Inclusion complexes of EMPO derivatives with 2,6-di-O-methyl-beta-cyclodextrin: synthesis, NMR and EPR investigations for enhanced superoxide detection

The free radical trapping properties of eight 5-alkoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO) type nitrones and those of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were evaluated for trapping of superoxide anion radicals in the presence of 2,6-di-O-methyl-beta-cyclodextrin (DM-beta-CD). (1)H-NMR titrations were performed to determine both stoichiometries and binding constants for the diamagnetic nitrone-DM-beta-CD equilibria. EPR titrations were then performed and analyzed using a two-dimensional EPR simulation program affording 1 : 1 and 1 : 2 stoichiometries for the nitroxide spin adducts with DM-beta-CD and the associated binding constants after spin trapping. The nitroxide spin adducts associate more strongly with DM-beta-CD than the nitrones. The ability of the nitrones to trap superoxide, the enhancement of the EPR signal intensity and the supramolecular protection by DM-beta-CD against sodium L-ascorbate reduction were evaluated.

Synthesis and spin-trapping behavior of 5-ChEPMPO, a cholesteryl ester analogue of the spin trap DEPMPO

[structure: see text] 5-(Cholesteryloxyethoxyphosphoryl)-5-methylpyrroline N-oxide (5-ChEPMPO), a DEPMPO analogue bearing a cholesterol group on the phosphorus atom, has been prepared and used to trap peroxyl-, alkoxyl-, thiyl-, and carbon-centered radicals in organic solvent. The important steric hindrance in 5-ChEPMPO does not affect the properties of 5-ChEPMPO in comparison to DEPMPO for the spin trapping of an enantiopure linoleic acid hydroperoxide. The 5-ChEPMPO-OOL spin adduct was observed by ESR and confirmed by ESI-MS/MS experiments. The relaxation terms of the 5-ChEPMPO-lipid peroxyl spin adduct were compared with those of other peroxyl spin adducts, and it was shown that the cholesteryl group has only a weak influence on the exchange rate between adduct conformers.

Homolysis ofN-alkoxyamines: A Computational Study

During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (Tc), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions.

Assignment of the EPR spectrum of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) superoxide spin adduct

[structure: see text] Spin trapping consists of using a nitrone or a nitroso compound to "trap" an unstable free radical as a long-lived nitroxide that can be characterized by electron paramagnetic resonance (EPR) spectroscopy. The formation of DMPO-OOH, the spin adduct resulting from trapping superoxide (O(2)(*)(-)) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), has been exploited to detect the generation of superoxide in a wide variety of biological and chemical systems. The 12-line EPR spectrum of DMPO-OOH has been either reported or mentioned in more than a thousand papers. It has been interpreted as resulting from the following couplings: A(N) approximately 1.42 mT, A(H)beta approximately 1.134 mT, and A(H)gamma(1H) approximately 0.125 mT. However, the DMPO-OOH EPR spectrum has an asymmetry that cannot be reproduced when the spectrum is calculated considering a single species. Recently, it was proposed that the 0.125 mT splitting was misassigned and actually results from the superimposition of two individual EPR spectra associated with different conformers of DMPO-OOH. We have prepared 5,5-dimethyl-[3,3-(2)H(2)]-1-pyrroline N-oxide (DMPO-d(2)), and we showed that the EPR spectrum of the corresponding superoxide spin adduct is composed of only six lines, in agreement with the assignment of the 0.125 mT splitting to a gamma-splitting from a hydrogen atom bonded to carbon 3 of DMPO. This result was supported by DFT calculations including water solvation, and the asymmetry of the DMPO-OOH EPR spectrum was nicely reproduced assuming a chemical exchange between two conformers.

Inclusion Complexes of PBN-Type Nitrone Spin Traps and Their Superoxide Spin Adducts with Cyclodextrin Derivatives: Parallel Determination of the Association Constants by NMR Titrations and 2D-EPR Simulations

(1)H NMR and electron paramagnetic resonance (EPR) titrations were used to determine the association constants of the complexes of alpha-phenyl-N-tert-butylnitrone (PBN) analogues and their superoxide spin adducts, respectively, with methylated beta-cyclodextrins. A 1:1 stoichiometry for the nitrones with randomly methylated beta-cyclodextrin and 2,6-di-O-methyl-beta-cyclodextrin and 1:1 and 1:2 stoichiometries for the corresponding cyclodextrin-nitroxide complexes were observed. After the superoxide radical spin trapping reaction, EPR titrations afforded the association constants of the corresponding cyclodextrin-nitroxide complexes. Two-dimensional EPR simulations indicated a bimodal inclusion of the nitroxide free radical spin adducts into the cyclodextrins. For all the nitrone-cyclodextrin and nitroxide-cyclodextrin complexes, the association constants were always higher for the nitroxide complexes than for the nitrone complexes. A cooperative system concerning the complexation of the nitroxide spin adduct with a cyclodextrin was evidenced by EPR titrations. The efficiency of the cyclodextrin inclusion technique to trap superoxide and to resist bioreduction by sodium l-ascorbate was also investigated.

Diastereoselective Synthesis and ESR Study of 4-PhenylDEPMPO Spin Traps

[reaction: see text] The cis and trans diastereoisomers of 5-(diethoxyphosphoryl)-5-methyl-4-phenylpyrroline N-oxide (4-PhDEPMPOt 8 and 4-PhDEPMPOc 9) were prepared stereoselectively and used as spin traps for hydroxyl and superoxide radicals. The spin adduct formed by reaction of the cis stereoisomer 9with superoxide radical anion exhibited an 8-line ESR spectrum, showing only a reduced alternating line width phenomenon. This spectrum is simpler than the 12-line spectrum of DEPMPO-OOH, which exhibits a strong alternating line width phenomenon. The half-life times of the 4-PhDEPMPOc-OOH and DEPMPO-OOH adducts were of the same order: 14.5 and 15.5 min, respectively.

Factors Influencing C−ON Bond Homolysis in Alkoxyamines: Unexpected Behavior of SG1 (N-(2-methyl-2-propyl)- N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl)-Based Alkoxyamines

Alkoxyamines and persistent nitroxides are important regulators of nitroxide-mediated radical polymerization (NMP). Since the polymerization time decreases with the increasing equilibrium constant K (k(d)/k(c)), i.e., the increasing rate constant k(d) of the homolysis of the C-ON bond between the polymer chain and the nitroxide moiety, the factors influencing the cleavage rate constants are of considerable interest. SG1-based alkoxyamines have turned out to be the most potent alkoxyamine family to use for NMP of various monomers. Therefore, it is of high interest to determine the factors which make SG1 derivatives better regulators than TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) derivatives. Contrary to what we had observed with TEMPO derivatives, we observed two relationships for the plot E(a) vs BDE(C-H), one for the nonpolar released alkyl radicals (E(a) (kJ/mol) = -133.0 + 0.72BDE) and the other one for the polar released alkyl radicals (E(a) (kJ/mol) = -137.0 + 0.69BDE). However, for both families (SG1 and TEMPO derivatives), the rate constants k(d) of the C-ON bond homolysis were correlated to the cleavage temperature T(c) (log(k(d)(s(-)(1))) = 1.51 - 0.058T(c)). Such correlations should help to design new alkoxyamines to use as regulators and to improve the tuning of NMP experiments.

ESR study of spin-trapping with two glycosylated analogues of PBN able to target cell membrane lectins

The spin trapping behaviour of the two galactosylated nitrones LAMPBN and TA1PBN, both of which are able to target cells through recognition by cell membrane lectins, were widely investigated on a variety of free radicals in aqueous media. The ESR spectra of the more amphiphilic nitrone, TA1PBN, were interpreted in the light of the LAMPBN trapping results. The spin adducts of nitrone TA1PBN, which may be better distributed inside the cell, were surrounded by two distinct environments due to the tensioactive organisation of the trap and gave two different ESR signals for each radical trapped.

Synthesis and preliminary biological evaluations of ionic and nonionic amphiphilic alpha-phenyl-N-tert-butylnitrone derivatives

In this study we report the synthesis of a series of new amphiphilic compounds derived from alpha-phenyl-N-tert-butylnitrone (PBN). The nitrone function was fitted into the core of the molecule between its polar and apolar groups. The polar head consisted of a lactobionamide, an ammonium, or a carboxylate group. The hydrophobic part consisted of a hydro- or a perfluorocarbon chain. The hydrophobic chain was linked to the tert-butyl group of the PBN derivatives using an urethane, a thioether, or an amide bond. The impact of these different parameters on the hydrophilic lipophilic balance of these compounds and their spin trap activity were studied. The various ESR measurements indicated that the aromatic and tert-butyl functional groups of PBN did not affect its spin trap properties. Moreover, these compounds were found to increase the viability of cultured human skin fibroblasts harboring the neurogenic ataxia retinitis pigmentosa mutation and presenting a severe ATPase deficiency.

Long-Range Polar Effect on the C-ON Bond Homolysis in (tert-Butyl[1-(diethylphosphonyl)-2,2-dimethylpropyl]aminoxyl) SG1-Based Alkoxyamines

Alkoxyamines and persistent nitroxyl radicals are important regulators of nitroxide-mediated radical polymerization. Because the polymerization times decrease with the increasing rate constant of the homolysis of the C-ON bond between the polymer chain and the nitroxyl moiety, the factors influencing the cleavage rate constant are of considerable interest. Because alkyl acrylate monomers are among the most used in polymerization, we present the measures of the rate constants (kd) of the C-ON bond cleavage for new SG1 based alkoxyamine models containing para-substituted aromatic acrylates (4-XC6H4OC(O)C(Me)H-SG1). It appears that the values of kd increase with the electron-withdrawing properties of the para-substituent groups (4-X) of the ester SG1-based alkoxyamines.

Diastereomeric excess upon cleavage and reformation of diastereomeric alkoxyamines

The thermal reactions of several TEMPO and DEPN (N-(2-methylpropan-2-yl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-aminoxyl) based alkoxyamines were studied by means of (1)H and (31)P NMR spectroscopy aiming to distinguish the contributions of diastereoselective homolysis and coupling to the total equilibrium diastereomeric distribution. The TEMPO-based compounds reveal no diastereomeric excess while DEPN based compounds show a moderate excess both upon homolysis and coupling. The diastereomeric preference of homolysis for DEPN-(propionate-like) adducts does not depend on the ester group and it is mainly affected by the size of beta-substituents. The diastereoselective coupling is sensitive to the total recombination rate constant k(c) and diastereoselectivity increases with the decrease of k(c). Small diastereoselective coupling is found in the recombination of DEPN with sec-butyl isobutyrate radical, where no prochiral centers are formed upon cleavage of corresponding alkoxyamine.