Phenol, enzymic oxidative polymerization

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Oxidative polymerization of phenol derivatives is also important pathway in vivo, and one example is the formation of melanin from tyrosine catalyzed by the Cu enzyme, tyrosinase. The pathway from tyrosine to melanin is described by Raper (7) and Mason (8) as Scheme 8 the oxygenation of tyrosine to 4-(3,4-dihydro-xyphenyl)-L-alanin (dopa), its subsequent oxidation to dopaqui-none, its oxidative cyclization to dopachrome and succeeding decarboxylation to 5,6-dihydroxyindole, and the oxidative coupling of the products leads to the melanin polymer. The oxidation of dopa to melanin was attempted here by using Cu as the catalyst. 

In order to preserve enzyme activity during the reaction process, special attention must be paid on the substrates that cause direct or indirect enzyme inactivation. Since peroxide is a strong peroxidase inhibitor, a low peroxide/enzyme ratio must be selected. When treating phenolic compounds, the polymeric products obtained from the action of peroxidases also cause enzyme inactivation [9]. If the enzyme is inactivated, not only is the reaction hindered but, sometimes, there is a direct oxidation of the substrate by the peroxide, which causes an enantioselective reduction in some synthetic reactions [10, 11]. In these cases, an appropriate enzyme concentration and usually an adequate enzyme addition strategy are considered [8],... 

Oxidative polymerization of phenols has been carried out extensively by using horseradish peroxidase and other enzymes. However, these interesting topics lie far beyond the scope of this chapter. 

So far, several oxidoreductases, peroxidase, laccase, polyphenol oxidase (tyrosinase), and so on have been reported to catalyze oxidative polymerization of phenol derivatives among which peroxidase is most often used [165,166]. Peroxidase is an enzyme that catalyzes the oxidation of a donor to an oxidized donor by the action of hydrogen peroxide, liberating two water molecules. Horseradish peroxidase (HRP) is a single-chain p-type hemoprotein that catalyzes the decomposition of hydrogen peroxide at the expense of aromatic proton donors. 

Enzymic reactions in SC-CO2 cover oxidations and solvolyses. Good yields (75 % at a residence time of only 13 s) were reported for the enzymic oxidation (immobilized cholesterol oxidase from G. chrysocreas) of cholesterol in supercritical CO2/O2 (9 1) (Scheme 8). Cosolvents, like tert-butyl alcohol, that increase the solubility and, to an even larger extent, those that assist aggregate formation, increase the rate of the reaction (fourfold in this case). [31] However, it appears that this line has not been pursued any further. Horseradish peroxidase was used in the oxidative polymerization of / -cresol by H2O2 in SC-CO2. Cosolvents were useful. The method was evaluated for manufacture of phenolic resins without incorporating formaldehyde. [47]... 

Phenolic polymers and phenol-formaldehyde resins are of great commercial interest for a number of electronic and industrial applications (7). However, there have been serious concerns regarding their use due to various toxic effects of formaldehyde and harsh synthesis environments (2). Peroxidase-catalyzed oxidative polymerization of phenol and substituted phenols provides an alternate route for the synthesis of phenolic polymers (3,4), The increased interest in this type of enzyme-based polymerization is mostly due to its environmental compatibility and potential for producing industrial polymers in high yield (5). 

Enzyme-Catalyzed Polymerization. Horseradish peroxidase (HRP) has been used for the oxidation of a wide variety of compounds, eg, phenol derivatives in the presence of hydroperoxide (44). HRP is a kind of heme glycoprotein that catalyzes the oxidation of phenol to phenoxy radicals. Subsequently, the resulting phenoxy radicals couple each other to oligo or polyphenol derivatives step by step (see Oxidative Polymerization). 

A chemoenzymatic way to produce poly(hydroquinone) was achieved by enzymatic oxidative polymerization of 4-hydroxyphenyl benzoate, followed by alkahne hydrolysis of the resulting polymer [45]. HRP and SBP were used as enzymes. The molecular weight of the resulting poly(4-hydroxyphenyl benzoate) varied between 1100 and 2400 g/mol. The structure was said to consist of phenylene and oxyphenylene moieties, which was found by IR analysis and titration of the residual amount of phenolic groups in the polymer. Other phenol polymers have shown their potential for electronic applications as well. Besides hydroquinone, catechol has also been used as substrate for peroxidase-catalyzed polymerization. The molecular weights of the reac-... 

So far, most peroxidase-catalyzed oxidative polymerizations have been carried out using the enzyme horseradish peroxidase (HRP). Another useful peroxidase that catalyzes the oxidative polymerization of phenols is soybean peroxidase (SBP). While the use of either HRP or SBP may often lead to similar products and results [77], the enzyme activity, yield, and molecular weight of the resulting polymers can also sometimes depend strongly on the type of enzyme used for the polymerization process. For example, SBP was foimd to be superior to HRP for the efficient polymerization of bisphenol A [140], but the polymerization of phenol with SBP afforded... 

During the last ten years, many research results have shown that oxidative polymerization catalyzed by peroxidases is a convenient, resource-saving, and environmentally friendly method for synthesizing phenol polymers. In contrast to the conventional synthesis of phenol-formaldehyde resins, the peroxidase-catalyzed polymerization of phenol proceeds under mild reaction conditions (room temperature, neutral pH). The polymerization of toxic phenols has promising potential for the cleaning of wastewaters. Moreover, the polymerization of phenols from renewable resources is expected to attract much attention in times of worldwide demand for the replacement of petroleum-derived raw materials. Besides the environment-protecting aspects of this innovative type of polymerization, the enzyme-catalyzed polymerization represents a convenient method to reahze new types of functional polyaromatic polymers. Phenol polymers made by peroxidase catalysis should have much potential for electronic and optical apphcations. The synthesis of functional phenol polymers is facihtated by the fact that poly-... 

The first chapter by Reihmann and Ritter reviews the recent developments of peroxidase-catalyzed oxidative polymerization of phenol and derivatives with a phenolic OH group. The importance of enzymatic polymerization in general is emphasized. Properties of product polyphenols, characteristics of the enzyme catalysis, and significance of the process and the product are discussed. The second chapter by Uyama and Kobayashi is concerned with the oxidative polymerization of polyphenols, which are compounds containing more than two phenolic OH groups. These compounds include catechols and flavonoids... 

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