Picric acid (2,4,6-Trinitrophenol), a derivative of phenol, is a yellow solid. Picric acid is mainly used in manufacture of explosives and as an intermediate in dye manufacturing. Both 2,4,6-trinitrophenol and 2,4-dinitrophenol are found in the waste streams released into the environment by aniline production plants.
The three nitro groups of picric acid create a highly positive redox potential that is susceptible to reductive rather than oxidative processes. An initial reductive attack on the aromatic ring converts picric acid to the hydride-TNP Meisenheimer complex. In Rhodococcus (opacus) erythropolis HL PM-1 (Lenke et al., 1992), Nocardioides sp. strain CB 22-2 (Behrend and Heesche-Wagner, 1999) and Nocardioides simplex (formerly Arthrobacter) FJ2-1A, (Ebert et al., 1999) elimination of nitrite is thought to proceed through this intermediate. However, Rieger et al. (1999) states this elimination requires an unlikely C-3 to C-2 hydrogen migration. Studies show that C-2 protonation of the hydride species occurs readily at physiological pH and suggests the protonated hydride complex is the reactive intermediate for nitrite elimination to 2-dinitrophenol. Reduction of the aromatic nucleus in 2,4-DNP forms the hydride-DNP Meisenheimer complex.
A second pathway involves a two component enzyme system which catalyzes two hydration reactions of picric acid to form a dihydride Meisenheimer-complex (Heiss et al. 2002). The nitro form of this dihydride complex remains as a dead-end metabolite; the aci form undergoes elimination of nitrite, after which the two metabolic pathways of picric acid merge (Ebert et al., 2001). The enzyme catalyzing denitration of the dihydride Meisenheimer-complex of picric acid to form the hydride Meisenheimer-complex of 2,4-dinitrophenol and also the hydrolase enzyme for subsequent reaction have been recently purified (Hofmann et al., 2004). It has been known for more than a decade that 4,6-dinitrohexanoate undergoes further mineralization, with a minor amount remaining as a dead-end metabolite (Lenke et al., 1992). However the further metabolites have not yet been identified.
The following is a text-format Picric Acid 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 (16k) format.
2,4,6-Trinitrophenol (Picric Acid) Nocardioides simplex FJ2-1A Rhodococcus erythropolis HL PM-1 Nocardioides sp. strain CB22-2 | | | hydride | transferase II | v TNP hydride Meisenheimer complex ----------------------+ | | | | | hydride | TNP hydride | transferase I | denitrase | | dihydride TNP | | tautomerase v v TNP dihydride <-----------> TNP dihydride 2,4-Dinitrophenol Meisenheimer complex Meisenheimer complex | (nitro form) (aci form) | | | | | hydride | TNP dihydride | transferase II | denitrase | | | v | 2,4-DNP hydride <------------------------+ Meisenheimer complex | | | hydride | transferase I | v 2,4-DNP Dihydride Meisenheimer complex | | | spontaneous | | v 2,4-Dinitrocyclohexanone | | | 2,4-dinitrocyclo- | hexanone hydrolase | v 4,6-Dinitrohexanoate | | v A | | v Carbon dioxide
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