Jean-Claude Bouteiller

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.

PPN-type nitrones: preparation and use of a new series of β-phosphorylated spin-trapping agents†

The synthesis of six members of a new series of β-phosphorylated nitrones derived from α-phenyl-N-tert-butylnitrone (PBN)‡ is described. A new method, based on HPLC, is used to evaluate their n-octanol–phosphate buffer partition coefficient. As is the case for other PBN-type traps, the lipophilicity of these nitrones is found to be greatly dependent on the structure of their aromatic moiety. The capacity of these compounds to act as spin trapping agents is surveyed by using them to trap various free radicals in organic or aqueous media, and the EPR parameters of the different spin adducts obtained are reported herein.

Preparation and use as spin trapping agents of new ester-nitrones

The synthesis of two new nitrones, N-benzylidene-1,1-bis(ethoxycarbonyl)ethylamine N-oxide (DEEPN) and N-[(1-oxidopyridin-1-ium-4-yl)methylidene]-1-ethoxycarbonyl-1- methylethylamine N-oxide (EPPyON), is described. Measurement of their n-octanol-phosphate buffer partition coefficient permitted evaluation of their lipophilicity. Their capacity to act as spin trapping agents was investigated in aqueous media. Although these nitrones were unsuitable for detecting hydroxyl radical, they efficiently trapped various carbon- and oxygen-centred radicals, including superoxide, in aqueous media. The half-lives of their superoxide adducts were determined at pH 5.8 and 7.2. At neutral pH, the superoxide spin adduct of DEEPN was found to be as persistent as that of 5-diethoxyphosphoryl-5-methyl-3,4-dihydropyrrole N-oxide (DEPMPO). Consequently, DEEPN was believed to be an efficient trap for superoxide detection in aqueous media.

Characteristics and use as spin trapping agent of a β-phosphorylated nitroso compound, DEPNP

2-(Diethylphosphonate)-nitrosopropane (DEPNP), prepared by oxidation of the corresponding aminophosphonate, was found to essentially exist as monomer in both water and organic solvents. The mechanisms of its degradation under 80°C heating or visible light exposure were studied by EPR spectroscopy: its decomposition gave rise to paramagnetic by-products, which have been identified as DEPNP / ·C(CH3)2[P(O)(OC2H5)2] and DEPNP / ·P(O)(OC2H5)2 spin adducts. Despite this drawback, DEPNP was successfully used as spin trapping agents to scavenge various carbon — and phosphorus-centred free radicals both in aqueous and organic media, giving rise to intense EPR spectra characteristic of the species trapped.

Electrochemical characterisation of β-phosphorylated nitrone spin traps

Oxidation and reduction potentials have been determined for a series of thirteen β-phosphorylated nitrones in water and in acetonitrile. The effect of structural modifications on potential window is briefly discussed.

Spin-trapping of free radicals by PBN-type β-phosphorylated nitrones in the presence of SDS micelles†

Three PBN-type β-phosphorylated nitrones, PPN 1, 4-PyOPN 2, and 4-ClPPN 3, have been used to trap two carbon centred free radicals, CH3˙ and CO2˙–, in a water–SDS biphasic system. The location of the traps and their adducts has been found by EPR spectroscopy. 4-PyOPN is found to remain in water, while 4-ClPPN is preferentially sequestered into the micellar structure, and PPN partitioned between the two phases. With the three nitrones, the CO2˙– spin adduct always remains in the bulk aqueous phase. In contrast, the three CH3˙ spin adducts seem to be present in both phases. However, 4-PyOPN-CH3 is essentially located in water, and 4-ClPPN–CH3 in the micelles, while PPN–CH3 is clearly shown to partition between the aqueous environment and the micellar interior. For all these spin adducts, the phosporus hyperfine splitting constant, aP, is found to be a good indicator of aminoxyl behaviour in the presence of micelles. For the various sodium dodecyl sulfate concentrations, the PPN–CH3 affinity for the micellar phase was evaluated from the average aP value.

Poly(β-phosphorylated nitrones): preparation and characterisation of a new class of spin trap

Four new poly(β-phosphorylated nitrones), the 1,3,5-tris[N-(1-diethylphosphono-1-methylethyl)-N-oxidoiminiomethyl]benzene (TN) 1, the 1,3-bis[N-(1-diethylphosphono-1-methylethyl)-N-oxidoiminiomethyl]benzene (MDN) 2, the 1,4-bis[N-(1-diethylphosphono-1-methylethyl)-N-oxidoiminiomethyl]benzene (PDN) 3, and the 1,2-bis[N-(1-diethylphosphono)-1-methylethyl)-N-oxidoiminiomethyl]benzene (ODN) 4, derived from the mono-nitrone N-benzylidene-1-diethoxyphosphoryl-1-methylethylamine N-oxide (PPN) 5, were synthesised. The capacity of 1–4 to act as spin-trapping agents was investigated in phosphate buffers at pH 5.8 and 7.2. Complex EPR spectra were obtained for the spin adducts with ortho-dinitrone 4. The three other compounds efficiently trapped superoxide and several carbon-centred radicals, giving mono-spin adducts, although only weak signals were obtained with the hydroxyl radical. When the spin trap concentration was kept below 1 mmol dm−3, the formation of di-radicals also occurred. The half-lives of the superoxide spin adducts of 1–3 were in the range of 5–12 min and did not change significantly between pH 5.8 and 7.2. A competitive kinetic study showed that the trinitrone 1 trapped the methyl radical 1.9 times more rapidly than either α-(1-oxidopyridin-1-ium-4–yl)-N-tert-butylnitrone (POBN) or 5-diethoxyphosphoryl-5-methyl-4,5-dihydro-3H-pyrrole N-oxide (DEPMPO) at pH 7.2.

Stable β-phosphorylated cyclic aminoxyl radicals in SDS micelles

The behaviour of a series of stable cyclic aminoxyl radicals in the presence of SDS micelles was studied by EPR spectroscopy. Six different compounds, i.e. two non-phosphorylated and four β-phosphorylated, were investigated. Except in the case of the strongly hydrophilic 3-CP 1, which always remained in the bulk aqueous phase, all the radicals were found to exchange between water and micelles. Their partition coefficients were evaluated from computer simulations of the EPR spectra and in the case of two aminoxyls TOMER-Et 5 and TOMER-Pri 6, the variation of the rate constant with temperature allowed us to estimate the exchange activation energy.

EPR study of β-phosphorylated stable aminoxyl radicals in the presence of liposomes

The behaviour of four stable β-phosphorylated aminoxyl radicals of the pyrrolidinoxyl series has been studied by EPR spectroscopy in the presence of dimyristoyl-phosphatidylcholine (DMPC) unilamellar liposomes. The affinity of these compounds for the liposome structure was found to be more or less high depending on their hydrophobicity. In each case, computer simulations of experimental EPR spectra permitted the determination of the hyperfine coupling constants of the radical inserted in the phospholipid bilayer.