Laccase-Initiated Polymerization

The influence of [02], [laccase] on the performance of the laccase-initiated polymerization of vinyl monomers is essentially the same as observed with peroxidases (6.3.2) [69], Thus, the polymer characteristics are inversely correlated with [laccase] and [monomer], [Acac] exceeds only in minor effect on the polymer weight with the exception of very low [Acac] [69], Here, no polymerization was observed unless the [02] was reduced significantly. This indicates an 02-dependent irreversible inactivation mechanism of the Acac radical. The mechanism of this inactivation however remains to be elucidated. On the other hand, [02] may be an efficient handle to control the average molecular weight of the resulting... 

The authors reported the generation of polymers in the presence of a large excess of oxidant (monomer oxidant 1.6 1.0 vol/vol) without any information on the mechanism of polymerization, polymer molecular weight, or role and influence of oxidant on the enzyme or reaction kinetics. Subsequently, the laccase-catalyzed polymerization of acrylamide in water without any initiator (b-diketone) at temperatures ranging from 50 to 80 °C was reported however, the reactions were efficiently carried out at room temperature with the use of laccase/b-diketone (2,4-pentanedione) initiator to produce polyacrylamide in 97% yield (Mn = 2.3 x 10 ) [24,25]. The molecular weight of the polymer in these studies was determined by size exclusion chromatography. At 50, 65, and 80 °C when the polymerization was carried out for 4h, polyacrylamides with M = 9.4,9.2, and 10 x 10, respectively, were produced with the highest yield of 70% at 65 °C. 

Kobayashi and coworkers reported on the mediator-free direct polymerization of acryl amide using the laccase from Pycnoporus coccineus suggesting a simplified polymerization scheme [67]. Using the laccase from Myceliophthora ther-mophilia, this was not achieved [68,69] and a follow-up study validating Kobayashi s initial study is still awaited. 

Some oxidoreductases have been reported to induce polymerization of vinyl monomers. HRP, xanthin oxidase, alcohol oxidase, chloroperoxidase, and laccase could catalyze the pol5merization of acrylamide and hydroxyethyl methacrylate (331,332). Glucose oxidase mediated the initiation of vinyl polymerization in the presence of Fe + and dissolved oxygen (333). 

On the basis of the observed products and literatures on the oxidation of phenolic conqpounds by laccase, the reaction scheme for 8-hydroxybentzon oxidation by laccase is proposed as shown in Fig.5. Initially the phenol moiety of 8-hydroxybentazon is oxidized by laccase and formed phenoxy radicals. Further reactions of these phenoxy radicals are cross coupled with each other and formed dimeric products. Ketone moiety of 8-hydroxybentazon-derived dimers are converted to enol in acidic condition. The coupling reactions most likely occur via C-C coupling at para positions because of their electronic (resonance and inductive) and satiric effects. Proposed reaction pathway leading to the formation of co-polymerization product appears to be initiated by die coupling of two catechol radicals, and subsequent oxidation of that dimmer, resulting in catechol-derived benzoquinone dimer. It appears that a tetrameric copolymerization product is formed by the non-enzymatic addition of two 8-hydroxybentazon to a catechol-derived benzoquinone dimer (Fig. 5). 

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