Oxidation of phenolic compounds

PO performs vitally important functions in the plant cell and is mainly associated with the oxidation of phenolic compounds and with the formation and strengthening of the cell wall (Passardi et al., 2004). PO is involved in the oxidative transformation of molecules in growth-regulating or signalling activities and - as a result - can also perform regulatory functions in the cell. Plant POs are represented by genetically different proteins with the same enzymatic activity (Welinder et al., 2002). 

Alnaizy R, Akgerman A (2000) Advanced oxidation of phenolic compounds. Adv Environ Res 4(3) 233-244... 

An excellent classification of this type of enzymes was that of Burton (2003). Because of their importance in polyphenol degradation, we have studied four enzymes closely related with the oxidation of phenolic compounds polyphenoloxidase, peroxidase, laccase, and lipoxygenase. 

The multi-copper oxidases include laccase, ceruloplasmin, and ascorbate oxidase. Laccase can be found in tree sap and in fungi ascorbate oxidase, in cucumber and related plants and ceruloplasmin, in vertebrate blood serum. Laccases catalyze oxidation of phenolic compounds to radicals with a concomitant 4e reduction of O2 to water, and it is thought that this process may be important in the breakdown of lignin. Ceruloplasmin, whose real biological function is either quite varied or unknown, also catalyzes oxidation of a variety of substrates, again via a 4e reduction of O2 to water. Ferroxidase activity has been demonstrated for it, as has SOD activity. Ascorbate oxidase catalyzes the oxidation of ascorbate, again via a 4e reduction of O2 to water. Excellent reviews of these three systems can be found in Volume 111 of Copper Proteins and Copper Enzymes (Lontie, 1984). 

The accumulation or oxidation of phenolic compounds principally in stored agricultural products normally gives rise to the physiological disorders that result in a major loss of commercial value of the products. Among the most common of these disorders in which phenol metabolism is involved is fruit browning and the russet spotting (RS) in harvested lettuce. 

EPR experiments on carbon-centred radicals with either a- or /J-boronic ester substituents have been reported.168 While the a-substituted radicals were modestly thermodynamically stable, the /J-substituted radicals underwent easy /J-climination. An EPR experiment on the photo-oxidation of phenolic compounds containing at least one free ortho position has indicated the formation of persistent secondary radicals derived from dimerization or polymerization from C-0 coupling.169 The structure of the succinimidyl radical has been re-examined using density functional theory with a variety of basis sets. The electronic ground state was found to be of cr-symmetry allowing for facile -scission. These conclusions were also predicted using MP2 but... 

Oxidation of phenols is one of the most important aspects of these compounds to the biologist. Oxidation of phenolic compounds can result in the browning of tissues. Well-known examples are the browning of lfuits after they have been cut. Oxidation can also result in the formation of metabolites that are toxic to animals and plants, and that can account for spoilage of foods in processing. On the other hand, toxic compounds formed from the oxidation of phenolics can inhibit pathogenic microorganisms. Certain phenols are used as retardants or antioxidants to prevent the oxidation of fatty acids. 

An alternative mechanism for the oxidation of phenolic compounds is enzyme-catalyzed oxidation. Several classes of enzymes can catalyze this reaction. According to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB), these enzymes are part of the E C. 1 class of oxidoreductases (see the Internet web site http //www.chem.qmul.ac.uk/iubmb/enzyme/ECl). The three main classes of enzymes that catalyze the oxidation of phenolic compounds are the oxidoreductases that use oxygen as electron acceptor (E.C. 1.10.3), the peroxidases (E.C. 1.11.1), and monophenol monooxygenase (E.C. 

Oxidation of Phenolic Compound, 3 1 Hydrogen Peroxide Phenol Mole Ratio... 

It is well known that the oxidation of phenolic compounds at solid electrodes produces phenoxy radicals, which couple to form a passivating polymeric film on the electrode surfaces [20,21]. The anodic reaction proceeds through an initial one-electron step to form phenoxy radicals, which subsequently can undergo either polymerization or further oxidation with the transfer of oxygen from hydroxyl radicals at the electrode... 

Properties estimated by interpolation within the range of conditions over which a QSAR was calibrated should be reliable, but extrapolations beyond this range cannot be made with certainty. A similar restriction applies to experimental variables factored out of the training set data before deriving the QSAR (e.g., the effect of pH on oxidation of phenolic compounds, or the effect of surface area on reductions with Fe°). 

Co catalysts are used to catalyze a series of oxidations that involve one-electron redox reactions, such as oxidation of phenolic compounds, thiol oxidation, and oxyfunctionalization of saturated hydrocarbon groups. 

Monzani E, Gatti AL, Profumo A, Casella L, Gullotti M (1997) Oxidation of phenolic compounds by lactoperoxidase. Evidence for the presence of a low-potential compound II during catalytic turnover. Biochemistry 36 1918-1926... 

Batch reactors based on peroxidases are mainly applied for degradation purposes (see Chap. 8). LiP, manganese peroxidase (MnP), HRP, SBP, and CPO were used for the oxidation of phenolic compounds [3, 6, 7, 9, 20, 38, 74, 75, 95], decoloriza-tion of dye-containing effluents [5, 22], and pulp biobleaching [59]. In the field of synthesis, CPO is the most versatile and promising of the peroxidases (see Chap. 6). It was applied in discontinuous operation for epoxidations [78,79], enantioselective oxidations of alcohols to aldehydes [14,48], halogenations [77,80], hydroxylations, and oxidation of indole to oxindole, which is an important drug precursor [96]. 

Pattenden and coworkers have recently evaluated the relative merits of LTA and electrochemical oxidation of phenolic compounds with particular reference to synthesis of the antiallergic compounds sodium chromoglycate (lOTAL 53) and proxicromil (54), which are used for the prophylactic treatment of asthma. The 2-carboxychromone moieties in the compounds (53) and (54) are synthesized from the appropriate 2, 6 -dihydroxyacetophenones. Oxidation of the 2 -hydroxyacetophenone (55) by LTA in (U-chloromethane gave almost exclusively the quinol acetate (56), which was subsequently converted to the 2, 6 -dihydroxyacetophenone (57), a precursor to proxicromil (54 Scheme 21). By contrast, electro-... 

Oloroso wines are only obtained by oxidative aging, which is accomplished by fortifying the initial wine to an ethanol content of about 18 vol.% in order to prevent growth of flor yeasts. Under such conditions, oloroso wine acquires a dark colour by effect of the oxidation of phenol compounds. 

Mechanism of reaction. Under oxidative conditions and for grapes containing high levels of hydroxycinnamic acids and low of levels of glutathione (a compound that easily reacts with o-quinones, preventing oxidation of phenolic compounds in wines) (Sarni-Manchado et al. 1995), some phenolics are able to react with the o-quinones of caftaric acid (Cheynier et al. 1986) (see also Chapter 9B). 

These reactions involve, on one hand, the nucleophilic A-rings of flavonoids such as flavanols, and on the other hand, electrophilic quinones arising from enzymatic or chemical oxidation of phenolic compounds. 

A crnde extract of sweet potato Ipomoea-batatas (L.) Lam.) was nsed as a source of phenol oxidases (polyphenoloxidase, tyrosinase, catecholoxidase, EC 1.14.18.1). The extract was directly placed in the carrier of a FIA system with UVD, to promote oxidation of phenolic compounds to o-quinones that condense to form melanin-like pigments with a strong absorption at 410 nm. The determination of phenols in industrial wastewaters showed good agreement with conventional methods (correlation coefficient 0.9954) LOD was 10 p,M, with RSD <2.7% (w = 6). Under optimal storage conditions the enzymatic activity did not vary for at least five months . 

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