4-Alkylphenols are amphiphilic compounds commonly used as surfactants in many industrial production processes. They can have many functions, such as surfactants in a detergent or emulsifying agents in plastic or latex production. 4-Nonylphenols, an important class of 4-alkylphenols, have an estimated use of 650,000 tons annually (Guenther et al., 2002). This is a major concern as 4-nonylphenols are not easily degraded and their buildup can be toxic to an array of species from aquatic organisms to mammals (Sonnenschein and Soto, 1998). There can be as many as or more than forty isomers in technical grade 4-nonylphenol (Moeder et al., 2006).
Microbes have evolved to biodegrade some of the relatively resilient 4-nonylphenol isomers. The process appears to depend on molecular oxygen (Ekelund et al., 1993), suggesting activation of the compound by incorporation of oxygen-bearing functional groups. To better understand 4-nonylphenol catabolism, one particular isomer, 4-(1-ethyl-1,4-dimethyl-pentylphenol) or NPL, is shown here in detail, with the R stereoisomer as the initial substrate. Both enantiomers are degraded, but not much is currently known regarding stereoselectivity.
Sphingomonas spp. carry out the initial biodegradation of NPL. A major and a minor degradation pathway have been postulated, both ultimately leading to hydroquinone and a nonanol that retains the structure of the original nonyl side chain. Sphingomonas spp. can grow on the aromatic part as a carbon and energy source, but are not capable of further catabolizing the nonyl alcohol, and it is discarded as a non-catabolic byproduct (Gabriel et al., 2005).
Both the minor and major pathways are catalyzed initially by an ipso-hydroxylation, i.e., hydroxylation at the anchor carbon atom of the alkyl substituent, forming 4-nonyl-4-hydroxy-cyclohexadienone (Gabriel et al., 2005). 18O-labeling experiments show that the ipso-hydroxy group is derived from molecular oxygen and that the major pathway for cleavage of the alkyl moiety is through cleaving the nonyl chain as alkyl cation (stabilized by the α-quaternary C atom of the nonyl group) and combination of the cation with a molecule of water to yield the nonyl alcohol and hydroquinone. In the minor pathway (not shown), a 1,2-C,O shift of the alkyl moiety is postulated to yield a 4-alkoxyphenol intermediate. A second ipso-hydroxylation leads to a hemiketal, which then spontaneously decomposes to p-benzoquinone and the nonyl alcohol. p-Quinone is quickly converted to hydroquinone by a reducing system (Gabriel et al., 2007).
The following is a text-format 4-(1-ethyl-1,4-dimethyl-pentyl)phenol pathway map. Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions. This map is also available in graphic (3K) form.
4-(1-Ethyl-1,4-dimethyl-pentyl)phenol Sphingomonas sp. Sphingomonas xenophaga Sphingomonas xenophagum Sphingomonas xenophagum 6383 Sphingomonas xenophagum 6985 Sphingomonas xenophagum 8566 Sphingomonas xenophagum 14677 Sphingomonas TTNP3 Sphingobium cloacae | | | 4-(1-ethyl-1,4-dimethyl- | pentyl)phenol | monooxygenase | | v Hydroquinone + 3,6-Dimethylheptan-3-ol | | | v to the 4-Nitrophenol Pathway
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