Paul Tordo

Dynamic Nuclear Polarization with a Rigid Biradical

A new polarizing agent with superior performance in dynamic nuclear polarization experiments is introduced, and utilizes two TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) moieties connected through a rigid spiro tether (see structure). The observed NMR signal intensities were enhanced by a factor of 1.4 compared to those of TOTAPOL, a previously described TEMPO-based biradical with a flexible tether.

Highly Efficient, Water‐Soluble Polarizing Agents for Dynamic Nuclear Polarization at High Frequency

Well polarized: Two new polarizing agents PyPol and AMUPol soluble in glycerol/water mixtures are used for dynamic nuclear polarization (DNP) NMR spectroscopy. The enhancement factors (e) are about 3.5 to 4 times larger than for the established agent TOTAPOL at 263 and 395 GHz. For AMUPol, the temperature dependence of e allows DNP experiments to be performed at temperatures significantly higher than for typical high-field DNP NMR experiments.

Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K

A series of seven functionalized nitroxide biradicals (the bTbK biradical and six derivatives) are investigated as exogenous polarization sources for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and with ca. 100 K sample temperatures. The impact of electron relaxation times on the DNP enhancement (ε) is examined, and we observe that longer inversion recovery and phase memory relaxation times provide larger ε. All radicals are tested in both bulk 1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and the difference in ε between the two cases is discussed. The impact of the sample temperature and magic angle spinning frequency on ε is investigated for several radicals each characterized by a range of electron relaxation times. In particular, TEKPol, a bulky derivative of bTbK with a molecular weight of 905 g·mol(-1), is presented. Its high-saturation factor makes it a very efficient polarizing agent for DNP, yielding unprecedented proton enhancements of over 200 in both bulk and materials samples at 9.4 T and 100 K. TEKPol also yields encouraging enhancements of 33 at 180 K and 12 at 200 K, suggesting that with the continued improvement of radicals large ε may be obtained at higher temperatures.

Detection of superoxide anion using an isotopically labeled nitrone spin trap: potential biological applications

We describe the synthesis and biological applications of a novel nitrogen‐15‐labeled nitrone spin trap, 5‐ethoxycarbonyl‐5‐methyl‐1‐pyrroline N‐oxide ([15N]EMPO) for detecting superoxide anion. Superoxide anion generated in xanthine/xanthine oxidase (100 nM min−1) and NADPH/calcium‐calmodulin/nitric oxide synthase systems was readily detected using EMPO, a nitrone analog of 5,5′‐dimethyl‐1‐pyrroline N‐oxide (DMPO). Unlike DMPO‐superoxide adduct (DMPO–OOH), the superoxide adduct of EMPO (EMPO–OOH) does not spontaneously decay to the corresponding hydroxyl adduct, making spectral interpretation less confounding. Although the superoxide adduct of 5‐(diethoxyphosphoryl)‐5‐methyl‐pyrroline N‐oxide is more persistent than EMPO–OOH, the electron spin resonance spectra of [14N]EMPO–OOH and [15N]EMPO–OOH are less complex and easier to interpret. Potential uses of [15N]EMPO in elucidating the mechanism of superoxide formation from nitric oxide synthases, and in ischemia/reperfusion injury are discussed.

A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials

A new nitroxide-based biradical having a long electron spin-lattice relaxation time (T(1e)) has been developed as an exogenous polarization source for DNP solid-state NMR experiments. The performance of this new biradical is demonstrated on hybrid silica-based mesostructured materials impregnated with 1,1,2,2-tetrachloroethane radical containing solutions, as well as in frozen bulk solutions, yielding DNP enhancement factors (ε) of over 100 at a magnetic field of 9.4 T and sample temperatures of ~100 K. The effects of radical concentration on the DNP enhancement factors and on the overall sensitivity enhancements (Σ(†)) are reported. The relatively high DNP efficiency of the biradical is attributed to an increased T(1e), which enables more effective saturation of the electron resonance. This new biradical is shown to outperform the polarizing agents used so far in DNP surface-enhanced NMR spectroscopy of materials, yielding a 113-fold increase in overall sensitivity for silicon-29 CPMAS spectra as compared to conventional NMR experiments at room temperature. This results in a reduction in experimental times by a factor >12,700, making the acquisition of (13)C and (15)N one- and two-dimensional NMR spectra at natural isotopic abundance rapid (hours). It has been used here to monitor a series of chemical reactions carried out on the surface functionalities of a hybrid organic-silica material.

2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-1-oxide: Evaluation of the spin trapping properties

The 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-l-oxide (EMPO), an easily prepared pyrroline-N-oxide has been tested as a free radical scavenger. Spin adducts of superoxide, hydroxyl radical, and other free radicals were characterized in phosphate buffer at pH 7.0 and 5.6. At pH 7 in phosphate buffer, the EMPO/O(2)(-*) spin adduct was estimated to be about five times more persistent than its DMPO analogue. Furthermore, its decay does not produce the EMPO/HO&z.rad; adduct.

Dynamic Nuclear Polarization Enhanced Solid‐State NMR Spectroscopy of Functionalized Metal–Organic Frameworks

Dynamic nuclear polarization (DNP) is applied to enhance the signal of solid-state NMR spectra of metal-organic framework (MOF) materials. The signal enhancement enables the acquisition of high-quality 1D 13C solid-state NMR spectra, 2D 1H-13C dipolar HETCOR and 1D 15N solid-state NMR spectra with natural isotopic abundance in experiment times on the order of minutes.

Kinetic subtleties of nitroxide mediated polymerization

Due to the academic and industrial interest of Nitroxide Mediated Polymerization (NMP), a lot of investigations have focused on the kinetics of this process. During the last decade, although the simplified kinetic scheme--equilibrium reactions between dormant species (alkoxyamine) and active species (alkyl radicals and nitroxides), propagation reaction of the macro-alkyl radical, and termination reactions--was suitable to predict the main trends at the macromolecular level, it has become obvious that it was not sufficient to describe all the kinetic effects involved in the NMP process. Indeed, like the conventional radical polymerization, NMP should be described as a 3 stage process including initiation, propagation, and self- and cross-termination. These two types of radical polymerization differ by the occurrence during NMP of an activation/deactivation process involving the dormant species in both the initiation and propagation stages. Evidence is provided of the importance of the rate of homolysis of the initiator (alkoxyamines) and of the rate of the first alkyl radical addition onto the monomer for the success of NMP. Thus, the fundamental kinetics of the main reactions involved in NMP as well as side-reactions are also discussed in this tutorial review.

Scavenging Free Radicals To Preserve Enhancement and Extend Relaxation Times in NMR using Dynamic Nuclear Polarization

This enhance-ment arises from thermal mixing, which is brought about bymicrowavesaturationoftheEPRtransitionsofstableradicalsthat are mixed with the sample under investigation beforefreezing. In dissolution DNP, the sample is usually polarizedat low temperatures and moderate magnetic fields (T=1.2 Kand B

Non-aqueous solvents for DNP surface enhanced NMR spectroscopy

A series of non-aqueous solvents combined with the exogenous biradical bTbK are developed for DNP NMR that yield enhancements comparable to the best available water based systems. 1,1,2,2-tetrachloroethane appears to be one of the most promising organic solvents for DNP solid-state NMR. Here this results in a reduction in experimental times by a factor of 1000. These new solvents are demonstrated with the first DNP surface enhanced NMR characterization of an organometallic complex supported on a hydrophobic surface.