Characterisation of coupling products formed by biotransformation of biphenyl and diphenyl ether by the white rot fungus
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O R I G I N A L PA P E R
Ulrike Jonas · Elke Hammer · Erhard T. K. Haupt · Frieder Schauer
Characterisation of coupling products formed by biotransformation of biphenyl and diphenyl ether by the white rot fungus Pycnoporus cinnabarinus Received: 10 April 2000 / Revised: 4 September 2000 / Accepted: 8 September 2000 / Published online: 3 November 2000 © Springer-Verlag 2000
Abstract Cells of the white rot fungus Pycnoporus cinnabarinus grown in glucose were able to hydroxylate biphenyl and diphenyl ether, although growth was inhibited by these substrates at concentrations above 250 µM. 2- and 4-Hydroxybiphenyl were detected as products of biphenyl metabolism and 2- and 4-hydroxydiphenyl ether as products of diphenyl ether metabolism in the culture media. After addition of 2-hydroxydiphenyl ether and 2-hydroxybiphenyl to cell-free supernatants containing laccase as the only ligninolytic enzyme, different coloured precipitates were formed. HPLC analysis revealed the formation of additional hydrophobic metabolites with one major product per transformation. Mass spectrometric analysis of the methyl derivatives of the polymer mixture indicated dimers and trimers with different binding types. The main products were identified as dimers with carboncarbon bonds in para-position to the hydroxyl group of the monomers by mass spectroscopy and nuclear magnetic resonance spectroscopy. Keywords Diphenyl ether · Biphenyl · Laccase · Pycnoporus cinnabarinus · White rot fungi · Coupling
Introduction Halogenated derivatives of biphenyl and diphenyl ether are very toxic substances. They are widespread in industrial applications as heat transfer fluids or flame retardants and serve as the basic structure for many herbicides, such as Nitrofen (2,4-dichlorophenyl-4′-nitrophenylether) and Bifenox (5-(2,4-dichlorophenoxy)-2-nitrobenzoic acid).
These compounds show similarity to some structures in lignin, a very recalcitrant wood component. Since white rot fungi are able to mineralise lignin, they were also used for the degradation of these aromatic compounds. Zeddel et al. (1993) described a degradation rate of 95% for chlorinated biphenyls containing up to four chlorine ions by the white rot fungus Trametes versicolor. However, the mineralisation rate achieved by several white rot fungi was not above 10% (Thomas et al. 1992; Dietrich et al. 1995; Vyas et al. 1994; Beaudette et al. 1998). This difference could be explained by the formation of products that are not easy to analyse. One possible fate of xenobiotics in soil during incubation with white rot fungi is that they are bound to soil by laccases or peroxidases (Mahro and Kästner 1993; Bollag 1992a; Bollag and Bollag 1990). These enzymes are able to oligomerise a number of phenol compounds (Bollag 1992a; Dec and Bollag 1990; Jonas et al. 1998). Although many types of reactions have been described in recent years, the role of laccase in lignin biodegradation is still confusing. However it seems that one function of laccase is to detoxify low molecular weight phenols released during
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