The 5-diethoxyphosphonyl-5-methyl-1-pyrroline N-oxide superoxide spin adduct (DEPMPO-OOH) is much more persistent (about 15 times) than the 5,5-dimethyl-1-pyrroline N-oxide superoxide spin adduct (DMPO-OOH). The diethoxyphosphonyl group is bulkier than the methyl group and its electron-withdrawing effect is much stronger. These two factors could play a role in explaining the different half-lifetimes of DMPO-OOH and DEPMPO-OOH. The trifluoromethyl and the diethoxyphosphonyl groups show similar electron-withdrawing effects but have different sizes. We have thus synthesized and studied 5-methyl-5-trifluoromethyl-1-pyrroline N-oxide (5-TFDMPO), a new trifluoromethyl analogue of DMPO, to compare its spin-trapping performance with those of DMPO and DEPMPO. 5-TFDMPO was prepared in a five-step sequence by means of the Zn/AcOH reductive cyclization of 5,5,5-trifluoro-4-methyl-4-nitropentanal, and the geometry of the molecule was estimated by using DFT calculations. The spin-trapping properties were investigated both in toluene and in aqueous buffer solutions for oxygen-, sulfur-, and carbon-centered radicals. All the spin adducts exhibit slightly different fluorine hyperfine coupling constants, thereby suggesting a hindered rotation of the trifluoromethyl group, which was confirmed by variable-temperature EPR studies and DFT calculations. In phosphate buffer at pH 7.4, the half-life of 5-TFDMPOOOH is about three times shorter than for DEPMPO-OOH and five times longer than for DMPO-OOH. Our results suggest that the stabilization of the superoxide adducts comes from a delicate balance between steric, electronic, and hydrogen-bonding effects that involve the β group, the hydroperoxyl moiety, and the nitroxide.

The 2-ethoxycarbonyl-2-methyl-3,4-dihydro2H-pyrrole-1-oxide (EMPO), an easily prepared pyrroline-N-oxide has been tested as free radical scavenger. Spin adducts of superoxide, hydroxyl radical and other free radical s were characterized in phosphate buffer at pH 7.0 and 5.6. At pH 7 in phosphate buffer, the EM PO/O2 -• spin adduct was estimated to be about 5 times more persistent than its DMPO analogu e. F rthermore, its decay does not produce the EMPO/ HO • adduct.

We have prepared a series of dinitroxides and we investigated their properties as polarizing agents for solid-state nuclear magnetic resonance/dynamic nuclear polarization applications at 100 K, 9.34 T (263 GHz electron paramagnetic resonance and 400 MHz 1H nuclear magnetic resonance). Our results show that a rigid structure with an orthogonal relative orientation of electron g tensors and the appropriate orientation of the two \( {\rm N} - {\rm O} \cdot \) bonds are required to obtain maximum polarization enhancements. In addition, with dinitroxides exhibiting a long T 1e, the saturation of the irradiated electron spin packet is favored leading to more efficient dynamic nuclear polarization.

We have investigated the properties of a series of dinitroxides as polarizing agents for SS NMR/DNP applications. Our results clearly establish that an orthogonal relative orientation of electron g tensors is a crucial requirement to obtain high enhancement DNP factors. Moreover, the ratio of the proton Larmor frequency over the e(-)-e(-) dipolar coupling (omega(H)/omega(D)) influences the efficiency of the cross effect (CE) mechanism, thus the < Ree > distance between the unpaired electrons must be adapted to omega(H).

Perdeuteration of biological macromolecules for magic angle spinning solid-state NMR spectroscopy can yield high-resolution 2H–13C correlation spectra and the method is therefore of great interest for the structural biology community. Here we demonstrate that the combination of sample deuteration and dynamic nuclear polarization yields resolved 2H–13C correlation spectra with a signal enhancement of e Z 700 compared to a spectrum recorded with microwaves off and otherwise identical conditions. To our knowledge, this is the first time that 2H-DNP has been employed to enhance MAS-NMR spectra of a biologically relevant system. The DNP process is studied using several polarizing agents and the technique is applied to obtain 2H–13C correlation spectra of U-[2H, 13C] proline.

We have characterized the rigid binitroxide radical bis-TEMPO–bis-Ketal (bTbK) by continuous-wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopy performed at X-band (9 GHz) and G-band (180 GHz) frequencies. bTbK has been successfully used for dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SS-DNP) experiments based on the cross-effect, which involves two electrons and one nuclear spin, and gave very high signal enhancements. For a quantitative description of the polarization enhancements and their excitation frequency profile, a detailed information about the values and relative orientation of the magnetic hyperfine-, dipolar-, g-tensors and the exchange interaction of the two unpaired electron spins within the molecule is mandatory. We have determined these tensors and their relative orientation by CW-EPR spectra and pulsed electron double resonance experiments in frozen solution. The potential of using the cross-effect also for DNP in liquid solutions has been experimentally investigated by room-temperature high-field DNP experiments performed at 9.2 T.

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.

A complete thermodynamic characterization of the chair-to-chair interconversion in beta-diphosphorylated piperidine-N-oxyl radicals was achieved by means of the analysis of temperature-dependent ESR spectra. A new two-dimensional simulation method was developed with the coordinates temperature and magnetic field, in which the entire set of spectra was simulated simultaneously by adjusting the coefficients in the power expansion, giving the temperature-dependent ESR parameters and the thermodynamic and kinetic parameters determining the site populations and exchange rates, respectively. The new method promotes elimination of the ambiguities inherent in the spectroscopic determination of thermodynamic parameters. Labile solvent-solute interactions can strongly influence the chemical exchange, producing a complex network of symmetric and asymmetric interconversions. The solvent dependence of magnetic relaxation was also analyzed.

ESR spectra of cis- and trans-beta-diphosphorylated pyrrolidine-N-oxyl radicals, c-1 and t-1, were studied in liquid and frozen solution. The expected 1:2:1 triplet (a(P) (2)) of the 1:1:1 triplet (a(N)) was observed for t-1; however, for c-1, the inner lines of the 1:2:1 triplet showed a dramatic broadening characteristic of chemical exchange between two equivalent conformations. Owing to the large difference in the hyperfine splitting constants (hfsc) of the exchanging phosphorus (Deltaa(P) approximate to 21 G), the coalescence temperature was unusually high (193 K, in n-pentane), and the potential barrier for ring interconversion (10.5 U mol(-1)) was easily obtained from the temperature dependence of the exchange rate. This value was in very good agreement with the value obtained for an empirical pseudorotational potential (11 kJ mol(-1)) that was adjusted to fit the temperature dependence of the phosphorus hfsc. For c-1, molecular mechanics calculations gave similar characteristics for the pseudorotational potential and indicated the existence of two identical minima with distorted geometries lying between T-3(4) and E-3 or T-4(3) and E-3. For t-1, only the T-3(4) conformer was found to be significantly populated. Frozen solution spectra showed that the phosphorus hfsc anisotropy is higher when the C-P bond is pseudoaxial; this result can be explained by a geometry-dependent delocalization of the impaired electron into the phosphorus 3p orbitals.

Persistent nitroxides and their corresponding alkoxyamines are important regulators of living radical polymerization. Here we describe the synthesis of beta-phosphorus nitroxides bearing a P-hydrogen, that present very interesting properties for the control of the radical polymerization of styrenes, acrylates and other monomers. A large series of alkoxyamines derived from these nitroxides was prepared, and Electron Spin Resonance (ESR) was used to determine both the temperature (T-c) and the rate constant (k(d)) for their homolysis. For the whole series of alkoxyamines (27 compounds), a very good linear correlation was found between T-c and logk(d). Satisfactory linear correlations were found between T-c and calculated (PM3 method) Bond Dissociation Energy (BDE) of the NO-C bond, for series of alkoxyamines with the same type of leaving radical. The characteristics of free radical polymerization of styrene carried out in the presence of these new nitroxides and alkoxyamines will be discussed.

Addition of one equivalent of dialkylphosphite to gamma -diketones yielded new 5-phosphorylated 1-pyrrolines 3. Depending on the experimental conditions, the addition of a second equivalent of dialkylphosphite to 3, led either to (+/-)-2,5-bis-phosphorylated pyrrolidines 4 or a mixture of (+/-)- and meso-4. Pyrrolidines 4 were isolated and subsequently oxidized to the corresponding stable (+/-)- or meso-nitroxides, 5. Coupling of (+/-)-5a with the prochiral 2-phenyl-ethyl radical was investigated. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.

Oxygen-centered radicals are suspected to play an important role in number of pathological processes as inflammatory processes 1 , during the ischemia-reperfusion 2 …The ESR spin-trapping method has been extensively used to study their role. Numerous spin-traps have been prepared 3,4 , but the most widely used was the 2,2-dimethyl-3,4-dihydro-2 H -pyrrole-1-oxide (DMPO) 1 (scheme 1). In the search of new nitrones, we previously reported the synthesis and the features of the spin-trap 2-diethoxyphosphoryl-2-methyl-3,4-dihydro-2 H -pyrrole-1-oxide (DEPMPO) 2 (scheme 1) 5 , whose superoxyde adduct was shown to be about 15 times more persistent than that of DMPO in phosphate buffer at pH 7 . The introduction of a phenyl group in position 5 will increase the lipophilicity, which can favour the trapping of radicals generated in lipid-rich locations. Furthermore, the potentialities of lipophilic nitrones as neuroprotective agents 6,7 is largely investigated. We report here, the synthesis of a new spin-trap, the 2-diethoxyphosphoryl-2-methyl-5-phenyl-3,4-dihydro-2