A Density Functional Theory Study toward Ring-Opening Reaction Mechanisms of 2,4,6-Trinitrotoluene's Meisenheimer Complex for the Biodegradation of Old Yellow Enzyme Flavoprotein Reductase

Abstract

The subsequent degradation pathway of the dihydride-Meisenheimer complex (2H^--TNT), which is the metabolite of 2,4,6-trinitrotoluene (TNT) by old yellow enzyme flavoprotein reductases of yeast and bacteria, was investigated computationally at the SMD/TPSSH/6-311+G(d,p) level of theory. Combining the experimentally detected products, a series of protonation, addition, substitution (dearomatization), and ring-opening reaction processes from 2H^--TNT to alkanes were proposed. By analyzing reaction free energies, we determined that the protonation is more advantageous thermodynamically than the dimerization reaction. In the ring-opening reaction of naphthenic products, the water molecule-mediated proton transfer mechanism plays a key role. The corresponding activation energy barrier is 37.7 kcal·mol^-1, which implies the difficulty of this reaction. Based on our calculations, we gave an optimum pathway for TNT mineralization. Our conclusions agree qualitatively with available experimental results. The details on transition states, intermediates, and free energy surfaces for all proposed reactions are given and make up for a lack of experimental knowledge.

Figure 1

Two major reported pathways for the transformation of dihydride–Meisenheimer complex 2H^––TNT of TNT. The above one (blue) is protonation followed by hydrolysis, and the other (red) is protonation followed by dimerization.

Figure 2

Structures and reaction free energy ΔG_protonation (abbreviated as ΔG_prot) of the protonation reaction for 2H^–-TNT (P0), its seven first-step protonation products (P1–P7, which corresponds to blue hydrogen), and four second-step protonation products (P8–P11, pink hydrogen); the unit of ΔG_protonation is kcal·mol^–1.

Figure 3

Distribution of the Fukui function isosurface on different atoms of 2H^––TNT and its protonated product P1,.

Scheme 1. Fate of Protonation Products of Dihydride Meisenheimer Complex 2H^–-TNT of TNT

Figure 4

Reaction pathway of the conversion from P8 to H1 and the corresponding free-energy profiles. (a) H₂O addition (to the benzene ring) followed by the substitution mechanism for the conversion of P8 to H1 through TS1 and TS2; (b) H₂O addition (to the nitro group) followed by the substitution mechanism for the conversion of P8 to H2 through TS3 and TS4; (c) first substitution followed by the H₂O addition and the proton transfer reaction for the conversion of P8 to H1 through TS5, TS6, and TS7. The distances are given in Angstroms.

Scheme 2. Corresponding Products for Subsequent Ring-Opening Reaction of Experiment-Detected Products H1 and H4

Figure 5

Free-energy profiles, transition states, and products for the ring-opening reaction of H1.The process includes two transition states (TS8 and TS9) and an intermediate (IN3). The distances are given in Angstroms.

Scheme 3. Four Possible Pathways for Dimerization Process of 2H^––TNT

Figure 6

Free-energy profiles for two conversion pathways of the dihydride–Meisenheimer complex (2H^–−TNT). The first pathway includes protonation and dimerization processes (black line).The second pathway includes protonation, hydrolysis reaction, and ring-opening reaction (blue line). The structures of transition states and stationary points can be seen in Figures 4 and 5 and Figure S1 (see the Supporting Information), respectively.