [26] However, the unpaired spin density in a trityl radical is highly delocalized, [33] [34] which means that in a TEMTriPol biradical the exchange interaction between the two radical moieties can easily be large. Using X-band EPR spectroscopy in solution at T = 279 K, Liu et al determined the average strength of this interaction for the TEMTriPol biradicals (see Table 1).…”

Section: Introductionmentioning

confidence: 99%

Efficient Dynamic Nuclear Polarization at 800 MHz/527 GHz with Trityl‐Nitroxide Biradicals

et al. 2015

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Cross-effect (CE) dynamic nuclear polarization (DNP) is a rapidly developing technique that enhances the signal intensities in magic angle spinning (MAS) NMR spectra. We report CE DNP experiments at 211, 600 and 800 MHz using a new series of biradical polarizing agents referred to as TEMTriPols, in which a nitroxide (TEMPO) and a trityl radical are chemically tethered. The TEMTriPol molecule with the optimal performance yields a record 1 H NMR signal enhancement of 65 at 800 MHz at a concentration of 10 mM in a glycerol/water solvent matrix. The CE DNP enhancement for the TEMTriPol biradicals does not decrease as the magnetic field is increased in the manner usually observed for bis-nitroxides. Instead, the relatively strong exchange interaction between the trityl and nitroxide moieties determines the magnetic field at which the optimum enhancement is observed. Graphical abstractA series of biradicals consisting of a nitroxide chemically tethered to a trityl are employed for cross-effect DNP at 211, 600 and 800 MHz. The relatively strong exchange interaction between the trityl and nitroxide moieties determines the field strength at which the enhancement is optimized, and yields a record 1 H NMR signal enhancement of 65 at 800 MHz.Correspondence to: Guinevere Mathies; Yangping Liu; Robert G. Griffin. # Current address: Dr. Kan-Nian Hu: Vertex Pharmaceuticals Incorporated, 50 Northern Avenue, Boston, MA 02210 (USA); Dr. Marc A. Caporini: Amgen Inc., 360 Binney Street, Cambridge, MA 02142 (USA) Supporting information for this article is given via a link at the end of the document. In these experiments, unpaired electrons (introduced into the NMR sample in the form of polarizing agents) are irradiated with microwaves at or near the electron Larmor frequency, and induce mutual electron-nuclear spin flips effecting a transfer of the electron spin polarization to the surrounding nuclei. Since its first application to MAS NMR at high field, [11] the efficiency of the DNP method has been increasing steadily through the development of dedicated highpower, high-frequency gyrotrons as microwave sources, specialized NMR/DNP cryo-probes, and optimization of sample preparation. Moreover, major increases in the enhancement factors have been derived from the chemical synthesis of improved polarizing agents. HHS Public AccessThe DNP mechanism that is currently most successful in high-field MAS NMR experiments is the cross-effect (CE It requires two interacting electrons, one of which interacts with a nearby nucleus, commonly a proton. The difference between the electron Larmor frequencies of these two electrons must be equal to the 1 H Larmor frequency, ω 0e1 -ω 0e2 = ±ω 01H . At this simplified CE matching condition (vide infra), strong state mixing occurs and consequently there is a high probability that microwave radiation resonant with electron 1 will flip electron 2 and the coupled proton together. [18] Initially, CE DNP experiments were performed with samples containing high concentrations of nitroxide monoradicals,...

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“…Therefore, radicals with minimized intramolecular hyperfine couplings are expected to give significantly improved DNP enhancements especially at low magnetic fields/low microwave frequencies. Such a radical is the so-called Finland trityl, for which it was shown that in methanol-D 4 intramolecular hyperfine couplings have coupling constants of less than 1.4 MHz and isotropic averages of less than 0.15 MHz [33], experiments along these lines will be performed in the near future. A detailed discussion of DNP with PB and BDPA radical at 9.7 GHz will be presented in a forthcoming publication.…”

Section: Resultsmentioning

confidence: 99%