TAILIEUCHUNG - Báo cáo khoa học: First evidence of catalytic mediation by phenolic compounds in the laccase-induced oxidation of lignin models

The sulfonephthalein indicator, phenol red, exhibits an unusually slow rate of oxidation by laccase fromPolipo-rus pinsitus, in spite of the fact that it is a phenol and therefore a natural substrate for this phenoloxidase enzyme. Nevertheless, after prolonged exposure to laccase (24 h) phenol red is oxidized by more than 90%. We found that phenol red, which can be oxidatively converted into a reso-nance-stabilized phenoxy radical, performs as amediator in the laccase-catalyzed oxidation of a nonphenolic substrate (4-methoxybenzyl alcohol) and also of a hindered phenol (2,4,6-tri-tert-butylphenol). . | Eur. J. Biochem. 270 3634-3640 2003 FEBS 2003 doi First evidence of catalytic mediation by phenolic compounds in the laccase-induced oxidation of lignin models Francesca d Acunzo and Carlo Galli Dipartimento di Chimica Universitci La Sapienza Roma Italy IMC-CNR Sezione Meccanismi di Reazione The sulfonephthalein indicator phenol red exhibits an unusually slow rate of oxidation by laccase from Polipo-rus pinsitus in spite of the fact that it is a phenol and therefore a natural substrate for this phenoloxidase enzyme. Nevertheless after prolonged exposure to laccase 24 h phenol red is oxidized by more than 90 . We found that phenol red which can be oxidatively converted into a resonance-stabilized phenoxy radical performs as a mediator in the laccase-catalyzed oxidation of a nonphenolic substrate 4-methoxybenzyl alcohol and also of a hindered phenol 2 4 6-tri-tert-butylphenol . In particular phenol red was found to be at least 10 times more efficient than 3-hydr-oxyanthranilate a reported natural phenolic mediator of laccase in the oxidation of 4-methoxybenzyl alcohol. Other phenols which do not bear structural analogies to phenol red underwent rapid degradation and did not perform as laccase mediators. On the other hand several variously substituted sulfonephthaleins of different pK2 values mediated the laccase catalysis the most efficient being dichlorophenol red which has the lowest pK2 of the series. The mediating efficiency of phenol red and dichlorophenol red was found to be pH dependent as was their oxidation Ep value determined by cyclic voltammetry . We argue that the relative abundance of the phenoxy anion which is easier to oxidize than the protonated phenol may be one of the factors determining the efficiency of a phenolic mediator together with its ability to form relatively stable oxidized intermediates that react with the desired substrate before being depleted in undesired routes. Keywords laccase phenolic mediators .

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