Comparison of the Correlation of MEP and the Free Energies of Reaction with Nitro Group Reduction of Nitrotoluene Congeners

Abstract

The nitro group is a strong electrophile that is amenable to reduction by both aerobic and anaerobic microorganisms. The rate and position of nitro group reduction depend on the parent compound's structure. Microbial experiments were conducted to determine relative rates of nitro group reduction and compared with thermodynamic analyses and calculated molecular electrostatic potential values. Rates of nitro group reduction were evaluated in anaerobic batch bioassays using the microbial isolate OSU-G.8 for 2,4,6-trinitrotoluene (246TNT), 2,4-dinitrotoluene (24DNT), 2,6-dinitrotoluene (26DNT), 4-nitrotoluene (4NT), and 2-nitrotoluene (2NT). No ortho reductions were observed, and rates for para reduction increased with nitro group substitution. For the reduction of a nitrotoluene to an aminotoluene, the free energy for the reaction suggests preferential reduction at the ortho position. Reduction at this position is inconsistent with the pathway that is commonly observed. The molecular electrostatic potential (MEP) was calculated for each nitrotoluene congener; the MEP is a measure of the magnitude of partial charges on a molecule. A correlation between MEP and nitrotoluene biotransformation rates was developed. Using the developed correlation, nitro group reduction rates were predicted for other nitrotoluene congeners. The nitro group reduction rate is expected to decrease in the following order 23DNT > 25DNT > 24DNT > 35DNT > 34DNT > 26DNT. For the trinitrotoluene congeners, the reduction rate is expected to decrease as follows: 234TNT > 345TNT > 236TNT > 235TNT > 246TNT > 245TNT.