Mandal, Sucharita, and Snorri Th. Sigurdsson. “Water-Soluble BDPA Radicals with Improved Persistence.” Chemical Communications, 2020, 10.1039.D0CC04920D.
1,3-Bis(diphenylene)-2-phenylallyl (BDPA) radicals are promising polarizing agents for increasing the sensitivity of NMR spectroscopy through dynamic nuclear polarization (DNP), but have low persistence and solubility in aqueous media. New tetraalkyl/aryl-ammonium derivatives of BDPA are soluble in polar solvents and are highly persistent, with 5–20-fold lower initial rates of degradation than BDPA.
Succinyl-DOTOPA: An effective triradical dopant for low-temperature dynamic nuclear polarization with high solubility in aqueous solvent mixtures at neutral pH
Yau, Wai-Ming, Jaekyun Jeon, and Robert Tycko. “Succinyl-DOTOPA: An Effective Triradical Dopant for Low-Temperature Dynamic Nuclear Polarization with High Solubility in Aqueous Solvent Mixtures at Neutral PH.” Journal of Magnetic Resonance 311 (February 2020): 106672.
We report the synthesis of the nitroxide-based triradical compound succinyl-DOTOPA and the characterization of its performance as a dopant for dynamic nuclear polarization (DNP) experiments in frozen solutions at low temperatures. Compared with previously described DOTOPA derivatives, succinyl-DOTOPA has substantially greater solubility in glycerol/water mixtures with pH > 4 and therefore has wider applicability. Solid state nuclear magnetic resonance (ssNMR) measurements at 9.39 T and 25 K, with magic-angle spinning at 7.00 kHz, show that build-up times of DNP-enhanced, cross-polarized 13C ssNMR signals are shorter and that signal amplitudes are larger for glycerol/water solutions of L-proline containing succinyl-DOTOPA than for solutions containing the biradical AMUPol, with electron spin concentrations of 15 mM or 30 mM, resulting in greater net sensitivity gains from DNP. In similar measurements at 90 K, AMUPol yields greater net sensitivity, apparently due to its longer electron spin-lattice and spin-spin relaxation times. One- and two-dimensional 13C ssNMR measurements at 25 K on the complex of the 27-residue peptide M13 with the calcium-sensing protein calmodulin, in glycerol/water with 10 mM succinyl-DOTOPA, demonstrate the utility of this compound in DNP-enhanced ssNMR studies of biomolecular systems.
Biradical rotamer states tune electron J coupling and MAS dynamic nuclear polarization enhancement #DNPNMR
Tagami, Kan, Asif Equbal, Ilia Kaminker, Bernard Kirtman, and Songi Han. “Biradical Rotamer States Tune Electron J Coupling and MAS Dynamic Nuclear Polarization Enhancement.” Solid State Nuclear Magnetic Resonance 101 (September 2019): 12–20.
Cross Eﬀect (CE) Dynamic Nuclear Polarization (DNP) relies on the dipolar (D) and exchange (J) coupling interaction between two electron spins. Until recently only the electron spin D coupling was explicitly included in quantifying the DNP mechanism. Recent literature discusses the potential role of J coupling in DNP, but does not provide an account of the distribution and source of electron spin J coupling of commonly used biradicals in DNP. In this study, we quantiﬁed the distribution of electron spin J coupling in AMUPol and TOTAPol biradicals using a combination of continuous wave (CW) X-band electron paramagnetic resonance (EPR) lineshape analysis in a series of solvents and at variable temperatures in solution – a state to be vitriﬁed for DNP. We found that both radicals show a temperature dependent distribution of J couplings, and the source of this distribution to be conformational dynamics. To qualify this conformational dependence of J coupling in both molecules we carry out “Broken Symmetry” DFT calculations which show that the biradical rotamer distribution can account for a large distribution of J couplings, with the magnitude of J coupling directly depending on the relative orientation of the electron spin pair. We demonstrate that the electron spin J couplings in both AMUPol and TOTAPol span a much wider distribution than suggested in the literature. We aﬃrm the importance of electron spin J coupling for DNP with density matrix simulations of DNP in Liouville space and under magic angle spinning, showcasing that a rotamer with high J coupling and “optimum” relative g-tensor orientation can signiﬁcantly boost the DNP performance compared to random orientations of the electron spin pair. We conclude that moderate electron spin J coupling above a threshold value can facilitate DNP enhancements.
Efficiency of Water-Soluble Nitroxide Biradicals for Dynamic Nuclear Polarization in Rotating Solids at 9.4 T: bcTol-M and cyolyl-TOTAPOL as New Polarizing Agents #DNPNMR
Geiger, Michel-Andreas, Anil P. Jagtap, Monu Kaushik, Han Sun, Daniel Stöppler, Snorri T. Sigurdsson, Björn Corzilius, and Hartmut Oschkinat. “Efficiency of Water-Soluble Nitroxide Biradicals for Dynamic Nuclear Polarization in Rotating Solids at 9.4 T: BcTol-M and Cyolyl-TOTAPOL as New Polarizing Agents.” Chemistry – A European Journal 24, no. 51 (September 12, 2018): 13485–94.
Nitroxide biradicals are very efficient polarizing agents in magic angle spinning (MAS) cross effect (CE) dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR). Many recently synthesized, new radicals show superior DNP-efficiency in organic solvents but suffer from insufficient solubility in water or glycerol/water for biological applications. We report DNP efficiencies for two new radicals, the water-soluble bcTol-M and cyolyl-TOTAPOL, and include a comparison with three known biradicals, TOTAPOL, bcTol, and AMUPol. They differ by linker groups, featuring either a 3-aminopropane-1,2-diol or a urea tether, or by the structure of the alkyl substituents that flank the nitroxide groups. For evaluating their performances, we measured both signal enhancements e and DNP-enhanced sensitivity k, and compared the results to electron spin relaxation data recorded at the same magnetic field strength (9.4 T). In our study, differences in DNP efficiency correlate with changes in the nuclear polarization dynamics rather than electron relaxation.
The ratios of their individual e and k differ by up to 20%, which is explained by starkly different nuclear polarization build-up rates. For the radicals compared here empirically, using proline standard solutions, the new radical bcTol-M performs best while being most soluble in water/glycerol mixtures.
Biradicals are very important polarizing agents used in DNP-enhanced NMR spectroscopy. Specifically in solid-state experiments they often out-perform monoradicals. Understanding the influence of all the different interactions present in a biradical is still ongoing research and only by using liquid and solid state EPR spectroscopy is it possible to characterize, understand and finally optimize polarizing agents for DNP.
Eaton, Sandra S., Lukas B. Woodcock, and Gareth R. Eaton. “Continuous Wave Electron Paramagnetic Resonance of Nitroxide Biradicals in Fluid Solution.” Concepts in Magnetic Resonance Part A, May 25, 2018, e21426.
Nitroxide biradicals have been prepared with electron-electron spin-spin exchange interaction, J, ranging from weak to very strong. EPR spectra of these biradicals in fluid solution depend on the ratio of J to the nitrogen hyperfine coupling, AN, and the rates of interconversion between conformations with different values of J. For relatively rigid biradicals EPR spectra can be simulated as the superposition of AB splitting patterns arising from different combinations of nitrogen nuclear spin states. For more flexible biradicals spectra can be simulated with a Liouville representation of the dynamics that interconvert conformations with different values of J on the EPR timescale. Analysis of spectra, factors that impact J, and examples of applications to chemical and biophysical problems are discussed.
Ranallo, Simona, Alessia Amodio, Andrea Idili, Alessandro Porchetta, and Francesco Ricci. “Electronic Control of DNA-Based Nanoswitches and Nanodevices.” Chem. Sci. 7, no. 1 (2016): 66–71.
The exchange (J) interaction of organic biradicals is a crucial factor controlling their physiochemical properties and potential applications, and can be modulated by changing the nature of the linker. In the present work, we for the first time demonstrate the effect of chiral configurations of radical parts on the J values of trityl-nitroxide (TN) biradicals. Four diastereoisomers (TNT1, TNT2, TNL1 and TNL2) of TN biradicals were synthesized and purified by conjugation of a racemic (R/S) nitroxide with the racemic (M/P) trityl radical via L-proline. The absolute configurations of these diastereoisomers were assigned by comparing experimental and calculated electronic circular dichroism (ECD) spectra as (M, S, S) for TNT1, (P, S, S) for TNT2, (M, S, R) for TNL1 and (P, S, R) for TNL2. Electron paramagnetic resonance (EPR) results showed that the configuration of the nitroxide part instead of the trityl part is dominant in controlling the exchange interactions and the order of the J values at room temperature is TNT1 (252 G) > TNT2 (127 G) >> TNL2 (33 G) > TNL1 (14 G). Moreover, the J values of TNL1/ TNL2 with the S configuration in the nitroxide part vary with temperature and polarity of solvents due to their flexible linker, whereas the J values of TNT1/ TNT2 are almost insensitive to these two factors due to the rigidity of their linkers. The distinct exchange interactions between TNT1,2 and TNL1,2 in frozen state led to strongly different high-field dynamic nuclear polarization (DNP) enhancements with ε = 7 for TNT1,2 and 40 for TNL1,2 at 800 MHz DNP conditions.
Audran, G., et al., Trityl-based alkoxyamines as NMP controllers and spin-labels. Polym. Chem., 2016. 7(42): p. 6490-6499.
Recently, new applications of trityl-nitroxide biradicals were proposed. In the present study, attachment of a trityl radical to alkoxyamines was performed for the first time. The rate constants kd of C-ON bond homolysis in these alkoxyamines were measured and found to be similar to those for alkoxyamines without a trityl moiety. The electron paramagnetic resonance (EPR) spectra of the products of alkoxyamine homolysis (trityl-TEMPO and trityl-SG1 biradicals) were recorded, and the corresponding exchange interactions were estimated. The decomposition of trityl-alkoxyamines showed more than an 80% yield of biradicals, meaning that the C-ON bond homolysis is the main reaction. The suitability of these labelled initiators/controllers for polymerisation was exemplified by means of a successful nitroxide-mediated polymerisation (NMP) of styrene. Thus, this is the first report of a spin-labelled alkoxyamine suitable for NMP.
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.
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.
Kubicki, D.J., et al., Rational design of dinitroxide biradicals for efficient cross-effect dynamic nuclear polarization. Chem. Sci., 2015.
A series of 37 dinitroxide biradicals have been prepared and their performance studied as polarizing agents in cross effect DNP NMR experiments at 9.4 T and 100 K in 1,1,2,2-tetrachloroethane (TCE). We observe that in this regime the DNP performance is strongly correlated with the substituents on the polarizing agents, and electron and nuclear spin relaxation times, with longer relaxation times leading to better enhancements. We also observe that deuteration of the radicals generally leads to better DNP enhancement but with longer buildup time. One of the new radicals introduced here provides the best performance obtained so far under these conditions.
Fernandez-de-Alba, C., et al., Matrix-free DNP-enhanced NMR spectroscopy of liposomes using a lipid-anchored biradical. Chemistry, 2015. 21(12): p. 4512-7.
Magic-angle spinning dynamic nuclear polarization (MAS-DNP) has been proven to be a powerful technique to enhance the sensitivity of solid-state NMR (SSNMR) in a wide range of systems. Here, we show that DNP can be used to polarize lipids using a lipid-anchored polarizing agent. More specifically, we introduce a C16-functionalized biradical, which allows localization of the polarizing agents in the lipid bilayer and DNP experiments to be performed in the absence of excess cryo-protectant molecules (glycerol, dimethyl sulfoxide, etc.). This constitutes another original example of the matrix-free DNP approach that we recently introduced.