Oxidation chemistry

A series of heterocyclic compounds possessing imidazolo[1,2-a]pyridine moiety, namely, ethyl 7-methylimidazolo[1,2-a] pyridine-2-carboxylate L1; 2-(3-nitrophenyl)imidazo[1,2-a]pyridine L2; 3-(imidazo[1,2-a]pyridine-2-yl)aniline L3; 2-phenylimidazolo[1,2-a]pyridine-3carbaldehyde L4; and 2-phenylimidazo [1,2-a]pyridine L5 were synthesized. The in situ generated copper (II), iron (II), and zinc (II) complexes of these compounds (L1-L5) were examined for their catalytic activities and were found to be effective catalysts for the oxidation of catechol to o-quinone with the atmospheric oxygen. The present study reveals that the rate of oxidation depends on four parameters: the nature of the ligand, transition metals, ion salts, and the concentration of the complex. The combination L2(Cu(CH 3 COO) 2) gives the highest rate.

Methods to improve the oxidative stability of B100 biodiesel were studied. A combination of a peroxide destroyer-triphenylphosphine-and a chain-breaking antioxidant-propyl gallate-was found to largely prevent oxidative damage in soy-and waste oil-derived biodiesels. Peroxides were shown to react rapidly with propyl gallate and render it ineffective. Triphenylphosphine alone provided some protection from oxidation, but not to the extent of the combination. Propyl gallate was found to be superior to other potential surface-active or chain-breaking antioxidants tested, such as ascorbyl palmitate, butylated hydroxytoluene (BHT), Trolox, and other gallate esters.

Methods to improve the oxidative stability of B100 biodiesel were studied. A combination of a peroxide destroyer-triphenylphosphine-and a chain-breaking antioxidant-propyl gallate-was found to largely prevent oxidative damage in soy-and waste oil-derived biodiesels. Peroxides were shown to react rapidly with propyl gallate and render it ineffective. Triphenylphosphine alone provided some protection from oxidation, but not to the extent of the combination. Propyl gallate was found to be superior to other potential surface-active or chain-breaking antioxidants tested, such as ascorbyl palmitate, butylated hydroxytoluene (BHT), Trolox, and other gallate esters.

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of c...

Recent decades have seen increasingly restrictive regulations applied to gasoline engines. Gasoline combustion chemistry must be investigated to achieve a better understanding and control of internal combustion engine efficiency and emissions. In this work, several gasoline fuels, namely the FACE (Fuel for Advanced Combustion Engines) gasolines, were selected as targets for oxidation study in jet-stirred reactors (JSR). The study is facilitated by formulating various gasoline surrogate mixtures with known hydrocarbon compositions to represent the real gasolines. Surrogates included binary mixtures of n-heptane and iso-octane, as well as more complex multicomponent mixtures. The oxidation characteristics of FACE gasolines and their surrogates were experimentally examined in JSR-1 and numerically simulated under the following conditions: pressure 1 bar, temperature 500-1050K, residence time 1.0 and 2.0 s, and two equivalence ratios (ϕ=0.5 and 1.0). In the high temperature region, all real fuels and surrogates showed similar oxidation behavior, but in the low temperature region, a fuel's octane number and composition had a significant effect on its JSR oxidation characteristics. Low octane number fuels displayed more low temperature reactivity, while fuels with similar octane number but a larger number of nalkane components were more reactive. A gasoline surrogate kinetic model was examined with FACE gasoline experiments either measured in JSR-2, or taken from previous work under the following conditions: pressure 10 bar, temperature 530-1200K, residence time 0.7s, and three equivalence ratios (ϕ=0.5, 1.0 and 2.0). Comparison between FACE gasoline experimental results with surrogate model predictions showed good agreement, demonstrating considerable potential for surrogate fuel kinetic modeling in engine simulations.

In-situ metal complexes of some 1,2,4-triazole Schiff base ligands (L1 – L11) were examined for their catalytic activities. The dioxygen complexes of metals were generated by stirring metal salts and the 1,2,4-triazoles Schiff base ligands, it has been found that the oxidation of catechol is very efficient to give the appropriate oquinone. We have found that ligand structure, solvent, concentration and substrate have proven huge influence on the combinations metal/ligand reactivities. The highest rate activity is given by the in-situ complex formed by ligand L10 and Cu(NO3)2 which is equal to 11.52 μmol.L -1 min -1 . Also, the solvent has huge influence on the kinetic oxidation reaction system.

In order to mimic the function of catecholase activity, in-situ copper (II) complexes of three ligands containing different numbers of benzyl groups: (E)-4methoxy-N-(4-methoxybenzylidene)aniline L1, (E)-N-(4-methoxybenzylidene) naphthalene-1-amine L2, (E)-N-(4-methoxybenzylidene)aniline L3, were reported and examined, in combination with different cupric salts, for their catecholase activities at ambient conditions. The highest rate activity was given by the combination of two equivalents of L3 and one equivalent of Cu(CH3COO)2 in methanol. The Michaelis Menten model was applied to obtain the kinetic parameters of the best catalyst.

Background: The effect of some selected laboratory prepared fruit wines (Pineapple wine, Cashew wine and Banana wine) at different alcoholic content on haematological parameters of albino rat (Wistar strain) were investigated. The Pack cell volume (PCV), Haemoglobin (Hb), Red blood cell (RBC), Mean cell haemoglobin (MCH), Mean cell volume (MCV), Mean cell haemoglobin concentration (MCHC), Total white blood cell (TWBC), Percentage Lymphocyte (%LYM), percentage Eosinophil (%EOS) and percentage Neutrophil (% NEU) were determined to estimate the toxicological effect of the administered wine. Results: The results indicated that there was a significant reduction in PCV and Hb count of rats fed with 7.5% of cashew wine and 10% of cashew wine and pineapple wine respectively when compared with the red wine (p< 0.05), while there was a significant reduction in RBC count of the rats fed with 7.5% and 10.% cashew wine when compared with the red wine (p< 0.05). Conclusion: There was little...

The conversion of monophenols to o-diphenols and the oxidation of resultant o-diphenols to o-benzoquinones are two activities widely exhibited by tyrosinases. Accordingly, during the oxidation of 3,4-dihydroxyphenylacetic acid, mushroom tyrosinase produced 4-carboxymethyl-o-benzoquinone as the initial product. Carboxymethyl-o-benzoquinone thus formed was found to be extremely unstable as compared to its lower homolog 4-carboxy-o-benzoquinone and underwent novel transformations to produce 3,4-dihydroxymandelic acid and a new compound identified as 2,5,6-trihydroxybenzofuran, based on uv, ir, and NMR spectral and HPLC studies. Tyrosinase further oxidized the 3,4-dihydroxymandelate formed to 3,4dihydroxybenzaldehyde, as reported earlier [Sugumaran, M. (1986) Biochemistry 25,4489-44921, while 2,5,6-trihydroxybenzofuran was oxidized to insoluble, unidentifiable dark pigments. Incubation of synthetic carboxymethyl-o-benzoquinone in dimethyl sulfoxide at room temperature resulted in the production of 3,4-dihydroxymandelic acid as the sole catecholic product. Based on these findings, possible mechanisms for the unusual conversion of carboxymethyl-o-benzoquinone to 3,Cdihydroxymandelic acid and 2,5,6-trihydroxybenzofuran are presented. o 1989 Academic PKSS, I~C.

Insect phenoloxidases participate in three physiologically important processes, viz., cuticular hardening (sclerotization), defense reactions (immune reaction), and wound healing. Arrest or even delay of any of these processes compromises the survival of insects. Since the products of phenoloxidase action, viz., quinones, are cytotoxic, uncontrolled phenoloxidase action is deleterious to the insects. Therefore, the activity of this important enzyme has to be finely controlled. A novel inhibition of insect phenoloxidases, which serves as a new regulatory mechanism for control of its activity, is described. The activity of phenoloxidases isolated from both Sarcophaga bullata and Manduca sexta is drastically inhibited by quinone isomerase (isolated from Calliphora), an enzyme that utilizes the phenoloxidase-generated 4-alkylquinones. In turn, phenoloxidase reciprocated the inhibition of isomerase. By forming a complex and controlling each other's activity, these two enzymes seem to regulate the levels of endogenously quinones. In support of this contention, an endogenous complex consisting of phenoloxidase, quinone isomerase, and quinone methide isomerase was characterized from the insect, Calliphora. This sclerotinogenic complex was isolated and purified by borate extraction of the larval cuticle, ammonium sulfate precipitation, and Sepharose 6B column chromatography. The complex exhibited a molecular mass of about 620-680 kDa, as judged by sizeexclusion chromatography on Sepharose 6B and HPLC and did not even enter 3% polyacrylamide gel during electrophoresis. The phenoloxidase activity of the complex exhibited a wide substrate specificity. Incubation of the complex with N-acetyldopamine rapidly generated N-acetylnorepinephrine, dehydro-N-acetyldopamine, and its dimers. In addition, transient accumulation of N-acetyldopamine quinone was also observed. These results confirm the presence of phenoloxidase, quinone isomerase, and quinone methide isomerase in the complex. Attempts to dissociate the complex with even trace amounts of SDS ended in the total loss of quinone isomerase activity. The complex does not seems to be made up of stoichiometric amounts of individual enzymes as the ratio of phenoloxidase to quinone isomerase varied from preparation to preparation. It is proposed that the complex formation between sequential enzymes of sclerotinogenic pathway is advantageous for the organism to effectively channel various reactive intermediates during cuticular hardening.

The oxidation chemistry of dopa in relation to mela-pounds is generally attributed to the reactivity of their nin biosynthesis has been extensively studied. How-oxidation products (13, 14). Hence it is important to ever, the oxidation of its lower homolog viz., 3,4-dihyexamine the oxidation chemistry of catecholamine dedroxyphenylglycine has not been described. Using 3,4rivatives. dimethoxybenzaldehyde as the starting material, the Due to the importance of melanin pigments in vitichemical synthesis of 3,4-dihydroxyphenylglycine was ligo, melanoma cancer, and skin color, a vast chemical accomplished by Strecker's synthesis and demethylliterature has been established on the oxidation chemation reactions. Tyrosinase readily oxidized this unistry of dopa (1-7). Enzymatic oxidation of dopa generusual amino acid to the expected quinone. The glycylates the corresponding dopaquinone, which is highly o-benzoquinone thus formed was highly unstable like unstable and exhibits a rapid intramolecular Michaelits higher analog, dopaquinone. However, unlike its 1,4-addition reaction to generate leucochrome. Trancounterpart, it failed to exhibit intramolecular cyclizasient leucochrome undergoes double decomposition tion reaction. Rather, glycyl-o-benzoquinone exhibited with dopaquinone generating dopachrome as the obfacile transformation(s) to ultimately generate 3,4-diservable intermediate. Isomerization of dopachrome to hydroxybenzaldehyde as an isolatable product. A quinone methide (15-17) and aromatization of the quiprobable mechanism involving intermediary formanone methide produces 5,6-dihydroxyindoles. Oxidation of unstable quinone methide and carbinolamine tive polymerization of dihydroxyindoles ultimately intermediates is proposed to account for the novel yields brown to black eumelanin pigments (7, 18-20).

Nitrite anions are formed in the human body and in the natural environment as intermediate chemical compounds during the reduction of nitrate, a ubiquitous anthropogenic contaminant introduced into the environment primarily through fertilizer use. Multiple reaction monitoring (MRM) in ion chromatography-electrospray ionization-tandem mass spectrometry (IC-ESI-MS/MS) is a promising new technique for quantifying and confirming the identity of anions in complex aqueous mixtures. In this article, we present the results of a short investigation devised to: (1) compare the signal generated by the MRM transitions for nitrite with those for nitrate, (2) isolate the source of the signal from these MRM transitions occurring within the IC-ESI-MS/MS instrument and (3) assess the relationship between the observed MRM signals for nitrite. The MRM transitions used in this study were m/z 62 (NO 3 − ) → m/z 46 (NO 2 − ) and m/z 46 (NO 2 − ) → m/z 46 (NO 2 − ). Results of the investigation revealed the association of both MRM transitions with the nitrite chromatographic peak, indicating the occurrence of nitrite oxidation to nitrate at the ESI interface before the first quadrupole. Calibrations for both MRM signals, as well as their sum, were found to be linear. However, the ratio of m/z 62 → m/z 46 to m/z 46 → m/z 46 (indicating an extent of oxidation) ranged from 35 to 56% over a nitrite concentration range of 10 to 100 ppm, showing no clear trend associated with concentration.

A series of heterocyclic compounds possessing imidazolo[1,2-a]pyridine moiety, namely, ethyl 7-methylimidazolo[1,2-a] pyridine-2-carboxylate L1; 2-(3-nitrophenyl)imidazo[1,2-a]pyridine L2; 3-(imidazo[1,2-a]pyridine-2-yl)aniline L3; 2-phenylimidazolo[1,2-a]pyridine-3carbaldehyde L4; and 2-phenylimidazo [1,2-a]pyridine L5 were synthesized. The in situ generated copper (II), iron (II), and zinc (II) complexes of these compounds (L1-L5) were examined for their catalytic activities and were found to be effective catalysts for the oxidation of catechol to o-quinone with the atmospheric oxygen. The present study reveals that the rate of oxidation depends on four parameters: the nature of the ligand, transition metals, ion salts, and the concentration of the complex. The combination L2(Cu(CH 3 COO) 2 ) gives the highest rate.