Transformations of six isomers of dimethylbenzothiophene by three Pseudomonas strains

Abstract

Dimethylbenzothiophenes are among the sulfur heterocycles in petroleum that are known to be degraded by microbial activity. Six of the 15 possible isomers of dimethylbenzothiophene were synthesized and used in bio-transformation studies with three Pseudomonas isolates that oxidize a variety of condensed thiophenes including methylbenzothiophenes and methyldibenzothiophenes. The isomers of dimethylbenzothiophene were chosen to have a variety of substitution patterns: both methyl groups on the thiophene ring (the 2,3-isomer); a methyl group on each of the rings (the 2,7-, 3,5-and 3,7-isomers); and both methyl groups on the benzene ring (the 4,6-and 4,7-isomers). Each isolate was grown on 1-methylnaphthalene or glucose in the presence of one of the dimethylbenzothiophenes and culture extracts were analyzed to identify nearly 30 sulfur-containing metabolites in total. Sulfoxides and sulfones were commonly found metabolites in culture extracts from the 2,3-, 2,7- and 3,7-isomers, whereas 2,3-diones, 3(2H)-ones and 2(3H)-ones were formed from the 4,6- and 4,7-isomers. High-molecular-weight products, some of which were tentatively identified as tetramethylbenzo[b]naphtho[1,2-d]thiophenes, were detected in the extracts of cultures incubated with 4,6- or 4,7-dimethylbenzothiophene. The methyl groups of all of the isomers, except 4,6-, were oxidized to give hydroxymethyl-methylbenzothiophenes and methylbenzothiophene-carboxylic acids, and these were the only products detected from the oxidation of 3,5-dimethylbenzothiophene.

Bacterial transformations of naphthothiophenes

“…Isolate W1 is known to oxidize the methyl groups of some methyl-and dimethylbenzothiophenes to methanols and carboxylic acids (35,37) and of some methyldibenzothiophenes to methanols (49). However, in this study, there was no evidence of oxidation of the methyl group of 1-methylnaphtho [2,1-b] thiophene, although some of the minor sulfur-containing metabolites that were not identified may have been a methanol or carboxylic acid.…”

“…The bacterial metabolism of benzothiophene and of methyland dimethyl-substituted benzothiophenes that do not bear methyl groups on the thiophene ring has been studied (14,16,35,37,48). Those investigations showed that the thiophene ring is oxidized and cleaved to give 2-mercaptophenylglyox-alates, which are often detected as the benzothiophene-2,3diones formed by acid-catalyzed dehydration during culture extraction (14).…”

“…Thus, it is not surprising that most studies of the microbial metabolism of organosulfur compounds have focused on the condensed thiophenes that are found in the aromatic fraction of petroleum, especially those which are commercially available, namely, benzothiophene (5,14,16,21,35,50), 3-methylbenzothiophene (16,35), and dibenzothiophene (5, 9, 25-27, 29, 31, 32, 34, 38, 42, 43, 46, 54, 56). It is also not surprising that only a few studies have been done with alkyl-substituted benzothiophenes (35)(36)(37)48) and dibenzothiophenes (33,40,45,49), because these compounds are not commercially available and must be chemically synthesized before they can be used in biodegradation studies.…”

Identification of Disulfides from the Biodegradation of Dibenzothiophene

“…Using the procedures routinely employed in this laboratory (16,28,29,33,44,45), which included acidifying the medium to approximately pH 2, extracting with dichloromethane, drying the extract with anhydrous sodium sulfate, concentrating the extract, derivatizing with diazomethane, and analyzing by GC-MS with upper oven temperature of 250°C, failed to show any peaks other than benzothiophene-2,3-dione.…”

“…Methylbenzothiophene-2,3-diones have been identified in acidified extracts from cultures degrading methylbenzothiophenes (31,44), methyldibenzothiophenes (45), and dimethyldibenzothiophenes (29,37). Similarly, dimethylbenzothiophene-2,3-diones have been identified in acidified extracts from cultures degrading some dimethylbenzothiophenes (33). Thus, at neutral pH, a wide range of methyl-and dimethylsubstituted 2-mercaptophenylglyoxylates would presumably be produced while petroleum undergoes biodegradation.…”