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Trolox-C (TXC) is a water-soluble analogue of a-tocopherol that reduces the metal-bound superoxo complex (SOC), [[(N[H.sub.3]).sub.5]Co([micro]-O2)Co[(N[H.sub.3]).sub.5]]5+ in water-ethanol media. In aqueous-acid media ([H+] = 0.02-0.70 mol/L), TXC quantitatively reduces SOC under the pseudo first-order reaction condition where the concentration of TXC, [TXC], was at least 10 times or more than that of [SOC]. Variations in media ionic strength suggest for the reaction between two cationic species, and a positive kinetic isotope effect (KH/KD < 1) indicates for a pre-equilibrium step of protonation of the pyran-oxygen of TXC. Both TXC and the protonated TXC, i.e., TXCH, are the potential reductants under the experimental range of media acidity. The reduction of SOC takes place through the single-electron transfer and the kinetic analysis yields the value for second-order rate constants for TXC and TXCH as 9.7 and 3.6 * [10.sup.3] (mol/L)-1 [s.sup.-1] at 15 [degrees]C. Keywords: redox, mechanism, [O.sub.2.sup.-], vitamin, ROS, SET, 1. Introduction Trolox-C (6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid; TXC) is the analogue of a-tocopherol, which has the advantage over the latter that it is slightly more soluble in water. (1,2) Both TXC and [...]

In higher organisms, metalloenzymes like cytochrome P450, containing a Fe(III) metal center, play an active role in metabolism of paracetamol (APAP). Here, we have chosen a mimicking [mu]-oxo-diiron complex, [Fe[(III).sub.2]([mu]-O)[(phen).sub.4][([H.sub.2]O).sub.2]].sup.4+] (1, phen = 1,10-phenanthroline), to study spectrophotometrically the kinetics of the redox interactions with APAP. In acidic buffer media (pH = 3.4-5.1), APAP quantitatively reduces 1 following first-order reaction kinetics. Each molecule of 1 accepts two electrons from APAP and is reduced to ferroin [[Fe[(phen).sub.3]].sup.2+]. On oxidation, APAP produces N-acetyl-p-benzoquinone imine (NAPQI), which on hydrolysis results in a mixture of benzoquinone, quinone oxime, acetamide, and acetic acid. In reaction media due to successive deprotonations, 1 exists in equilibrium with the species [[Fe[(III).sub.2]([mu]-O)[(phen).sub.4]([H.sub.2]O)(OH)].sup.3+] (1a) and [[Fe[(III).sub.2]([mu]-O)[(phen).sub.4](OH).sub.2].sup.2+] (1b) (p[K.sub.a] = 3.71 and 5.28, respectively). The kinetic analyses suggest for an unusual reactivity order as 1 < 1a >> 1b. The mechanistic possibilities suggest that although 1 is reduced by concerted electron transfer (ET)--proton transfer (PT) mechanism, 1a and 1b may be reduced by a concerted PT--ET mechanism where a slow proton-abstraction step is followed by a rapid ET process. It seems that the initial activation of the bridging [mu]-oxo group by a proton-abstraction results in the higher reactivity of 1a. Key words: metalloenzymes, diiron, Fe(III), APAP, redox, PT-ET mechanism. Chez les organismes superieurs, les metalloenzymes qui contiennent un centre metallique a base de Fe(III), comme le cytochrome P450, jouent un rôle actif dans le metabolisme du paracetamol (APAP). Dans le cadre des presents travaux, nous avons choisi un complexe enzymomimetique de [mu]-oxo-difer, le p[Fe[(III).sub.2]([mu]-O)[(phen).sub.4][([H.sub.2]O).sub.2]].sup.4+] (1, phen = 1,10-phenanthroline), pour mener une etude spectrophotometrique de la cinetique de ses interactions redox avec l'APAP. Dans un tampon acide (pH = 3,4-5,1), l'APAP reduit quantitativement le complexe 1 suivant une cinetique reactionnelle de premier ordre. Chaque molecule du complexe 1 accepte 2 [e.sup.-] de l'APAP lorsqu'elle est reduite en ferroine, [[Fe[(phen).sub.3]].sup.2+]. En s'oxydant, l'APAP se transforme en N-acetyl-p-benzoquinone-imine (NAPQI), qui s'hydrolyse et produit un melange de benzoquinone, de quinone-oxime, d'acetamide et d'acide acetique. Dans le milieu reactionnel, les deprotonations successives font en sorte que le complexe 1 existe en equilibre avec les especes [[Fe[(III).sub.2]([mu]-O)[(phen).sub.4]([H.sub.2]O)(OH)].sup.3+] (1a) et [[Fe[(III).sub.2]([mu]-O)[(phen).sub.4][(OH).sub.2]].sup.2+] (1b) (p[K.sub.a] = 3,71 et 5,28, respectivement). Les analyses cinetiques laissent supposerun ordre de reactivite inhabituel, soit 1 < 1a >> 1b. Si l'on considere les mecanismes possibles, le complexe 1 pourrait être reduit selon un mecanisme concerte de transfert d'electron et transfert de proton (ET--PT), tandis que les especes 1a et 1b pourraient être reduites selon un mecanisme PT--ET dans lequel une etape lente d'abstraction du proton serait suivie d'un processus rapide de transfert d'electron. L'activation initiale du groupement pontant [mu]-oxo par l'abstraction d'un proton semble se traduire par une plus grande reactivite de l'espece 1a. [Traduit par la Redaction] Mots-cles: metalloenzymes, difer, Fe(III), APAP, redox, mecanisme PT--ET., Introduction In living organisms, metalloenzymes with heme and non-heme iron sites serve important purposes such as dioxygen binding and C-H activation. (1-3) Such mono- or di-iron cores were found to [...]

In acid media ([[H.sup.+]] = 0.01-0.06 M), 2-mercaptoethanol (HSC[H.sub.2]C[H.sub.2]OH, abbreviated as MERCAP) and thioglycolic acid (HSC[H.sub.2]COOH, abbreviated as TGA) reduce the superoxo complex [[(en)(dien)[Co.sup.III]([O.sub.2])[Co.sub.III](en)(dien)].sup.5+] (1) to the corresponding peroxo complex [[(en)(dien)[Co.sup.III](μ-[O.sub.2])[Co.sup.III](en)(dien)].sup.4+] (2). The observed rate ([k.sub.o]), although proportional to both [MERCAP] and [TGA], is higher for TGA than for MERCAP, which is contrary to the expected trend on the basis of standard reduction potential (-0.14 versus -0.26 V, respectively). Moreover, [k.sub.o] values decrease with increasing media ionic strength (I) and [[H.sup.+]] and thus, thiolate anions are supposed to be the reactive forms of the reductants. Under the experimental conditions, the concentrations of such reductants (-SC[H.sub.2]C[H.sub.2]OH and HOOCC[H.sub.2][S.sup.-], respectively) from MERCAP ([pK.sub.a] = 9.7) and TGA ([pK.sub.a1], [pK.sub.a2] = 3.53, 10.10) are very small. However, for TGA, the tautomerization between HSC[H.sub.2]CO[O.sup.-] and -SC[H.sub.2]COOH ([pK.sub.i] = 7.0) plays a significant role in increasing the concentration of HOOCC[H.sub.2][S.sup.-]. The rates of both of the reactions are limited by the rate of solvent diffusion and this fact is also supported by the relatively low activation energy ([E.sub.a]), for the reactions. The [E.sub.a] values for the reactions with MERCAP and TGA are close enough (29.6 ± 1.3 and 27.0 ± 0.4 kJ [M.sup.-1] respectively) to suggest a common reaction mechanism for both. Key words: cobalt(III), superoxide, redox, kinetics, mechanism. Resume: En milieu acide ([[H.sup.+]] = 0,01-0,06 M), le 2-mercaptoethanol (HSC[H.sub.2]C[H.sub.2]OH, designe par l'abreviation MERCAP) et l'acide thioglycolique (HSC[H.sub.2]COOH, designe par l'abreviation ATG) reduisent le complexe superoxo [[(en)(dien)[Co.sup.III]([O.sub.2])[Co.sup.III](en)(dien)].sup.5+] (1) en complexe peroxo [[(en)(dien)[Co.sup.III](μ-[O.sub.2])[Co.sup.III](en)(dien)].sup.4+] (2) correspondant. Bien que la vitesse ([k.sub.o]) observee soit proportionnelle aux concentrations de MERCAP et d'ATG, elle est plus elevee pour l'ATG que pour le MERCAP, ce qui va a l'encontre de la tendance prevue d'apres les potentiels de reduction standard (-0,14 et -0,26 V, respectivement). En outre, les valeurs [k.sub.o] diminuent en fonction de l'augmentation de la force ionique du milieu reactionnel (I) et de l'acidite [[H.sup.+]], et par consequent, les ions thiolate devraient constituer la forme reactive des agents reducteurs. En conditions experimentales, les concentrations de tels agents reducteurs (-SC[H.sub.2]C[H.sub.2]OH et HOOCC[H.sub.2][S.sup.-] respectivement) issus du MERCAP ([pK.sub.a] = 9,7) et de l'ATG ([pK.sub.a1] = 3,53, [pK.sub.a2] = 10,10) sont tres faibles. Toutefois, dans le cas de l'ATG, la tautomerisation entre les especes HSC[H.sub.2]CO[O.sup.-] et -SC[H.sub.2]COOH ([pK.sub.i] = 7,0) joue un role important dans l'augmentation de la concentration de l'espece HOOCC[H.sub.2][S.sup.-]. La vitesse des deux reactions est limitee par la vitesse de diffusion dans le solvant, et ce fait est egalement corrobore par une energie d'activation Ea relativement peu elevee. Les Ea des reactions avec le MERCAP et avec l'ATG sont assez rapprochees (29,6 ± 1,3 et 27,0 ± 0,4 kJ [M.sup.-1] respectivement) pour laisser supposer un mecanisme commun aux deux reactions. [Traduit par la Redaction] Mots-cles: cobalt(III), superoxyde, redox, cinetiques, mecanisme., Introduction Superoxide ([O.sub.2.sup.-]), the one-electron reduced metabolized product of dioxygen ([O.sub.2]), (1-3) is a long known potent cellular poison. (4,5) Uncontrolled production of the reactive oxygen species (ROS) like [O.sub.2.sup.-] [...]