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Badoni S, Berruyer P, Emsley L. Optimal sensitivity for 1H detected relayed DNP of organic solids at fast MAS. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2024; 360:107645. [PMID: 38401477 DOI: 10.1016/j.jmr.2024.107645] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/03/2023] [Revised: 01/30/2024] [Accepted: 02/09/2024] [Indexed: 02/26/2024]
Abstract
Dynamic nuclear polarization (DNP) combined with high magnetic fields and fast magic angle spinning (MAS) has opened up a new avenue for the application of exceptionally sensitive 1H NMR detection schemes to study protonated solids. Recently, it has been shown that DNP experiments at fast MAS rates lead to slower spin diffusion and hence reduced DNP enhancements for impregnated materials. However, DNP enhancements alone do not determine the overall sensitivity of a NMR experiment. Here we measure the overall sensitivity of one-dimensional 1H detected relayed DNP experiments as a function of the MAS rate in the 20-60 kHz regime using 0.7 mm diameter rotors at 21.2 T. Although faster MAS rates are detrimental for the DNP enhancement on the target material, due to slower spin diffusion, we find that with increasing spinning rates the gain in sensitivity due to 1H line-narrowing and the folding-in of sideband intensity compensates a large part of the loss of overall hyperpolarization. We find that sensitivity depends on the atomic site in the molecule, and is maximised at between 40 and 50 kHz MAS for the sample of L-histidine.HCl·H2O studied here. There is a 10-20 % difference in sensitivity between the optimum MAS rate and the fastest rate currently accessible (60 kHz).
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Affiliation(s)
- Saumya Badoni
- Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
| | - Pierrick Berruyer
- Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
| | - Lyndon Emsley
- Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
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Chatterjee S, Venkatesh A, Sigurdsson ST, Mentink-Vigier F. Role of Protons in and around Strongly Coupled Nitroxide Biradicals for Cross-Effect Dynamic Nuclear Polarization. J Phys Chem Lett 2024; 15:2160-2168. [PMID: 38364262 DOI: 10.1021/acs.jpclett.3c03472] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/18/2024]
Abstract
In magic angle spinning dynamic nuclear polarization (DNP), biradicals such as bis-nitroxides are used to hyperpolarize protons under microwave irradiation through the cross-effect mechanism. This mechanism relies on electron-electron spin interactions (dipolar coupling and exchange interaction) and electron-nuclear spin interactions (hyperfine coupling) to hyperpolarize the protons surrounding the biradical. This hyperpolarization is then transferred to the bulk sample via nuclear spin diffusion. However, the involvement of the protons in the biradical in the cross-effect DNP process has been under debate. In this work, we address this question by exploring the hyperpolarization pathways in and around bis-nitroxides. We demonstrate that for biradicals with strong electron-electron interactions, as in the case of the AsymPols, the protons on the biradical may not be necessary to quickly generate hyperpolarization. Instead, such biradicals can efficiently, and directly, polarize the surrounding protons of the solvent. The findings should impact the design of the next generation of biradicals.
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Affiliation(s)
- Satyaki Chatterjee
- University of Iceland, Department of Chemistry, Science Institute, Dunhaga 3, 107 Reykjavik, Iceland
| | - Amrit Venkatesh
- National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States
| | - Snorri Th Sigurdsson
- University of Iceland, Department of Chemistry, Science Institute, Dunhaga 3, 107 Reykjavik, Iceland
| | - Frédéric Mentink-Vigier
- National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States
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Tatman BP, Franks WT, Brown SP, Lewandowski JR. Nuclear spin diffusion under fast magic-angle spinning in solid-state NMR. J Chem Phys 2023; 158:2890210. [PMID: 37171196 DOI: 10.1063/5.0142201] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2023] [Accepted: 04/10/2023] [Indexed: 05/13/2023] Open
Abstract
Solid-state nuclear spin diffusion is the coherent and reversible process through which spin order is transferred via dipolar couplings. With the recent increases in magic-angle spinning (MAS) frequencies and magnetic fields becoming routinely applied in solid-state nuclear magnetic resonance, understanding how the increased 1H resolution obtained affects spin diffusion is necessary for interpretation of several common experiments. To investigate the coherent contributions to spin diffusion with fast MAS, we have developed a low-order correlation in Liouville space model based on the work of Dumez et al. (J. Chem. Phys. 33, 224501, 2010). Specifically, we introduce a new method for basis set selection, which accounts for the resonance-offset dependence at fast MAS. Furthermore, we consider the necessity of including chemical shift, both isotropic and anisotropic, in the modeling of spin diffusion. Using this model, we explore how different experimental factors change the nature of spin diffusion. Then, we show case studies to exemplify the issues that arise in using spin diffusion techniques at fast spinning. We show that the efficiency of polarization transfer via spin diffusion occurring within a deuterated and 100% back-exchanged protein sample at 60 kHz MAS is almost entirely dependent on resonance offset. We additionally identify temperature-dependent magnetization transfer in beta-aspartyl L-alanine, which could be explained by the influence of an incoherent relaxation-based nuclear Overhauser effect.
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Affiliation(s)
- Ben P Tatman
- Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
- Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom
| | - W Trent Franks
- Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
| | - Steven P Brown
- Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
| | - Józef R Lewandowski
- Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom
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Perras FA, Matsuki Y, Southern SA, Dubroca T, Flesariu DF, Van Tol J, Constantinides CP, Koutentis PA. Mechanistic origins of methyl-driven Overhauser DNP. J Chem Phys 2023; 158:154201. [PMID: 37093991 DOI: 10.1063/5.0149664] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2023] [Accepted: 03/31/2023] [Indexed: 04/26/2023] Open
Abstract
The Overhauser effect in the dynamic nuclear polarization (DNP) of non-conducting solids has drawn much attention due to the potential for efficient high-field DNP as well as a general interest in the underlying principles that enable the Overhauser effect in small molecules. We recently reported the observation of 1H and 2H Overhauser effects in H3C- or D3C-functionalized Blatter radical analogs, which we presumed to be caused by methyl rotation. In this work, we look at the mechanism for methyl-driven Overhauser DNP in greater detail, considering methyl librations and tunneling in addition to classical rotation. We predict the temperature dependence of these mechanisms using density functional theory and spin dynamics simulations. Comparisons with results from ultralow-temperature magic angle spinning-DNP experiments revealed that cross-relaxation at temperatures above 60 K originates from both libration and rotation, while librations dominate at lower temperatures. Due to the zero-point vibrational nature of these motions, they are not quenched by very low temperatures, and methyl-driven Overhauser DNP is expected to increase in efficiency down to 0 K, predominantly due to increases in nuclear relaxation times.
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Affiliation(s)
- Frédéric A Perras
- Chemical and Biological Sciences Division, Ames National Laboratory, Ames, Iowa 50011, USA
- Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA
| | - Yoh Matsuki
- Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA
- Institute for Protein Research, Osaka University, Suita, Osaka 565-0871, Japan
| | - Scott A Southern
- Chemical and Biological Sciences Division, Ames National Laboratory, Ames, Iowa 50011, USA
| | - Thierry Dubroca
- National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA
| | - Dragos F Flesariu
- Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus
| | - Johan Van Tol
- National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA
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Gan Z. An analytical treatment of electron spectral saturation for dynamic nuclear polarization NMR of rotating solids. J Chem Phys 2023; 158:024114. [PMID: 36641384 DOI: 10.1063/5.0109077] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Saturation of electron magnetization by microwave irradiation under magic-angle spinning (MAS) is studied theoretically. The saturation is essential for dynamic nuclear polarization (DNP) enhancement of nuclear magnetic resonance signals. For a spin with a large g-anisotropy and a long T1 relative to the rotor period, the sample rotation distributes saturation to the whole powder sample spectrum. Analytical expressions for the saturation and frequency profiles are obtained. For a pair of coupled electrons such as those in bis-nitroxides, which are commonly used for MAS DNP, an el-er model (where el and er stand for electrons on the left and the right, respectively, in their spectral positions) is introduced to simplify the analysis of a coupled two-spin system under MAS. For such a system, strong electron couplings exchange magnetization during dipolar/J rotor events when the two electron frequencies cross each other. The exchange is equivalent to a swap of the el and er electrons. This allows for the treatment of a coupled spin pair as two independent spins such that an analytical solution can be obtained for the steady-state magnetization and the difference between the two electrons. The theoretical study with its analytical result provides a simple physical picture of electron saturation under MAS and of how radical properties and experimental parameters affect cross-effect DNP. The effects of depolarization and the extension to more coupled electron spins are also discussed using this approach.
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Affiliation(s)
- Zhehong Gan
- National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, USA
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Perras FA, Flesariu DF, Southern SA, Nicolaides C, Bazak JD, Washton NM, Trypiniotis T, Constantinides CP, Koutentis PA. Methyl-Driven Overhauser Dynamic Nuclear Polarization. J Phys Chem Lett 2022; 13:4000-4006. [PMID: 35482607 DOI: 10.1021/acs.jpclett.2c00748] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
The Overhauser effect is unique among DNP mechanisms in that it requires the modulation of the electron-nuclear hyperfine interactions. While it dominates DNP in liquids and metals, where unpaired electrons are highly mobile, Overhauser DNP is possible in insulating solids if rapid structural modulations are linked to a modulation in hyperfine coupling. Herein, we report that Overhauser DNP can be triggered by the strategic addition of a methyl group, demonstrated here in a Blatter's radical. The rotation of the methyl group leads to a modulation of the hyperfine coupling to its protons, which in turn facilitates electron-nuclear cross-relaxation. Removal of the methyl protons, through deuteration, quenches the process, as does the reduction of the hyperfine coupling strength. This result suggests the possibility for the design of tailor-made Overhauser DNP polarizing agents for high-field MAS-DNP.
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Affiliation(s)
| | - Dragos F Flesariu
- Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus
| | | | | | - J David Bazak
- Physical & Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States
| | - Nancy M Washton
- Physical & Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States
| | | | - Christos P Constantinides
- Department of Natural Sciences, University of Michigan-Dearborn, 4901 Evergreen Road, Dearborn, Michigan 48128-1491, United States
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