Phenol, enzymatic reactions oxidation

Chemical Properties. Lignin is subject to oxidation, reduction, discoloration, hydrolysis, and other chemical and enzymatic reactions. Many ate briefly described elsewhere (51). Key to these reactions is the ability of the phenolic hydroxyl groups of lignin to participate in the formation of reactive intermediates, eg, phenoxy radical (4), quinonemethide (5), and phenoxy anion (6) ... 

The low specificity of electron-donating substrates is remarkable for laccases. These enzymes have high redox potential, making them able to oxidize a broad range of aromatic compounds (e.g. phenols, polyphenols, methoxy-substituted phenols, aromatic amines, benzenethiols) through the use of oxygen as electron acceptor. Other enzymatic reactions they catalyze include decarboxylations and demethylations [66]. 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

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]. 

Chi the basis of E. values we can make a conclusion about the difhision factors which are some of the most conqilicated points concerning catalysis with immobilized enzymes. The value for the activation energy on peroxidase oxidation of phenol with catalase immobilized on "NORIT" soot is E, =10.95 kJ.mof which is an indication that the process takes place under diSusion regime. The latter means that the enzymatic reaction rate is determined by the mass tranfer of substrate to the surfoce of the carrier particles and its diffiision into the carrier. 

There are numerous examples in which enzymatic oxidation might produce a reactive species in Diels-Alder reactions. Oxidations include dehydrogenation giving 1,3-diene, and phenol oxidation affording cyclic dienone, which is acted as a diene or a dienophile. 

In nature, the oxidative dimerization of phenols is controlled by enzymes, as is demonstrated by the axial chirality of the 6,8 -coupled juglone derivative isodiospyrin. In synthesis, however, phenol oxidation only proceeds in high yields when the enzymatic reaction control is replaced by substituent control, that is, if all but one of the positions with high spin density in the radical (ortho-and para positions) are blocked. 

From tyrosine, a series of enzymatic reactions including ring hydroxyla-tion, deciurboxylation, deamination, and D-methylation are responsible for over 20 majof metabolites of neurochemical interest. All of the above metabolites retain at least an electroactive phent nudeus, and most provide enhanced detectability via vanillyl or catechol functional groups. Table III provides cyclic voltammctric data in 90% 0.1 M citrate (pH 4)/10% methanol for a series of these metabolites. Substituent effects are evident. Gener-aUy the catecholamines with their OKlihydroxyphenyl structures are easiest to oxidize, followed by vanillyl dmvatives such as normetanephrine and vanillylmandelic acid. Simple phenols such as tyrosine remain the most difficult to oxidize in this series. 

The three core skeletons obtained from the phenol coupling steps form the basis of further alkaloid diversity. A complex network of enzymatic reactions exists to produce a spectrum of compoimds that differs between species, varieties and cultivars and even between the different tissues and vegetative phases of the same plant. These biochemical modifications are achieved by a multitude of enzymes catalyzing various types of reactions, such as C-C and C-0 bond formations, O- and A-methylations, demethylations, hydroxylations, oxidations and reductions. The various products obtained from these reactions yields the several hundred of structurally related AAs known to date (Table 1, Figures 1-2) [3-6, 13]. 

The said study has paved the way for realizing an artificial system which resembles a natural enzymatic reaction process and which is needed for conditioning the existence of biomaterials. It is expected to be applied to the industrial-use oxidation process in which benzene and paraffin are transformed into phenol and alcohol, respectively. ... 

The enzymatic reaction kinetics on the HRP-catalyzed oxidation of p-cresol in aqueous 1,4-dioxane or methanol showed that the cataljdic turnover niunber and Michaelis constant were larger than those in water (235). Numerical and Monte Carlo simulations of the peroxidase-catalyzed polymerization of phenols were demonstrated (236). The simulations predicted the monomer reactivity and polymer molecular weight, leading to synthesis of polymers with specific molecular weight and index. In an aqueous 1,4-dioxane, the formation of monomer aggregate was observed (237), which might elucidate the specific polymerization behaviors in such a medium. 

We have recently begun studies geared at the analytical application of enzymes immobilized on microelectrodes. We had previously demonstrated that 4-amino phenyl phosphate could be employed as a substrate for the determination of the enzymatic activity of alkaline phosphatase by following the oxidation of 4-amino-phenol which is the product of the enzymatic reaction [28]. In the present case, the interest was in determining whetiier such an approach could be transposed to a microelectrode and to determine if the immobilization process gave rise to an enzymatically active interface. 

Hydroxy derivatives or phenolics are frequent target for method development using carbon paste electrodes based either on direct oxidation of hydroxy group on an aromatic ring or on enzyme-based reactions with the final determination of the product of the enzymatic reaction involved. The reason lies both in their usual ease of oxidation and in their relatively frequent occurrence as hydroxy derivatives of aromatic hydrocarbons form the base of various biologically active organic compounds used as disinfectants, pharmaceuticals, herbicides, and pesticides. CPE and CPE modified with humic acids were used for the determination of pentachloro-phenol. With modified electrode lower determination limit was reached due to better accumulatiOTi of the analyte. Formation of a quinone-Uke compound during... 

An interesting combination of enzymatic with non-enzymatic transformation in a one-pot three-step multiple sequence was reported by Waldmann and coworkers [82]. Phenols 125 in the presence of oxygen and enzyme tyrosinase are hydroxylated to catechols 126 which are then oxidized in situ to ortho quinones 127. These intermediates subsequently undergo a Diels-Alder reaction with inverse electron demand by reaction with different dienophiles (Table 4.19) to give endo bicyclic 1,2-diketones 128 and 129 in good yields. 

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