Phenols/polyphenols reactions during

Polymer Modification by Oxidoreductases. Tyrosinase (polyphenol oxidase, a copper-containing monooxygenation enzyme) was used as catalyst for modification of chitosan. The enzymatic treatment of chitosan film in the presence of tyrosinase and phenol derivatives produced a new material of chitosan derivative (309). During the reaction, imstable o-quinones were formed, followed by the reaction with chitosan to give the modified chitosan. In the enzymatic treatment of p-cresol with a low concentration of chitosan (<1%), the reaction solution was converted into a gel (310). 

The substantial drawback of BPA synthesis from phenol and acetone is the formation of more than 30 various b3q>roducts in the side reactions occurring during the process (Figure 7.9) [53,54]. These are mainly the isomers of BPA, i.e., the o,p-isomers of BPA, the Dianin s compound, chromans, trisphenols and polyphenols as shown in Figure 7.10. 

Tyrosinase is an enzyme complex (phenolase, polyphenol oxidase are other names which have been used for this enzyme), which catalyses of the ortho hydroxylation of monohydric phenols. The enzyme, which should not be confused with L-tyrosine hydroxylase mentioned above, contains Cu (I) and catalyses two distinct reactions—the hydroxylation of monohydric phenols to o-diphenols (cresolase activity) and the oxidation of o-diphenols to o-quinones (catecholase or catechol oxidase activity) . Most enzymes of this type, which are widely distributed in both the plant and animal kingdoms, exhibit both cataljrtic functions. Thus typically, the conversion of L-tyrosine (5) to L-dopa (15) and dopaquinone (36) which occurs in melanin biosynthesis is catalysed by an enzyme of the tyrosinase category. The two activities appear, in the majority of cases, to be functions of the same enzyme. However, certain o-diphenol oxidases such as those from tea , sweet potato and tobacco have been reported to show no capacity to catalyse the hydroxylation reaction but this is most probably due to destruction of the cresolase activity during purification. 

In 1953, Kenten and Mann, during their survey of various donors active in the oxidatic reaction (see Section III), showed that p-cresol promotes the oxida tion of dihydroxyfumarate. Kenten later (1955) demonstrated that in the case of indoleacetic acid, some phenolic compounds (monophenols, resorcinol, etc.) are stimulatory while others are inhibitory (most polyphenols) despite the fact that all were substrates for peroxidase. Similar results have been obtained for pyridine nucleotides (Akazawa and Conn, 1958). Yamazaki analyzed the behavior of phenols and other hydrogen donors effective in the peroxidatic activity of the enzyme, and has shown that they fall into two classes, which he terms redogenic and oxidogenic. Table IV gives a list of the donors... 

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