9,10-Phenanthrenedione biodegradation by a soil bacterium and identification of transformation products by LC/ESI-MS/MS

•A soil bacterium transformed cytotoxic 9,10-phenanthrenedione.•Three oxidized ring cleavage products were revealed by LC/ESI(-)-MS/MS.•One product was confirmed to be 2,2′-diphenic acid by LC/ESI(-)-MS/MS methods.•The first pathway for bacterial 9,10-phenanthrenedione transformation was proposed.

Transformation of 9,10-phenanthrenedione, a cytotoxic derivative of phenanthrene, was shown to occur by a soil bacterium belonging to the genus Sphingobium. Phenanthrene-grown cells of this strain were exposed to 50 mg L−1 9,10-phenanthrenedione in liquid cultures, extracted, and extracts were analyzed by liquid chromatography electrospray ionization mass spectrometry in negative ionization mode. Full scan analyses of exposed cells over the range from m/z 50 to m/z 500 were compared to abiotic and biotic controls. Product and precursor ion scan mode analyses indicated that at least three aromatic ring-cleavage transformation products of 9,10-phenanthrenedione were present and structures for these products, corresponding to [M−H]− = 271, [M−H]− = 241, and [M−H]− = 339 were proposed to be 4-(1-hydroxy-3,4-dioxo-2-naphthyl)-2-oxo-but-3-enoic acid, 2,2′-diphenic acid and 2-[(6-carboxy-2,3-dihydroxy-phenyl)-hydroxy-methyl]-5-oxo-hex-3-enedioic acid. The identity of 2,2′-diphenic acid was confirmed by comparison to an authentic standard and when the strain was exposed to 50 mg L−1 2,2′-diphenic acid in separate assays, a transformation product with a similar mass spectrum as 9,10-phenanthrenedione-derived [M−H]− = 339 was revealed. Based upon these results, pathways for the transformation of 9,10-phenanthrenedione by strain KK22 were proposed. Strain KK22 appeared unable to use 9,10-phenanthrenedione as a growth substrate under these conditions. This is the first report of potential biotransformation pathways of 9,10-phenanthrenedione by a bacterium.