The degradation of α-quaternary nonylphenol isomers by Sphingomonas sp. strain TTNP3 involves a type II ipso-substitution mechanism

Abstract

The degradation of radiolabeled 4(3',5'-dimethyl-3'-heptyl)-phenol [nonylphenol (NP)] was tested with resting cells of Sphingomonas sp. strain TTNP3. Concomitantly to the degradation of NP, a metabolite identified as hydroquinone transiently accumulated and short-chain organic acids were then produced at the expense of hydroquinone. Two other radiolabeled isomers of NP, 4(2',6'-dimethyl-2'-heptyl)-phenol and 4(3',6'-dimethyl-3'-heptyl)-phenol, were synthesized. In parallel experiments, the 4(2',6'-dimethyl-2'-heptyl)-phenol was degraded more slowly than the other isomers of NP by strain TTNP3, possibly because of effects of the side-chain structure on the kinetics of degradation. Alkylbenzenediol and alkoxyphenol derivatives identified as metabolites during previous studies were synthesized and tested as substrates. The derivatives were not degraded, which indicated that the mineralization of NP does not proceed via alkoxyphenol as the principal intermediate. The results obtained led to the elucidation of the degradation pathway of NP isomers with a quaternary alpha-carbon. The proposed mechanism is a type II ipso substitution, leading to hydroquinone and nonanol as the main metabolites and to the dead-end metabolites alkylbenzenediol or alkoxyphenol, depending on the substitution at the alpha-carbon of the carbocationic intermediate formed.

Degradation Pathway of Bisphenol A: Does ipso Substitution Apply to Phenols Containing a Quaternary α-Carbon Structure in the para Position?

“…Studies of the degradation of NP have shown that this compound is degraded to CO 2 and that HQ is the intermediate used as the sole source of carbon for growth by Sphingomonas sp. strain TTNP3 (6,10). Further degradation compounds that support the hypothesis of a type II mechanism were identified.…”

“…Insights into the metabolism of NP by Sphingomonas strains have made clear that the ipso substitution mechanism is not restricted to halogen-substituted phenols (5,6,14). These bacteria are able to break highly stable C-C bonds between an aromatic carbon and the quaternary ␣-carbon of alkylphenol.…”

“…strain TTNP3 were prepared on standard I medium as reported previously (10). At the late growth phase, cells were harvested for the preparation of the resting cell suspension as previously described (6). Briefly, cultures were centrifuged and pellets were resuspended in 50 mM phosphate buffer (pH 7.0).…”

“…Investigations were carried out mainly with Sphingomonas sp. strain TTNP3 (6)(7)(8)(9)(10)(11) and S. xenophaga strain Bayram (13,14). In both strains, degradation pathways involve ipso-hydroxylation of NP as the first step (Fig.…”

“…1B) and proceed via an ipso substitution (type II) in Sphingomonas sp. strain TTNP3 (5,6). ipso substitutions are grouped into two types according to the nature of the substituent eliminated from the quinol (27).…”