Robert Lauricella

Decay of the hydroperoxyl spin adduct of 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide: an EPR kinetic study

The decay kinetics of the hydroperoxyl spin adduct of both 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO), a new β-phosphorylated cyclic nitrone and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were studied in various media by EPR spectroscopy. In organic solvents, both first- and second-order processes were shown to contribute to the decay of the two spin adducts. However, in aqueous solution the DMPO–hydroperoxyl spin adduct decay was pure first-order. The half-lives of the two spin adducts were determined in every medium tested and the DEPMPO–hydroperoxyl spin adduct was shown to be significantly more persistent (from five times in organic solvents to 30 times in a pH 5.6 buffer) than the DMPO–hydroperoxyl spin adduct.

Improved Spin Trapping Properties by β-Cyclodextrin−Cyclic Nitrone Conjugate

Spin trapping using a nitrone and electron paramagnetic resonance (EPR) spectroscopy is commonly employed in the identification of transient radicals in chemical and biological systems. There has also been a growing interest in the pharmacological activity of nitrones, and there is, therefore, a pressing need to develop nitrones with improved spin trapping properties and controlled delivery in cellular systems. The beta-cyclodextrin (beta-CD)-cyclic nitrone conjugate, 5-N-beta-cyclodextrin-carboxamide-5-methyl-1-pyrroline N-oxide (CDNMPO) was synthesized and characterized. 1-D and 2-D NMR show two stereoisomeric forms (i.e., 5S- and 5R-) for CDNMPO. Spin trapping using CDNMPO shows distinctive EPR spectra for superoxide radical anion (O2(*-)) compared to other biologically relevant free radicals. Kinetic analysis of O2(*-) adduct formation and decay using singular value decomposition and pseudoinverse deconvolution methods gave an average bimolecular rate constant of k = 58 +/- 1 M(-1) s(-1) and a maximum half-life of t(1/2) = 27.5 min at pH 7.0. Molecular modeling was used to rationalize the long-range coupling between the nitrone and the beta-CD, as well as the stability of the O2(*-) adducts. This study demonstrates how a computational approach can aid in the design of spin traps with a relatively high rate of reactivity to O2(*-), and how beta-CD can improve adduct stability via intramolecular interaction with the O2(*-) adduct.

5-Diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO): a new phosphorylated nitrone for the efficient In Vitro and In Vivo spin trapping of oxygen-centred radicals

5-Diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO), a newly synthesized spin trap, is used for the in vitro spin trapping of hydroxyl and superoxide radicals, and though the spin trapping rates are close to those reported for 5,5-dimethyl-1-pyrroline N-oxide (DMPO), the DEPMPO–superoxide spin adduct is significantly more persistent than its DMPO analogue, a difference which allows the detection of superoxide during the reperfusion of ischaemic isolated rat hearts.

A new kinetic approach to the evaluation of rate constants for the spin trapping of superoxide/hydroperoxyl radical by nitrones in aqueous media

A new kinetic approach to the evaluation of rate constants for the spin trapping of superoxide/hydroperoxyl radical by nitrones in buffered media is described. This method is based on a competition between the superoxide trapping by the nitrone and the spontaneous dismutation of this radical in aqueous media. EPR spectra are recorded as a function of time at various nitrone concentrations, and kinetic curves are obtained after treatment of these spectra using both singular value decomposition and pseudo-inverse deconvolution methods. Modelling these curves permits the determination of the rate constants k(T) and k(D) for the superoxide trapping and the adduct decay reactions, respectively. Kinetics parameters thus obtained with six nitrones, namely the 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide (EMPO), the 5-diethoxyphosphoryl-5-methyl-3,4-dihydro-5H-pyrrole N-oxide (DEPMPO), the 5,5-dimethyl-3,4-dihydro-5H-pyrrole N-oxide (DMPO), the 1,3,5-tri[(N-(1-diethylphosphono)-1-methylethyl)-N-oxy-aldimine]benzene (TN), the N-benzylidene-1-ethoxycarbonyl-1-methylethylamine N-oxide (EPPN), and the N-[(1-oxidopyridin-1-ium-4-yl)methylidene]-1-ethoxycarbonyl-1-methylethylamine N-oxide (EPPyON), indicate that cyclic nitrones trapped superoxide faster than the linear ones. However, the low k(T) values obtained for compounds show that there is still a need for new molecules with better spin trapping capacities.

Evidence of overestimation of rate constants for the superoxide trapping by nitrones in aqueous media.

Since major disagreements exist regarding the kinetics of superoxide trapping by nitrones, the underlying theory of one of the most popular method used in these studies was reinvestigated. It involves a competition between the nitrone of interest and a superoxide scavenger, and implies that the superoxide spontaneous dismutation, the spin adduct decay, and the consumption of the competitor during the course of the experiments are negligible events. Evidences of the importance of these three unduly neglected reactions are given, and errors connected to their omission are estimated. Hence this Stern-Volmer-like method should be avoided in the determination of rate constants for the trapping of superoxide by nitrones.

Reactivities of substituted α-phenyl-N-tert-butyl nitrones.

In this work, a series of α-phenyl-N-tert-butyl nitrones bearing one, two, or three substituents on the tert-butyl group was synthesized. Cyclic voltammetry (CV) was used to investigate their electrochemical properties and showed a more pronounced substituent effect for oxidation than for reduction. Rate constants of superoxide radical (O2•–) reactions with nitrones were determined using a UV–vis stopped-flow method, and phenyl radical (Ph•) trapping rate constants were measured by EPR spectroscopy. The effect of N-tert-butyl substitution on the charge density and electron density localization of the nitronyl carbon as well as on the free energies of nitrone reactivity with O2•– and HO2• were computationally rationalized at the PCM/B3LYP/6-31+G**//B3LYP/6-31G* level of theory. Theoretical and experimental data showed that the rates of the reaction correlate with the nitronyl carbon charge density, suggesting a nucleophilic nature of O2•– and Ph• addition to the nitronyl carbon atom. Finally, the substituent effect was investigated in cell cultures exposed to hydrogen peroxide and a correlation between the cell viability and the oxidation potential of the nitrones was observed. Through a combination of computational methodologies and experimental methods, new insights into the reactivity of free radicals with nitrone derivatives have been proposed.

Preparation of N-aryl-ketonitrone spin traps

The syntheses of seven N-aryl-C,C-dialkoxycarbonylnitrones 1–7, six of which were original, were achieved from the appropriate aryl-nitroso compounds. These ketonitrones were found to trap efficiently carbon-centred free radicals in aqueous media, yielding stable aminoxyl radicals whose EPR spectra lasted several days. The two penta-deuterated compounds 6 and 7 were also found to be efficient at trapping methoxyl radical. Their various spin adducts showed simple three line signals, very sensitive to the polarity of the environment. This study represents the very first use of linear ketonitrones as spin traps.

CONFORMATIONAL ANALYSIS OF DIALKYL (2,5-DIALKYL PYRROLIDIN-2-YL)PHOSPHONATES AND DIETHYL(2,5,5-TRIMETHYL PYRROLIDIN-2-YL)PHOSPHONATE BY X-RAY ANALYSIS , NMR AND FORCE FIELD CALCULATIONS

Abstract Using force field calculations (MM2), 1H, 13C NMR spectroscopy and X-ray diffraction, conformational studies were performed on four substituted (pyrrolidin-2-yl) phosphonates 1–4. According to force field calculations, one type of conformation (twist form around C4 and C5 atoms) was found to be strongly favoured for compounds 2–4 while for 1 an equilibrium between two envelope forms on the C4 atom was established. 1H NMR parameters obtained from simulations of spectra with the LAOCOON program as well Karplus relationships were in good agreement with the calculated geometries.

Photochromic spin traps. Part 3. 1,2 A new phosphorylated spiro[indoline-naphthoxazine]†

The synthesis and characterisation of a new phosphorylated nitrone belonging to the spiro[indoline-naphthoxazinic] family are described. The results of spin trapping experiments which indicate the ability of this photochromic nitrone to react with carbon-, oxygen- and sulfur-centred radicals are also reported.