Oxidation of carbazole, N-ethylcarbazole, fluorene, and dibenzothiophene by the laccase of Coriolopsis gallica

Purified laccase from Coriolopsis gallica UAMH8260 oxidized carbazole, N-ethylcarbazole, fluorene, and dibenzothiophene in the presence of 1-hydroxybenzotriazole and 2,2'-azinobis (3-ethylbenzthiazoline-6-sulfonic acid) as free radical mediators. Susceptibility to laccase oxidation appears related to the ionization potential (IP) of the substrate: compounds with an IP above 8.52, dibenzofuran (IP = 8.77) and benzothiophene (IP = 8.73) were not attacked. Carbazole (IP = 7.68) was the most sensitive to oxidation with > 99% transformed with 10 milliunits of laccase after 1 h, though most reactions were carried out for 18 h. 9-Fluorenone was identified as the product of fluorene (IP = 8.52) oxidation, and dibenzothiophene sulfone from dibenzothiophene (IP = 8.44). Although carbazole and N-ethylcarbazole were both completely removed within 18 h, no oxidation or condensation metabolites were detected. This investigation is the first to report the oxidation of dibenzothiophene, carbazole, and N-ethylcarbazole by laccase.