Phenols, enzymatic oxidation

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). 

Most of the C-diazeniumdiolates are not NO donors since they hydrolyze to produce nitrous oxide directly [174]. However, it has been found that carefully selected compounds can serve as NO donors under physiological conditions via alternative reaction pathways. Many cupferron analogs release NO via enzymatic oxidation [175] as do Oi-alkylated diazeniumdiolates [176]. Several novel types of NO-releasing N-hydroxy-N-nitrosamines have been prepared. These new preparative methods have been described in earlier sections. The precursors are enamines (Scheme 3.10), phenolates (Scheme 3.12), nitriles, and N-hydroxyguanidines (Scheme 3.9). 

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

Trusov (1915,1916), as indicated in Section 1.4.1, introduced the concept that polyphenols and quinones contributed to the synthesis of HS. Subsequently, as the result of enzymatic oxidation (with oxidase enzymes from microorganisms), the phenols oxidize to quinones and these, through further condensation, are converted into dark-colored humic-type substances. 

The production of humic substances by microorganisms is an extracellular process, because the enzymes are secreted into the external solution that contains the phenolic compounds derived from lignin and tannic acid degradation and microbial and plant metabolites. These phenolic compounds can then be enzymatically oxidized to quinones, which can undergo further polymerization or polycondensation reactions with other biomolecules (e.g., amino acids) to form humic polymers (Stevenson, 1994 Bollag et al., 1998 Burton, 2003). 

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. 

Uyama H, Kurioka H, Kaneko I, Kobayashi S (1994) Synthesis of a new family of phenol resin by enzymatic oxidative polymerization. Chem Lett 423-426... 

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... 

Amplihcation factors of 8 to 12 were claimed for the determination of phenol in a FLA system by a cyclic process depicted in equations 5 (Section VI.A. 1). Phenol is converted to o-benzoquinone in contact with immobilized tyrosinase held in a fixed bed reactor the quinone reacts with ascorbic acid (91) to yield catechol and dehydroascorbic acid (136) catechol can be enzymatically oxidized again to o-benzoquinone and so forth. The accumulated dehydroascorbic acid forms with o-phenylenediamine (137) a highly fluorescent product (kex 345 nm, kfl 410 nm). LOD was ca 0.02 (xM for phenol and catechol the linear range for phenol was 0.1 to 2 p,M and for catechol 0.02 to 2... 

Phenoxyl and semiquinone radicals are important intermediates in numerous biological systems and ESR spectroscopy has been used to detect and identify them in such systems. Studies were carried out on enzymatic reduction of quinone derivatives and enzymatic oxidation of hydroquinone and phenol derivatives. This topic has been reviewed... 

A large amount of phenols is released in wastewater and can be lost to waste streams. A rapid increase in the distribution and abundance of plastic debris in the ocean around the world was reported, and the adverse influence of plastic s phenol residues has been of great interest Polluted water disinfection, enzymatic oxidation of chlorinated phenols, decomposition of alkylphenol polyethoxylates and combustion of phenols can lead to the formation of highly toxic compounds. High adsorption of phenols on sludge and sediments requires that their distribution in these systems also be followed. All of these facts have promoted extensive research on phenolic compounds and their fate in the environment. 

Figure 6. Oxidation of phenols catalyzed by tyrosinase showing the two enzymatic oxidations, the melanin pathway, and the ascorbate shuttle.

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