Category Archives: Polarizing Agents

Stability of nitroxide biradical TOTAPOL in biological samples

McCoy, Kelsey M., Rivkah Rogawski, Olivia Stovicek, and Ann E. McDermott. “Stability of Nitroxide Biradical TOTAPOL in Biological Samples.” Journal of Magnetic Resonance 303 (June 1, 2019): 115–20.

We characterize chemical reduction of a nitroxide biradical, TOTAPOL, used in dynamic nuclear polarization (DNP) experiments, specifically probing the stability in whole-cell pellets and lysates, and present a few strategies to stabilize the biradicals for DNP studies. DNP solid-state NMR experiments use paramagnetic species such as nitroxide biradicals to dramatically increase NMR signals. Although there is considerable excitement about using nitroxide-based DNP for detecting the NMR spectra of proteins in whole cells, nitroxide radicals are reduced in minutes in bacterial cell pellets, which we confirm and quantify here. We show that addition of the covalent cysteine blocker N-ethylmaleimide to whole cells significantly slows the rate of reduction, suggesting that cysteine thiol radicals are important to in vivo radical reduction. The use of cell lysates rather than whole cells also slows TOTAPOL reduction, which suggests a possible role for the periplasm and oxidative phosphorylation metabolites in radical degradation. Reduced TOTAPOL in lysates can also be efficiently reoxidized with potassium ferricyanide. These results point to a practical and robust set of strategies for DNP of cellular preparations.

Endogenous dynamic nuclear polarization NMR of hydride-terminated silicon nanoparticles #DNPNMR

Ha, Michelle, Alyxandra N. Thiessen, Ivan V. Sergeyev, Jonathan G.C. Veinot, and Vladimir K. Michaelis. “Endogenous Dynamic Nuclear Polarization NMR of Hydride-Terminated Silicon Nanoparticles.” Solid State Nuclear Magnetic Resonance 100 (August 2019): 77–84.

Silicon nanoparticles (SiNPs) are intriguing materials and their properties fascinate the broader scientific community; they are also attractive to the biological and materials science sub-disciplines because of their established biological and environmental compatibility, as well as their far-reaching practical applications. While characterization of the particle nanostructure can be performed using 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy, poor sensitivity due to low Boltzmann population and long acquisition times hinder in-depth studies of these potentially game-changing materials. In this study, we compare two dynamic nuclear polarization (DNP) NMR protocols to boost 29Si sensitivity in hydride-terminated SiNPs. First, we assess a traditional indirect DNP approach, where a nitroxide biradical (AMUPol or bCTbk) is incorporated into a glassing agent and transferred through protons (e− → 1H → 29Si) to enhance the silicon. In this mode, electron paramagnetic resonance (EPR) spectroscopy demonstrated that the hydride-terminated surface was highly reactive with the exogenous biradicals, thus decomposing the radicals within hours and resulting in an enhancement factor, ε, of 3 (TB = 15 s) for the 64 nm SiNP, revealing the surface components. Secondly, direct DNP NMR methods were used to enhance the silicon without the addition of an exogenous radical (i.e., use of dangling bonds as an endogenous radical source). With radical concentrations <1 mM, 29Si enhancements were obtained for the series of SiNPs ranging from 3 to 64 nm. The ability to use direct 29Si DNP transfer (e− → 29Si) shows promise for DNP studies of these inorganic nanomaterials (ε = 6 (TB = 79 min) for 64 nm SiNPs) with highly reactive surfaces, showing the sub-surface and core features. These preliminary findings lay a foundation for future endogenous radical development through tailoring the surface chemistry, targeting further sensitivity gains.

Proton polarization enhancement of up to 150 with dynamic nuclear polarization of plasma-treated glucose powder #DNPNMR

Katz, Itai, Akiva Feintuch, Raanan Carmieli, and Aharon Blank. “Proton Polarization Enhancement of up to 150 with Dynamic Nuclear Polarization of Plasma-Treated Glucose Powder.” Solid State Nuclear Magnetic Resonance 100 (August 2019): 26–35.

Dynamic nuclear polarization (DNP) for the enhancement of the NMR signals of specific metabolites has recently found applications in the context of magnetic resonance imaging (MRI). Currently, DNP signal enhancement is implemented in clinical systems through the use of exogenous stable organic free radicals, known as polarization agents (PAs), mixed in a solution with the metabolite of interest. These PAs are medically undesirable and thus must be filtered out prior to patient injection – a task that involves considerable technical complexity and consumes valuable time during which the polarization decays. Here, we aim to demonstrate DNP enhancements large enough for clinical relevance using a process free of exogenous PAs. This is achieved by processing (soft grinding) the metabolite in its solid form and subsequently exposing it to plasma in a dilute atmosphere to produce chemically-unstable free radicals (herein referred to as electrical-discharge-induced radicals EDIRs) within the powder. These samples are then subjected to the normal DNP procedure of microwave irradiation while placed under a high static magnetic field, and their NMR signal is measured to quantify the enhancement of the protons’ signal in the solid. Proton signal enhancements (measured as the ratio of the NMR signal with microwave irradiation to the NMR signal without microwave irradiation) of up to 150 are demonstrated in glucose. Upon fast dissolution, the free radicals are annihilated, leaving the sample in its original chemical composition (which is safe for clinical use) without any need for filtration and cumbersome quality control procedures. We thus conclude that EDIRs are found to be highly efficient in providing DNP enhancement levels that are on par with those achieved with the exogenous PAs, while being safe for clinical use. This opens up the possibility of applying our method to clinical scenarios with minimal risks and lower costs per procedure.

A biradical-tagged phospholipid as a polarizing agent for solid-state MAS Dynamic Nuclear Polarization NMR of membrane proteins #DNPNMR

Good, Daryl B., Maxim A. Voinov, David Bolton, Meaghan E. Ward, Ivan V. Sergeyev, Marc Caporini, Peter Scheffer, et al. “A Biradical-Tagged Phospholipid as a Polarizing Agent for Solid-State MAS Dynamic Nuclear Polarization NMR of Membrane Proteins.” Solid State Nuclear Magnetic Resonance, April 2019, S0926204018301140.

A novel Dynamic Nuclear Polarization (DNP) NMR polarizing agent ToSMTSL-PTE representing a phospholipid with a biradical TOTAPOL tethered to the polar head group has been synthesized, characterized, and employed to enhance solid-state Nuclear Magnetic Resonance (SSNMR) signal of a lipid-reconstituted integral membrane protein proteorhodopsin (PR). A matrix-free PR formulation for DNP improved the absolute sensitivity of NMR signal by a factor of ca. 4 compared to a conventional preparation with TOTAPOL dispersed in a glassy glycerol/water matrix. DNP enhancements measured at 400 MHz/263 GHz and 600 MHz/395 GHz showed a strong field dependence but remained moderate at both fields, and comparable to those obtained for PR covalently modified with ToSMTSL. Additional continuous wave (CW) X-band electron paramagnetic resonance (EPR) experiments with ToSMTSL-PTE in solutions and in lipid bilayers revealed that an unfavorable conformational change of the linker connecting mononitroxides could be one of the reasons for moderate DNP enhancements. Further, differential scanning calorimetry (DSC) and CW EPR experiments indicated an inhomogeneous distribution and/or a possibility of a partial aggregation of ToSMTSLPTE in DMPC:DMPA bilayers when the concentration of the polarizing agent was increased to 20 mol% to maximize the DNP enhancement. Thus, conformational changes and inhomogeneous distribution of the lipid-based biradicals in lipid bilayers emerged as important factors to consider for further development of this matrix-free approach for DNP of membrane proteins.

Efficient Hyperpolarization of U- 13C-Glucose Using Narrow-Line UV-Generated Labile Free Radicals #DNPNMR

DNP requires a paramagnetic polarizing agent. This is great for DNP but not so great for the NMR experiment, since the paramagnetic species often causes line broadening due to increase nuclear relaxation. To decrease the unwanted relaxation enhancement some researchers suggested to remove (e.g. filter out) the paramagnetic species after the dissolution step.

The article describes an elegant method, using UV generated radicals for polarization at low temperatures, which recombine once the sample is heated up during the dissolution process, effectively removing the paramagnetic enhanced relaxation process.

Capozzi, Andrea, Saket Patel, Christine Pepke Gunnarsson, Irene Marco-Rius, Arnaud Comment, Magnus Karlsson, Mathilde H. Lerche, Olivier Ouari, and Jan Henrik Ardenkjaer-Larsen. “Efficient Hyperpolarization of U- 13C-Glucose Using Narrow-Line UV-Generated Labile Free Radicals.” Angewandte Chemie, December 20, 2018.

Free radicals generated via irradiation with UV-light of a frozen solution containing a fraction of pyruvic acid (PA), have demonstrated their dissolution Dynamic Nuclear Polarization (dDNP) potential providing up to 30% [1-13C]PA liquid-state polarization. Moreover, their labile nature has proven to pave a way to nuclear polarization storage and transport. Herein, differently from the case of PA, we tackled the issue of providing dDNP UV-radical precursors, trimethylpyruvic acid (TriPA) and its methyl-deuterated form d9-TriPA, not involved in any metabolic pathway. The 13C dDNP performance was evaluated for hyperpolarization of [U-13C6,1,2,3,4,5,6,6-d7]-Dglucose. The generated UV-radical proved to be a versatile and highly efficient polarizing agent providing, after dissolution and transfer (10 s), a 13C liquid-state polarization up to 32%.

Influence of 13C Isotopic Labeling Location on Dynamic Nuclear Polarization of Acetate #DNPNMR

Niedbalski, Peter, Christopher Parish, Andhika Kiswandhi, Zoltan Kovacs, and Lloyd Lumata. “Influence of 13C Isotopic Labeling Location on Dynamic Nuclear Polarization of Acetate.” The Journal of Physical Chemistry A 121, no. 17 (May 4, 2017): 3227–33.

Dynamic nuclear polarization (DNP) via the dissolution method has alleviated the insensitivity problem in liquid-state nuclear magnetic resonance (NMR) spectroscopy by amplifying the signals by several thousand-fold. This NMR signal amplification process emanates from the microwavemediated transfer of high electron spin alignment to the nuclear spins at high magnetic field and cryogenic temperature. Since the interplay between the electrons and nuclei is crucial, the chemical composition of a DNP sample such as the type of free radical used, glassing solvents, or the nature of the target nuclei can significantly affect the NMR signal enhancement levels that can be attained with DNP. Herein, we have investigated the influence of 13C isotopic labeling location on the DNP of a model 13C compound, sodium acetate, at 3.35 T and 1.4 K using the narrow electron spin resonance (ESR) line width free radical trityl OX063. Our results show that the carboxyl 13C spins yielded about twice the polarization produced in methyl 13C spins. Deuteration of the methyl 13C group, while proven beneficial in the liquid-state, did not produce an improvement in the 13C polarization level at cryogenic conditions. In fact, a slight reduction of the solid-state 13C polarization was observed when 2H spins are present in the methyl group. Furthermore, our data reveal that there is a close correlation between the solid-state 13C T1 relaxation times of these samples and the relative 13C polarization levels. The overall results suggest the achievable solid-state polarization of 13C acetate is directly affected by the location of the 13C isotopic labeling via the possible interplay of nuclear relaxation leakage factor and cross-talks between nuclear Zeeman reservoirs in DNP.

BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T #DNPNMR

Wisser, Dorothea, Ganesan Karthikeyan, Alicia Lund, Gilles Casano, Hakim Karoui, Maxim Yulikov, Georges Menzildjian, et al. “BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T.” Journal of the American Chemical Society 140, no. 41 (October 17, 2018): 13340–49.

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and magic angle spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow electron paramagnetic resonance line radical, α,γ-bisdiphenylene-β-phenylallyl (BDPA), tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron–electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP probe, it yields enhancements of up to 185 at 18.8 T (800 MHz 1H resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz 1H resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field 27Al and 29Si DNP surface enhanced NMR spectra of amorphous aluminosilicates and 17O NMR on silica nanoparticles.

Electron-Spin Relaxation of Triarylmethyl Radicals in Glassy Trehalose #DNPNMR

Triarylmethyl radicals are commonly used dissolution-DNP experiments (dDNP). This article is a good reference source for the electronic relaxation times T1e and T2e in different solvents.

Kuzhelev, Andrey A., Olesya A. Krumkacheva, Ivan O. Timofeev, Victor M. Tormyshev, Matvey V. Fedin, and Elena G. Bagryanskaya. “Electron-Spin Relaxation of Triarylmethyl Radicals in Glassy Trehalose.” Applied Magnetic Resonance 49, no. 11 (November 2018): 1171–80.

Trehalose was recently proposed as a promising immobilizer of biomolecules for room-temperature electron paramagnetic resonance (EPR) structural studies. The most crucial parameter in these investigations is electron-spin relaxation (namely, phase memory time Tm). Recently, triarylmethyl (TAM) spin labels attached to DNA in trehalose were found to have the longest Tm at room temperature as compared to the existing spin labels and immobilizers. Therefore, in this work, we investigated TAM radicals in trehalose including Finland trityl (H36 form), perdeuterated Finland trityl (D36 form), and a deuterated version of OX063. The temperature dependence of electron-spin relaxation time of these radicals immobilized in trehalose was measured at X-band frequency, and possible mechanisms of relaxation were considered. OX063D in glassy trehalose has longer Tm up to 200 K as compared to Finland trityl, but at higher temperatures, OX063D is inferior in its relaxation properties, and the deuterated form of Finland trityl is preferable for pulse dipolar EPR spectroscopy experiments at 298 K. The influence of various deuterations (TAM or trehalose) on the observed relaxation times was studied, being controlled by the electron-spin-echo envelope modulation at room temperature.

Large-scale ab initio simulations of MAS DNP enhancements using a Monte Carlo optimization strategy #DNPNMR

Perras, Frédéric A., and Marek Pruski. “Large-Scale Ab Initio Simulations of MAS DNP Enhancements Using a Monte Carlo Optimization Strategy.” The Journal of Chemical Physics 149, no. 15 (October 21, 2018): 154202.

Magic-angle-spinning (MAS) dynamic nuclear polarization (DNP) has recently emerged as a powerful technology enabling otherwise unrealistic solid-state NMR experiments. The simulation of DNP processes which might, for example, aid in refining the experimental conditions or the design of better performing polarizing agents, is, however, plagued with significant challenges, often limiting the system size to only 3 spins. Here, we present the first approach to fully ab initio large-scale simulations of MAS DNP enhancements. The Landau-Zener equation is used to treat all interactions concerning electron spins, and the low-order correlations in the Liouville space method is used to accurately treat the spin diffusion, as well as its MAS speed dependence. As the propagator cannot be stored, a Monte Carlo optimization method is used to determine the steady-state enhancement factors. This new software is employed to investigate the MAS speed dependence of the enhancement factors in large spin systems where spin diffusion is of importance, as well as to investigate the impacts of solvent and polarizing agent deuteration on the performance of MAS DNP.

Conformation of Bis-nitroxide Polarizing Agents by Multi- frequency EPR Spectroscopy #DNPNMR

To optimize the DNP process it is crucial to understand the EPR properties of the polarizing agents. This article demonstrates the need of multi-frequency, high-field EPR spectroscopy to gain a deep understanding of all EPR parameters and how they influence the DNP process.

Soetbeer, Janne, Peter Gast, Joseph J Walish, Yanchuan Zhao, Christy George, Chen Yang, Timothy M Swager, Robert G Griffin, and Guinevere Mathies. “Conformation of Bis-Nitroxide Polarizing Agents by Multi- Frequency EPR Spectroscopy,”

The chemical structure of polarizing agents critically determines the efficiency of dynamic nuclear polarization (DNP). For cross-effect DNP, biradicals are the polarizing agents of choice and the interaction and relative orientation of the two unpaired electrons should be optimal. Both parameters are affected by the molecular structure of the biradical in the frozen glassy matrix that is typically used for DNP/MAS NMR and likely differs from the structure observed with X-ray crystallography. We have determined the conformations of six bis-nitroxide polarizing agents, including the highly efficient AMUPol, in their DNP matrix with EPR spectroscopy at 9.7 GHz, 140 GHz, and 275 GHz. The multi-frequency approach in combination with an advanced fitting routine allows us to reliably extract the interaction and relative orientation of the nitroxide moieties. We compare the structures of six bis-nitroxides to their DNP performance at 500 MHz/330 GHz.

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