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Irreversible Oxidation (irreversible + oxidation)
Selected AbstractsCobalt(II) Complexes with Substituted Salen-Type Ligands and Their Dioxygen Affinity in N,N -Dimethylformamide at Various TemperaturesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2005Andreas Huber Abstract Several unsymmetrically substituted salen-type cobalt(II) Schiff-base complexes CoL [H2L = 1,6-bis(2-hydroxyphenyl)-3,3-dimethyl-2,5-diaza-1,5-hexadiene (1); 1,6-bis(2-hydroxyphenyl)-3,3-dimethyl-2,5-diaza-1,5-heptadiene (2); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-6-(2-hydroxyphen-yl)-3,3-dimethyl-2,5-diaza-1,5-heptadiene (3); 1-(2-hydroxyphenyl)-6-methyl-2,5-diaza-1,5-nonadien-8-one (4); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-6-methyl-2,5-di-aza-1,5-nonadien-8-one (5); 1-(2-hydroxyphenyl)-3,3,6-trimethyl-2,5-diaza-1,5-nonadien-8-one (6); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-3,3,6-trimethyl-2,5-diaza-1,5-nonadien-8-one (7)] were prepared and characterized by their UV/Vis absorption spectra, magnetic moments, and oxidation potentials. Except for complex 4 (irreversible oxidation with t½ , 3 h), complexes 1,3 and 5,7 are remarkably resistant against irreversible auto-oxidation in air-saturated N,N -dimethylformamide (DMF) at ambient temperature. To characterize the Lewis acidity of the cobalt center in 1,7, the equilibrium constant Kpy was determined for monoadduct formation with pyridine (CoL + pyCoL·py). An O2 -sensitive optode was used to determine the Henry constant, KH, for the system O2/DMF in the temperature range 298,228 K. The formation of 1:1 adducts of complexes 1,7 with O2 in DMF, as characterized by the equilibrium constant K, was followed spectrophotometrically in the temperature range 298,228 K. The parameters ,Ho, ,So, and K are reported. At 298 K, K ranges from 21.9 M,1 (5) to 155 M,1 (7). The overall spectroscopic information, including EPR spectra obtained with frozen solutions of 3 and 7 in O2 -saturated DMF, confirm that the 1:1 adducts CoL·O2 are cobalt(III) superoxo compounds. The symmetrically substituted salen complex8 [H2L = 1,6-bis(3- tert -butyl-2-hydroxy-5-methylphenyl)-3,3,4,4-tetramethyl-2,5-diaza-1,5-hexadiene in 8] is shown to catalyze the oxidation of triphenylphosphane and 2,6-di- tert -butylphenol by O2 in DMF at ambient temperature. The correlation of the data obtained for K, Kpy, and the oxidation potential E½ is discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] The thioredoxin-independent isoform of chloroplastic glyceraldehyde-3-phosphate dehydrogenase is selectively regulated by glutathionylationFEBS JOURNAL, Issue 1 2007Mirko Zaffagnini In animal cells, many proteins have been shown to undergo glutathionylation under conditions of oxidative stress. By contrast, very little is known about this post-translational modification in plants. In the present work, we showed, using mass spectrometry, that the recombinant chloroplast A4 -glyceraldehyde-3-phosphate dehydrogenase (A4 -GAPDH) from Arabidopsis thaliana is glutathionylated with either oxidized glutathione or reduced glutathione and H2O2. The formation of a mixed disulfide between glutathione and A4 -GAPDH resulted in the inhibition of enzyme activity. A4 -GAPDH was also inhibited by oxidants such as H2O2. However, the effect of glutathionylation was reversed by reductants, whereas oxidation resulted in irreversible enzyme inactivation. On the other hand, the major isoform of photosynthetic GAPDH of higher plants (i.e. the AnBn -GAPDH isozyme in either A2B2 or A8B8 conformation) was sensitive to oxidants but did not seem to undergo glutathionylation significantly. GAPDH catalysis is based on Cys149 forming a covalent intermediate with the substrate 1,3-bisphosphoglycerate. In the presence of 1,3-bisphosphoglycerate, A4 -GAPDH was fully protected from either oxidation or glutathionylation. Site-directed mutagenesis of Cys153, the only cysteine located in close proximity to the GAPDH active-site Cys149, did not affect enzyme inhibition by glutathionylation or oxidation. Catalytic Cys149 is thus suggested to be the target of both glutathionylation and thiol oxidation. Glutathionylation could be an important mechanism of regulation and protection of chloroplast A4 -GAPDH from irreversible oxidation under stress. [source] Photophysical and electrochemical characterization of new poly(arylene vinylene) copolymers containing quinoline or bisquinoline segmentsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2009John A. Mikroyannidis Abstract Four new fluorescent conjugated vinylene-copolymers incorporating quinoline or bisquinoline segments along the backbone were synthesized by Heck coupling. Three of them were fluorenevinylene-copolymers and contained quinoline (PQFV, PQFVT) or bisquinoline segments (PBQFV). One of them (PBQPV) was phenylenevinylene-copolymer and contained bisquinoline segments. All the copolymers were soluble in common organic solvents and had relatively low glass transition temperature (Tg = 50,56 °C for fluorenevinylenes and Tg < 25 °C for phenylenevinylene). In THF solutions, the quinoline-containing copolymers showed absorption maxima at 411,420 nm while the bisquinoline-containing ones exhibited maxima at 357,361 nm. The emission maxima of solutions were 465,490 nm. The copolymers showed high quantum yields up to 64%. The films exhibited absorption and emission maxima in the range of 371,437 nm and 480,521 nm, respectively. All copolymers revealed reversible reduction with electron affinity of 2.66,3.53 eV and irreversible oxidation scans with ionization potential of 5.39,5.53 eV. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3370,3379, 2009 [source] Highly Conjugated p -Quinonoid ,-Extended Tetrathiafulvalene Derivatives: A Class of Highly Distorted Electron DonorsCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2004Marta C. Díaz Abstract A new class of ,-extended TTF-type electron donors (11,a,c) has been synthesized by Wittig,Horner olefination of bianthrone (9) with 1,3-dithiole phosphonate esters (10,a,c). In cyclic voltammetry experiments, donors 11,a,c reveal a single, electrochemically irreversible oxidation,yielding the corresponding dicationic products,at relatively low oxidation potentials (,0.7,0.8 V). Theoretical calculations, performed at the DFT level (B3,P86/6-31,G*), predict a highly-folded C2h structure for 11,a. In the ground state, the molecule adopts a double saddle-like conformation to compensate the steric hindrance. The calculations suggest that the intramolecular charge transfer associated with the HOMO,LUMO transition is responsible for an absorption band observed above 400 nm. While the radical cation 11,a.+ retains the folded C2h structure predicted for the neutral molecule as the most stable conformation, the dication 11,a2+ has a fully aromatic D2 structure, formed by an orthogonal 9,9,-bianthryl central unit to which two singly-charged dithiole rings are attached. The drastic conformational changes that compounds 11 undergo upon oxidation account for their electrochemical properties. By means of pulse radiolysis measurements, radical-induced one-electron oxidation of 11,a,c was shown to lead to the radical cation species (11,a,c.+), which were found to disproportionate with generation of the respective dication species (11,a,c2+) and the neutral molecules (11,a,c). Una nueva familia de moléculas dadoras de electrones de tipo TTF , -extendido, altamente conjugadas, (11,a,c) se han sintetizado mediante la reacción de olefinación de Wittig,Horner de la biantrona (9) con fosfonatos de 1,3-ditiol (10,a,c). En los experimentos de voltamperometría cíclica, los dadores 11,a,c muestran una única onda de oxidación electroquímicamente irreversible,dando lugar a los productos dicatiónicos,a potenciales relativamente bajos (,0.7,0.8 V). Cálculos teóricos, llevados a cabo a nivel DFT (B3,P86/6-31,G*), predicen una estructuraC2haltamente distorsionada para 11,a. La molécula adopta una conformación en forma de doble mariposa para aliviar el impedimento estérico. Los cálculos sugieren que la transferencia de carga intramolecular asociada a la transición HOMO,LUMO es responsable de la banda de absorción observada por encima de 400 nm en el espectro electrónico. El catión radical 11,a.+retiene la estructura C2hplegada predicha para la molécula neutra como la conformación más estable. Por el contrario, el dicatión 11,a2+muestra una estructuraD2totalmente aromática,formada por una unidad central de 9,9,-biantrilo ortogonal, unida a los anillos cargados de ditiol. Los profundos cambios conformacionales que experimentan los compuestos 11 tras la oxidación explican sus propiedades electroquímicas. Medidas de radiólisis de pulso, esto es, la oxidación monoelectrónica de 11,a,c inducida por radicales, conduce a las especies catión radical (11,a,c.+), las cuales dismutan para generar las respectivas especies dicatiónicas (11,a,c2+) y la molécula neutra (11,a,c). [source] | ||||||||||