Category Archives: Radicals

New NMR tools for protein structure and function: Spin tags for dynamic nuclear polarization solid state NMR #DNPNMR

Rogawski, R. and A.E. McDermott, New NMR tools for protein structure and function: Spin tags for dynamic nuclear polarization solid state NMR. Arch. Biochem. Biophys., 2017. 628: p. 102-113.

https://www.ncbi.nlm.nih.gov/pubmed/28623034

Magic angle spinning solid state NMR studies of biological macromolecules [1-3] have enabled exciting studies of membrane proteins [4,5], amyloid fibrils [6], viruses, and large macromolecular assemblies [7]. Dynamic nuclear polarization (DNP) provides a means to enhance detection sensitivity for NMR, particularly for solid state NMR, with many recent biological applications and considerable contemporary efforts towards elaboration and optimization of the DNP experiment. This review explores precedents and innovations in biological DNP experiments, especially highlighting novel chemical biology approaches to introduce the radicals that serve as a source of polarization in DNP experiments.

Persistent Radicals of Self-assembled Benzophenone bis-Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid-state DNP MAS Spectroscopy #DNPNMR #NMR

Most commonly nitroxide-based radicals are used in DNP. However, there are many other stable radicals. This one is a UV generated radical that is stable for weeks at room temperature. In addition it exhibits a fairly narrow linewidth. This radical, in combination with another narrow line radical (BDPA, trityl, etc.) could make a very efficient polarizing agent when mixed together or even covalently attached to each other.

DeHaven, B.A., et al., Persistent Radicals of Self-assembled Benzophenone bis-Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid-state DNP MAS Spectroscopy. Chemistry, 2017. 23(34): p. 8315-8319.

https://www.ncbi.nlm.nih.gov/pubmed/28423212

UV-irradiation of a self-assembled benzophenone bis-urea macrocycle generates mum amounts of radicals that persist for weeks under ambient conditions. High-field EPR and variable-temperature X-band EPR studies suggest a resonance stabilized radical pair through H-abstraction. These endogenous radicals were applied as a polarizing agent for magic angle spinning (MAS) dynamic nuclear polarization (DNP) NMR enhancement. The field-stepped DNP enhancement profile exhibits a sharp peak with a maximum enhancement of on/off =4 superimposed on a nearly constant DNP enhancement of on/off =2 over a broad field range. This maximum coincides with the high field EPR absorption spectrum, consistent with an Overhauser effect mechanism. DNP enhancement was observed for both the host and guests, suggesting that even low levels of endogenous radicals can facilitate the study of host-guest relationships in the solid-state.

EPR Imaging Spin Probe Trityl Radical OX063: A Method for Its Isolation from Animal Effluent, Redox Chemistry of Its Quinone Methide Oxidation Product, and in Vivo Application in a Mouse

Serda, M., et al., EPR Imaging Spin Probe Trityl Radical OX063: A Method for Its Isolation from Animal Effluent, Redox Chemistry of Its Quinone Methide Oxidation Product, and in Vivo Application in a Mouse. Chem Res Toxicol, 2016. 29(12): p. 2153-2156.

https://www.ncbi.nlm.nih.gov/pubmed/27989135

We report herein a method for the recovery, purification, and application of OX063, a costly, commercially available nontoxic spin probe widely used for electron paramagnetic resonance (EPR) imaging, as well as its corresponding quinone methide (QM) form. This precious probe can be successfully recovered after use in animal model experiments (25-47% recovery from crude lyophilizate with 98.5% purity), even from samples that are >2 years old. Significantly, the recovered trityl can be reused in further animal model EPR imaging experiments. The work also describes support for the observed formation of an air-sensitive radical derived from the QM under reducing conditions.

Effects of biradical deuteration on the performance of DNP: towards better performing polarizing agents

Perras, F.A., et al., Effects of biradical deuteration on the performance of DNP: towards better performing polarizing agents. Phys Chem Chem Phys, 2016. 18(1): p. 65-9.

http://www.ncbi.nlm.nih.gov/pubmed/26619055

We study the effects of the deuteration of biradical polarizing agents on the efficiency of dynamic nuclear polarization (DNP) via the cross-effect. To this end, we synthesized a series of bTbK and TOTAPol biradicals with systematically increased deuterium substitution. The deuteration increases the radicals\’ relaxation time, thus contributing to a higher saturation factor and larger DNP enhancement, and reduces the pool of protons within the so-called spin diffusion barrier. Notably, we report that full or partial deuteration leads to improved DNP enhancement factors in standard samples, but also slows down the build-up of hyperpolarization. Improvements in DNP enhancements factors of up to 70% and time savings of up to 38% are obtained upon full deuteration. It is foreseen that this approach may be applied to other DNP polarizing agents thus enabling further sensitivity improvements.

Dynamic nuclear polarization in solid samples by electrical-discharge-induced radicals

Katz, I. and A. Blank, Dynamic nuclear polarization in solid samples by electrical-discharge-induced radicals. J Magn Reson, 2015. 261: p. 95-100.

http://www.ncbi.nlm.nih.gov/pubmed/26547016

Dynamic nuclear polarization (DNP) is a method for enhancing nuclear magnetic resonance (NMR) signals that has many potential applications in chemistry and medicine. Traditionally, DNP signal enhancement is achieved through the use of exogenous radicals mixed in a solution with the molecules of interest. Here we show that proton DNP signal enhancements can be obtained for solid samples without the use of solvent and exogenous radicals. Radicals are generated primarily on the surface of a solid sample using electrical discharges. These radicals are found suitable for DNP. They are stable under moderate vacuum conditions, yet readily annihilate upon compound dissolution or air exposure. This feature makes them attractive for use in medical applications, where the current variety of radicals used for DNP faces regulatory problems. In addition, this solvent-free method may be found useful for analytical NMR of solid samples which cannot tolerate solvents, such as certain pharmaceutical products.

Dynamic nuclear polarization of a glassy matrix prepared by solid state mechanochemical amorphization of crystalline substances

This is a very interesting article describing the preparation of a sample for DNP by co-milling the analyte with the polarizing substrate. For a long time it was thought that just mechanically mixing the polarizing agent with the analyte will not result in a sample that is useful for DNP because the radical has to be mixed at an atomic level. Clearly this article demonstrate that this is not necessary. This will open up DNP to be used with a complete class of new materials.

Elisei, E., et al., Dynamic nuclear polarization of a glassy matrix prepared by solid state mechanochemical amorphization of crystalline substances. Chemical Communications, 2015. 51(11): p. 2080-2083.

http://dx.doi.org/10.1039/C4CC08348B

A mechanochemical \”solvent-free\” route is presented for the preparation of solid samples ready to be employed in the Dynamic Nuclear Polarization (DNP). 1H-DNP build-up curves at 3.46 T as a function of temperature and radical concentration show steady state nuclear polarization of 10% (0.5% TEMPO concentration at 1.75 K).

Diradicals

For solid-state DNP-NMR spectroscopy very large enhancement factors can be achieved when employing the cross-effect, which is especially sufficient when using bi-radicals (e.g. TOTAPOL,bTbk …). Bi-radicals (or Di-radicals) were not invented for DNP-NMR spectroscopy and a huge amount of EPR literature is available describing their magnetic resonance related properties.

The article cited below gives a comprehensive overview and can a source of new inspiration in the hunt for more efficient polarizing agents for DNP-NMR spectroscopy.

Abe, M., Diradicals. Chem. Rev., 2013. 113(9): p. 7011-7088.

Unfortunately no abstract available.

Temperature Dependence of Electron Spin Relaxation of 2,2-Diphenyl-1-Picrylhydrazyl in Polystyrene

Meyer, V., S. Eaton, and G. Eaton, Temperature Dependence of Electron Spin Relaxation of 2,2-Diphenyl-1-Picrylhydrazyl in Polystyrene. Appl. Magn. Reson., 2013. 44(4): p. 509-517.

http://dx.doi.org/10.1007/s00723-012-0417-7

The electron spin relaxation rates for the stable radical DPPH (2,2-diphenyl-1-picrylhydrazyl) doped into polystyrene were studied by inversion recovery and electron spin echo at X-band and Q-band between 20 and 295 K. At low concentration (340 muM, 0.01%) spin-lattice relaxation was dominated by the Raman process and a local mode. At high concentration (140 mM, 5%) relaxation is orders of magnitude faster than at the lower concentration, and 1/T1 is approximately linearly dependent on temperature. Spin lattice relaxation rates are similar at X-band and Q-band. The temperature dependence of spin echo dephasing was faster at about 140 K than at higher or lower temperatures, which is attributed to a wagging motion of the phenyl groups.

Temperature Dependence of Electron Spin Relaxation of 2,2-Diphenyl-1-Picrylhydrazyl in Polystyrene

Meyer, V., S. Eaton, and G. Eaton, Temperature Dependence of Electron Spin Relaxation of 2,2-Diphenyl-1-Picrylhydrazyl in Polystyrene. Appl. Magn. Reson., 2013. 44(4): p. 509-517.

http://dx.doi.org/10.1007/s00723-012-0417-7

The electron spin relaxation rates for the stable radical DPPH (2,2-diphenyl-1-picrylhydrazyl) doped into polystyrene were studied by inversion recovery and electron spin echo at X-band and Q-band between 20 and 295 K. At low concentration (340 muM, 0.01%) spin-lattice relaxation was dominated by the Raman process and a local mode. At high concentration (140 mM, 5%) relaxation is orders of magnitude faster than at the lower concentration, and 1/T1 is approximately linearly dependent on temperature. Spin lattice relaxation rates are similar at X-band and Q-band. The temperature dependence of spin echo dephasing was faster at about 140 K than at higher or lower temperatures, which is attributed to a wagging motion of the phenyl groups.

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