Spin Trapping of the Phosphorus-centered Radicals Generated from Hydrogen Abstraction Reaction by 2,2-Diphenyl-1-picrylhydrazyl

IntroductionSPin trapping technique has been widely used for the detection and identification of unstable radicals. As traps, nitrones and nitroso compounds are most widely usedll--4]. However,the identification of spin adducts by EPR spectroscopy is rather difficult because the variationof the hyperfine coupling constants(hfccs) of spin adducts caused by the structural changes oftrapped radicals is not very large. Recently, the ph osp horns- con t al m ng m t rox id e h a s att ra ctedmuch …     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS–XII. SPIN TRAPPING STUDY OF THE FREE RADICALS GENERATED DURING THE PHOTOLYSIS OF PHOTO ALLERGENS BITHIONOL and FENTICHLOR

Free radicals were trapped and observed by ESR when photoallergens bithionol and fentichlor were irradiated in the presence of spin traps N- t -butyl-α-phenylnitrone (PBN) and 5,5-dimethyl-pyrroline-N-oxide (DMPO). In the absence of air, both PBN and DMPO trapped a carbon-centered radical. The carbon-centered radical, which was capable of abstracting a hydrogen atom from cysteine, glutathione, ethanol and formate, was identified as an aryl radical derived from the homolytic cleavage of the carbon-chlorine bond. In the presence of air, both carbon-centered radicals and hydroxyl radicals were trapped by DMPO. Under similar conditions, the yield of the hydroxyl radicals was greater from bithionol than from fentichlor. The presence of the hydroxyl radical was confirmed by kinetic experiments employing hydroxyl radical scavengers (ethanol, formate). Superoxide and H₂O₂ were not involved. Experiments with oxygen-¹⁷O indicated that the hydroxyl radicals came exclusively from dissolved oxygen. The precursor of the hydroxyl radical is postulated to be a peroxy intermediate (ArOO*) derived from the reaction of an aryl radical (Ar*) with molecular oxygen. Both bithionol and fentichlor photoionized only when excited in the UVC (<270 nm) region. Free radicals have long been postulated in the photodechlorination of bithionol and fentichlor and the present study provides supporting evidence for such a mechanism. Aryl and hydroxyl radicals are reactive chemical species which may trigger a series of events that culminate in photoallergy.     

Muonium addition to DMPO and PBN sorbed in silica-gel

Nitroxyl radicals were formed by adding the light hydrogen isotope, muonium to the spin traps DMPO (5,5-dimethyl-1-pyrroline-N-oxide) and PBN (N-tert-butyl-alpha-phenylnitrone) sorbed as 30 wt% ethanol solutions in silica-gel; evidence is presented for a specific hydrogen-bonded interaction between the DMPO adduct and the silica surface; longitudinal-field muon spin relaxation measurements (LF-MuSRx) were performed which identified two distinct motional regimes in both samples.     

Influence of the glucosylated β-cyclodextrin inclusion on sulfur trioxide spin-adduct stabilizations and spin-trapping rate processes for DMPO-type spin traps

The effect of CD-inclusion on spin-trapping rates and spin-adduct decay rates for sulfur trioxide radical anion (SO₃ ^??) was investigated. SO₃ ^?? radical was produced with UV photolysis of sodium sulfite in basic aqueous solution, and spin-trapped with various spin traps, i.e., PBN (α-phenyl-N-t-butylnitrone), DMPO (5,5-dimethyl pyrroline-1-oxide), and three other phosphoryl DMPO-type spin traps. A modified β-CD, 6-O-α-d-glucosyl-β-cyclodextrin (G-β-CD) having better inclusion properties than β-CD, was employed. Upon adding excess G-β-CD, decay rates of SO₃ ^?? radical adducts significantly decreased in most spin traps. Half-lives of SO₃ ^?? radical adducts of phosphoryl spin traps were one to two orders of magnitude longer than that of PBN or DMPO, and the G-β-CD addition further extended the half-life time. The spin traps containing phosphoryl-group all showed higher SO₃ ^?? trapping rates than those of PBN and DMPO, but two phosphoryl spin traps achieved slower trapping rates by G-β-CD addition. In addition, the structures of CD-inclusion complexes of spin traps were established by means of 1D and 2D NMR measurements. Based on the results, the influences of inclusion on the spin-trapping rate processes and spin-adduct stabilizations were discussed. We conclude that substituents in DMPO-type spin traps may be modified to provide best spin-trapping capabilities in the presence or absence of CD.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS—IX. A SPIN TRAPPING STUDY OF THE PHOTOLYSIS OF AMIODARONE AND DESETHYLAMIODARONE

The photolysis of amiodarone (AM) and its major metabolite mono-N-desethylamiodarone (DEA), has been studied by absorption spectroscopy, electron spin resonance spectroscopy (spin trapping) and oxygraph measurements. Changes in the absorption spectrum of both AM and DEA upon UV irradiation indicate that both drugs undergo deiodination. Spin trapping experiments with 2-methyl-2-nitrosopropane (MNP), α-phenyl-N- tert -butyl-nitrone (PBN) and 5.5-dimethyl-1-pyrroline-N-oxide (DMPO) suggest the formation of an aryl radical from AM during UV irradiation. Amiodarone also undergoes photoionization. Under aerobic conditions the photoelectron is scavenged by oxygen to give superoxide, which is trapped by DMPO. Oxygraph measurements further confirmed the consumption of oxygen and the generation of superoxide during the irradiation of aqueous solutions of AM. Deiodination, photoionization and superoxide formation were all observed at wavelengths as low as 335 nm, suggesting that some or all of these processes may be involved in AM-induced photosensitivity. The aryl radical derived from AM during UV irradiation abstracted a hydrogen atom from suitable donors (ethanol, glutathione, cysteine, linoleic acid). Reaction of the dienyl radical derived from linoleic acid would yield the corresponding peroxy radical thereby initiating lipid peroxidation. This would explain the deposition of lipofuscin, a pigment formed from the products of lipid peroxidation, in the skin of patients receiving AM.     

炔丙醇电还原自由基中间产物的ESR研究

Electrochemical-ESR technique (ex situ method) with spin traps phenyl tert-butyl nitrone (PBN) and 5,5-dimethyl-1-Pyrroline-1-oxide (DMPO) has been applied to the detection of radical intermediates produced during electroreduction of propargyl alcohol (PA) at platinized platinum electrode in an acidified alcohol-aqua solution. Propargyl radical and H have been detected by PBN and DMPO, respectively. As a result of the discovery of these radicals, an evidence for a radical mechanism of the electroreduction of PA put forward by some authors might be obtained beyond all doubt.     

Kinetics and mechanism of oxidation of manganese(III) acidoporphyrin complexes with hydrogen peroxide

The reactions of manganese(III) acidotetraphenylporphyrin complexes with hydrogen peroxide in an aqueous-organic medium at 288–308 K are studied by spectrophotometry. The reaction is the oxidation of the manganese(III) complex. The spectral and kinetic data correspond to a multistep mechanism including the step of coordination of a hydrogen peroxide molecule by the central manganese atom. A possibility of formation of oxidized complexes without macrocycle destruction upon the reaction with H₂O₂ makes manganese(III) porphyrins quite promising for use as models of natural catalases.     

Preparation of iron amido complexes via putative Fe(IV) imido intermediates

The isolation and characterization of monomeric Fe(III) amido complexes with hybrid ureate/amidate ligands is described. An aryl azide serves as the source of the amido ligand in preparing the complexes from trigonal monopyramidal Fe(II) precursors. Aryl azides more commonly react with transition metal complexes by a two-electron oxidation process to yield imido complexes, suggesting that the Fe(III) amido complexes may be formed from high valent species by hydrogen atom abstraction from an external species. The mechanistic basis for formation of the amido complexes is investigated using substrates that readily donate hydrogen atoms. Results from these experiments suggest that the Fe(III) amido complexes are generated from Fe(IV) imido intermediates that can facilitate homolytic X-H bond cleavage. The Fe(III) amido complexes are high spin (S = 5/2) with a strong absorbance band at lambdamax approximately 600 nm and extinction coefficients between 2000 and 3000 M-1 cm-1. These complexes are hygroscopic, reacting with 1 equiv of water to produce the corresponding Fe(III)-OH complexes and p-toluidine.     

Cobalt(III) azide dimethylglyoximates with some sulfanilamide derivatives: Synthesis and crystal structures

A number of new Co(III) dimethylglyoximates containing N-pyrimidinylsulfanilamides were obtained. These complexes of the general formula [Co(N₃)(DH)₂L] (I?CV) (where DH is the dimethylglyoxime monoanion and L are N-pyrimidinylsulfanilamides NH₂-C₆H₄-SO₂-NH-R with different substituents R) are expected to have antibacterial properties. Their structures were examined by IR, electronic absorption, and NMR spectroscopy and X-ray diffraction. They were classified among cobalt(III) trans dioximates. In all the complexes obtained, the cobalt atom coordinates to four N atoms of two dimethylglyoxime residues. The octahedral environment of the metal atom is completed with the amino N atom of the neutral molecule L and the N atom of the azide anion. In the crystal structures, complexes I?CV are united through intermolecular hydrogen bonds N-H??O, N-H??N, and O-H??O, in which the groups ?NH₂ and =NH of the ligand L and water molecules (for I, IV, and V) serve as proton donors.     

Synthesis, structure, and physical properties for a series of monomeric iron(III) hydroxo complexes with varying hydrogen-bond networks

Mononuclear iron(III) complexes with terminal hydroxo ligands are proposed to be important species in several metalloproteins, but they have been difficult to isolate in synthetic systems. Using a series of amidate/ureido tripodal ligands, we have prepared and characterized monomeric Fe (III)OH complexes with similar trigonal-bipyramidal primary coordination spheres. Three anionic nitrogen donors define the trigonal plane, and the hydroxo oxygen atom is trans to an apical amine nitrogen atom. The complexes have varied secondary coordination spheres that are defined by intramolecular hydrogen bonds between the Fe (III)OH unit and the urea NH groups. Structural trends were observed between the number of hydrogen bonds and the Fe-O hydroxo bond distances: the more intramolecular hydrogen bonds there were, the longer the Fe-O bond became. Spectroscopic trends were also found, including an increase in the energy of the O-H vibrations with a decrease in the number of hydrogen bonds. However, the Fe (III/II) reduction potentials were constant throughout the series ( approximately 2.0 V vs [Cp 2Fe] (0/+1)), which is ascribed to a balancing of the primary and secondary coordination-sphere effects.     

A SPIN TRAPPING STUDY OF THE PHOTOCHEMISTRY OF 5,5-DIMETHYL-1-PYRROLINE N-OXIDE (DMPO)

The photochemistry of 5,5-dimethyl-l-pyrroline N -oxide (DMPO) has been studied in benzene, cyclohexane and aqueous buffer solutions (pH 7.4) by means of electron paramagnetic resonance (EPR) and the spin trapping technique. Ultraviolet irradiation of DMPO in aqueous buffer with unfiltered UV radiation from a Xe arc lamp results in photoionization of the spin trap and the generation of the DMPO cation radical, DMPO⁺. The aqueous electron, e_aq⁻, was trapped by DMPO and detected as the DMPO/H adduct. The DMPO⁺- reacted with the water to yield the DMPO/OH adduct. Ultraviolet irradiation of DMPO in nitrogen-saturated benzene gave an unidentified carbon-centered DMPO adduct that was replaced by hydroperoxyl and alkoxyl adducts of DMPO when oxygen was present. Experiments employing ¹⁷O₂ gas indicated that the oxygen in the DMPO alkoxyl adduct was derived from molecular oxygen. However, UV irradiation of DMPO in cyclohexane yielded the cyclohexyl and cyclohexyloxyl adducts of DMPO in nitrogen-saturated and air-saturated solutions, respectively. These observations suggest that in aprotic solvents UV irradiation of DMPO generates a carbon-centered radical (R⁻), derived from the trap itself, which in benzene reacts with oxygen to yield an alkoxyl radical (RO⁻), possibly via a peroxyl radical (ROO⁻) intermediate. In cyclohexane R⁻ abstracts a hydrogen atom from the solvent to yield the cyclohexyl radical in the absence of oxygen and the cyclohexyloxyl radical in the presence of oxygen. These findings indicate that when DMPO is used as a spin trap in studies employing short-wavelength UV radiation (λ < 300 nm) the photochemistry of DMPO cannot be ignored.     

Reactivity of Cr(III) μ-oxo compounds: catalyst regeneration and atom transfer processes

Oxidation of CpCr[(XylNCMe)(2)CH] (Xyl = 2,6-Me(2)C(6)H(3)) with pyridine N-oxide or air generated the μ-oxo dimer, {CpCr[(XylNCMe)(2)CH]}(2)(μ-O). The μ-oxo dimer was converted to paramagnetic Cr(III) CpCr[(XylNCMe)(2)CH](X) complexes (X = OH, O(2)CPh, Cl, OTs) via protonolysis reactions. The related Cr(III) alkoxide complexes (X = OCMe(3), OCMe(2)Ph) were prepared by salt metathesis and characterized by single crystal X-ray diffraction. The interconversion of the Cr(III) complexes and their reduction back to Cr(II) with Mn powder were monitored using UV-vis spectroscopy. The related CpCr[(DepNCMe)(2)CH] (Dep = 2,6-Et(2)C(6)H(3)) Cr(II) complex was studied for catalytic oxygen atom transfer reactions with PPh(3) using O(2) or air. Both Cr(II) complexes reacted with pyridine N-oxide and γ-terpinene to give the corresponding Cr(III) hydroxide complexes. When CpCr[(DepNCMe)(2)CH] was treated with pyridine N-oxide in benzene in the absence of hydrogen atom donors, a dimeric Cr(III) hydroxide product was isolated and structurally characterized, apparently resulting from intramolecular hydrogen atom abstraction of a secondary benzylic ligand C-H bond followed by intermolecular C-C bond formation. The use of very bulky hexaisopropylterphenyl ligand substituents did not preclude the formation of the analogous μ-oxo dimer, which was characterized by X-ray diffraction. Attempts to develop a chromium-catalyzed intermolecular hydrogen atom transfer process based on these reactions were unsuccessful. The protonolysis and reduction reactions of the μ-oxo dimer were used to improve the previously reported Cr-catalyzed radical cyclization of a bromoacetal.     

Constructing sandwich-type rare Earth double-decker complexes with N-confused porphyrinato and phthalocyaninato ligands

Reaction of the half-sandwich complexes M(III)(Pc)(acac) (M = La, Eu, Y, Lu; Pc = phthalocyaninate; acac = acetylacetonate) with the metal-free N-confused 5,10,15,20-tetrakis[(4-tert-butyl)phenyl]porphyrin (H(2)NTBPP) or its N2-position methylated analogue H(CH(3))NTBPP in refluxing 1,2,4-trichlorobenzene (TCB) led to the isolation of M(III)(Pc)(HNTBPP) (M = La, Eu, Y, Lu) or Y(III)(Pc)[(CH(3))NTBPP] in 8-15% yield. These represent the first examples of sandwich-type rare earth complexes with N-confused porphyrinato ligands. The complexes were characterized with various spectroscopic methods and elemental analysis. The molecular structures of four of these double-decker complexes were also determined by single-crystal X-ray diffraction analysis. In each of these complexes, the metal center is octa-coordinated by four isoindole nitrogen atoms of the Pc ligand, three pyrrole nitrogen atoms, and the inverted pyrrole carbon atom of the HNTBPP or (CH(3))NTBPP ligand, forming a distorted coordination square antiprism. For Eu(III)(Pc)(HNTBPP), the two macrocyclic rings are further bound to a CH(3)OH molecule through two hydrogen bonds formed between the hydroxyl group of CH(3)OH and an aza nitrogen atom of the Pc ring or the inverted pyrrole nitrogen atom of the HNTBPP ring, respectively. The location of the acidic proton at the inverted pyrrole nitrogen atom (N2) of the protonated double-deckers was revealed by (1)H NMR spectroscopy.     

INCORPORATION OF THE SPIN TRAP DMPO INTO CULTURED FETAL MOUSE LIVER CELLS

Abstract— The incorporation of four different spin traps into cultured fetal mouse liver cells was investigated using electron spin resonance spectroscopy. The cells were incubated in saline solutions of the traps, washed, and then irradiated in a saline solution containing hydrogen peroxide. The presence of a reproducible ESR signal was taken as evidence of spin trap incorporation. The spin trap DMPO was found to be incorporated, while PBN and 4-POBN were not. MNP was toxic to the cells.     

Asymmetric and symmetric cobalt(III) dioximates with pyrazinecarboxamide: Synthesis and crystal structure

A number of novel complexes of cobalt(III) trans-dioximates containing pyrazinecarboxamide were obtained: [CoBr(DfgH)₂(Pzca)] · 0.125H₂O (I), [CoCl(NioxH)₂(Pzca)] (II), [CoCl(DmgH)(NioxH)(Pzca)] · 0.5C₂H₅OH · 0.5H₂O (III), and [CoCl(DmgH)₂(Pzca)] (IV), where DmgH, DfgH, and NioxH are the monoanions of dimethylglyoxime, diphenylglyoxime, and cyclohexane-1,2-dione dioxime, respectively; Pzca is pyrazinecarboxamide. Complexes I?CIII were examined by X-ray diffraction. In these complexes, the Co(III) atom has an octahedral environment with the pseudomacrocyclic fragment (DioxH)₂ in the equatorial plane (DioxH is the ??-dioxime residue). The latter is stabilized by hydrogen bonds O-H?O. In all the complexes, the organic ligand is monodentate and coordinated through one N atom of the heterocycle so that it is trans to the halide anion.     

Thermal reaction of aquaammineruthenium(III) complex with amino acid or imidazole derivative in the solid state

Intermolecular thermal-substitution reaction between aquaammineruthenium(III) complex and amino acid or imidazole derivative has been investigated in the solid state by the TG-DTA method. Pentaammineruthenium(III) complexes containing amino acid or imidazole derivative have been obtained directly by the thermal reactions. Glycine, β-alanine, and γ-aminobutyric acid coordinate to Ru(III) through their carbonyl oxygen, and imidazole does through its N(3) atom. Distinct coordination site is provided in the complex with histidine and/or adenine: the bonding site depends on the outer-sphere anion of aquaammine complex. The N(3) atom of the histidine and N(7) atom of the adenine coordinate to Ru(III) taking the paratoluenesulphonate salt of aquaammineruthenium(III) into the reaction. When the methanesulphonate salt is used, the nitrogen atom in the side-chain amino-group participates in complexation. Direct chelation of the glycine, histidine, and adenine to the deaquated cis-diaquatetraammineruthenium(III) complex has been confirmed.     

η3:η1⇌η2-tautomerism in carbonyliron complexes. Influence of substituents

A number of chelate η³-allylcarbamoyl iron complexes (I) with different substituents (R in the allyl ligand and R′ at the nitrogen atom) were synthesized. The influence of structural features on the equilibrium between the complexes I and their η²-azadiene tautomers (II) was studied by IR spectroscopy. It was established that the equilibrium position is determined in the first place by steric factors. When R is a bulky substituent the equilibrium is shifted towards the cyclic form I, whereas the branching of the alkyl substituent R′ at the nitrogen atom favours the open olefinic form II. Furthermore π-σ-(N) rearrangement of complexes II to σ-(N) derivatives (III), and the conversion of III into η⁴-azadiene complexes (IV) also depend on the steric requirements of the substituents R and R′.     

Mechanistic studies of the reactions of nitrone spin trap PBN with glutathiyl radical

We performed mechanistic studies of the reaction of PBN with the physiologically relevant glutathiyl radical, GS*, formed upon oxidation of the intracellular antioxidant, glutathione, GSH. The scavenging rate constant of GS* by PBN has been measured directly by laser flash photolysis and indirectly by competitive EPR of the spin adduct of PBN and another spin trap, DMPO (5,5-dimethyl-1-pyrroline N-oxide), and was found to be 6.7 x 107 M(-1) s(-1). Reverse decomposition of the paramagnetic PBN-glutathiyl radical adduct to the nitrone and thiyl radical was observed for the first time. The rate constant for the reaction of the monomolecular decomposition of the radical adduct was found to be 1.7 s(-1). Diamagnetic, EPR-invisible products of PBN adduct degradation were studied by 1H NMR and 19F NMR using newly synthesized fluorine-substituted PBN.     

New pathway to triammine complexes of chromium(III): Crystal structure of 1-chloro-2,3-diaquo-4,5,6-triamminechromium(III) chloride

A new synthetic method to triammine complexes of Cr(III) is reported, via a hydrolytic process from tetrammine complexes. The crystal and molecular structure of 1 -chloro - 2,3 - diaquo - 4,5,6 - triamminchromium(III) chloride are described. The crystals are orthorombic, space group Pnma, with a = 13.230(2), b = 10.545(1), c = 6.754(1) Å, V = 942.3(3), Z = 4. Least-squares refinement of the structure based on 529 observations led to final discrepancy indices of R = 0.047 and R_w = 0.056. The Cr atom is bonded to two water molecules, three ammonia molecules and one chlorine atom in a distorted octahedral coordination showing a trans chloro-aquo configuration.     

Aqueous rhodium(III) hydrides and mononuclear rhodium(II) complexes

In aqueous solutions, as in organic solvents, rhodium hydrides display the chemistry of one of the three limiting forms, i.e. {Rh(I)+ H+}, {Rh(II)+ H.}, and {Rh(III)+ H-}. A number of intermediates and oxidation states have been generated and explored in kinetic and mechanistic studies. Monomeric macrocyclic rhodium(II) complexes, such as L(H2O)Rh2+ (L = L1 = [14]aneN4, or L2 = meso-Me6[14]aneN4) can be generated from the hydride precursors by photochemical means or in reactions with hydrogen atom abstracting agents. These rhodium(II) complexes are oxidized rapidly with alkyl hydroperoxides to give alkylrhodium(III) complexes. Reactions of Rh(II) with organic and inorganic radicals and with molecular oxygen are fast and produce long-lived intermediates, such as alkyl, superoxo and hydroperoxo complexes, all of which display rich and complex chemistry of their own. In alkaline solutions of rhodium hydrides, the existence of Rh(I) complexes is implied by rapid hydrogen exchange between the hydride and solvent water. The acidity of the hydrides is too low, however, to allow the build-up of observable quantities of Rh(I). Deuterium kinetic isotope effects for hydride transfer to a macrocyclic Cr(v) complex are comparable to those for hydrogen atom transfer to various substrates.