Geoscience Reference
In-Depth Information
Fig. 15.5
Oxidative coupling of 2,4 dichlorophenol (Bollag and Liu
1990
)
Fig. 15.6 Aromatic ring cleavage degradation pathways of endocrine disruptor nonylphenol
isomers by Sphingomonas sp. strain TTNP3 (Corvini et al.
2006
)
Epoxidation reactions define the insertion of an oxygen atom into a carbon-
carbon double bond, leading to the formation of a product with toxicity greater
than the parent chemical. Highly toxic organochlorinated pesticides are subject to
epoxidation in the presence of various microorganisms, such as Aspergillus and
P. niger, A. flavus, Penicillium chrysogenum and P. notatum, which catalyze the
reaction (Milles et al.
1969
).
Oxidative coupling involves condensation reactions catalyzed by phenol oxi-
dases. In oxidative coupling of phenol, for example, arloxy or phenolate radicals
are formed by the removal of an electron and a proton from an hydroxyl group.
The herbicide 2,4-D is degraded (Fig.
15.5
) to 2,4 dichlorophenol, which can be
oxidatively coupled by phenol oxidases (Bollag and Liu
1990
).
Aromatic and heterocyclic cleavage involves a hydrocarbon being separated
from an oxygen atom, which functions to link it to another moiety of the molecule;
this process may lead to a decrease in the initial toxicity. The metabolic effects of
microorganisms differ with the molecular configuration of the product, which
affect aromatic or heterocyclic ring cleavage differently.
Aromatic ring cleavage is a microorganism-mediated catabolic process
(Fig.
15.6
). The type of linkage, the specific substituents, their position, and their
number determine the susceptibility of an aromatic ring to fission. Usually, the
substituents must be modified or removed, and an hydroxyl group inserted in an
