Enzymatic oxidations of phenols

In the oxidative polymerization of phenols catalyzed by Cu complexes, the substrate coordinates to the Cu(II) complex and is then activated. The activated phenol couples in the next step. The Cu complex acts effectively as a catalyst at concentrations of 0.2-2 mol% compared to the substrate. The oxidation proceeds rapidly at room temperature under an air atmosphere to give poly(phenylene ether) in a quantitative yield. The polymerization follows Michaelis-Menten-type kinetics [55]. Enzymatic oxidation of phenols is an important pathway in the biosynthesis of lignin in plants [56] catalyzed by a metalloenzyme. 

Heck PE, Massey IJ, Aitken MD. Toxicity of reaction products from enzymatic oxidation of phenolic pollutants. Water Sci Technol 1992 26(9-11) 2369—2371. 

In addition to the enzymatic oxidation of phenolic acids, their autooxidation can also result in brown-colored pigments that are detrimental to food quality. It is dependent on the physicochemical environment and strongly increases with pH. For example, auto-oxidation of phenolic acids was responsible for color loss of carrot puree, and processing treatments that reduce residual oxygen may result in better color retention after processing and storage [132]. 

Figure 9.33 Non-enzymatic oxidation of phenols to o-diphenols and o-quinones.

Most winemakers prefer limiting air contact with crushed grapes and white juice as much as possible. An adapted sulfur dioxide addition to juice blocks the enzymatic oxidation of phenolic compounds. This philosophy is based on empirical observation juices of many grape varieties must conserve a green color dming the pre-fermentation phase to be transformed into fruity white wines. Oxidation phenomena must consequently be avoided as much as possible. 

Oxygen consumption in juice is essentially due to the enzymatic oxidation of phenolic compounds. Two oxidases (Section 11.6.2) are involved (Dubemet and Rib6reau-Gayon, 1973,... 

Diacetoxybiphenyl [2] was prepared via the reaction of acetic anhydride with 4,4 -dihydroxybiphenyl which in turn was obtained by enzymatic oxidation of phenol [1] by horseradish peroxidase.Stereospecific bis-hydroxylation of benzene [3] by Pseudomonas putida and subsequent acetylation gave cis l,2-diacetoxy-3,5-cyclohexadine [4]. Condensation of [TMA] and 4-aminobenzoic acid [6] in DMAc at 150 C gave N-(4-carboxyphenyl) trimellitimide [7] in excellent yield.22... 

Coutts IGC, Hamblin MR, Tinley EJ (1979) The enzymatic oxidation of phenolic tetrahydroiso-quinoline-1-carboxylic acids. J Chem Soc Perkin Trans 1 2744-2750 Davis VE, Cashaw JL, McMurtrey KD, Ruchirawat S, Nimit Y (1982) Metabolism of tetrahydro-isoquinolines and related alkaloids. In Bloom F, Barchas J, Sandler M, Usdin E (eds) Beta-carbolines and tetrahydroisoquinolines. Liss, New York, p 99 Furuya T, Nakano M, Yoshikawa T (1978) Biotransformation of (RS)-reticuline and morphinan alkaloids by cell cultures of Papaver somniferum. Phytochemistry 17 891-893 Gates M (1953) Conversion of codeinone to codeine. J Am Chem Soc 75 4340-4341 Graves JMH, Smith WK (1967) Transformation of pregnenolone and progesterone by cultured plant cells. Nature (London) 214 124 8-1249... 

Biochemical Routes. Enzymatic oxidation of benzene or phenol leading to dilute solution of dihydroxybenzenes is known (62). Glucose can be converted into quinic acid [77-95-2] by fermentation. The quinic acid is subsequently oxidized to hydroquinone and -benzoquinone with manganese dioxide (63). 

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). 

As described above, the enzymatic polymerization of phenols was often carried out in a mixture of a water-miscible organic solvent and a buffer. By adding 2,6-di-0-methyl-(3-cyclodextrin (DM-(3-CD), the enzymatic polymerization of water-insoluble m-substituted phenols proceeded in buffer. The water-soluble complex of the monomer and DM-(3-CD was formed and was polymerized by HRP to give a soluble polymer. In the case of phenol, the polymerization took place in the presence of 2,6-di-O-methyl-a-cyclodextrin (DM-a-CD) in a buffer. Only a catalytic amount of DM-a-CD was necessary to induce the polymerization efficiently. Coniferyl alcohol was oxidatively polymerized in the presence of a-CD in an aqueous solution. ... 

Waldmann et al. used tyrosinase which is obtained from Agaricus bisporus for the oxidation of phenols to give ortho-quinones via the corresponding catechols in the presence of oxygen (scheme 33).1881 A combination of this enzymatic-initiated domino process with a Diels-Alder reaction yields the functionalized bicyclic components 164 and 165 as a 33 1 mixture starting from simple p-methyl-phenol 160 in the presence of ethyl vinyl ether 163 as an electron rich dienophile via the intermediates 161 and 162 in an overall yield of 77%. 

Figure 4.3 Aromatic products of the enzymatic oxidation of benzene. Phenol is the major metabolite.

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. 

Perhaps the most well-known peroxidase-catalyzed reactions are those involving electron transfer, in which an aromatic substrate is oxidized in a mono-electronic oxidation up to its mono-radical, Eq. (1), which is capable of participating further in a variety of non-enzymatic reactions such as disproportionation, polymerization and electron transfer. These types of reactions are very common during the peroxidase-catalyzed oxidation of phenols and, in some cases, during the oxidation of alkaloids. For example, peroxidase is capable of dimerizing jatrorrhizine (IV) to 4,4 -bis-jatrorrhizine (V) in the presence of H2O2 (Scheme III) [50]. 

FIGURE 4. Operation principles of a biosensor based on enzymatic oxidation of a phenol (top) and electrochemical detection by determining oxygen or hydrogen peroxide (bottom left) or the oxidation products derived from the phenol (bottom right). Ph denotes a phenol, Ph an activated form of a phenol and Q a quinone... 

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