The fumarate addition reaction mechanism is central to the anaerobic biodegradation of several hydrocarbons, both aromatic (e.g., toluene, ethyl benzene) and aliphatic (e.g., <i>n-</i>hexane, dodecane) by bacterial cultures. Succinate synthase enzymes, which belong to the glycyl radical enzyme family, are the main facilitators of the fumarate addition mechanism. The catalytic mechanism that converts hydrocarbons into succinates for subsequent metabolism, involves three steps: 1) initial H-abstraction from the hydrocarbon by the radical enzyme, 2) addition of the resulting hydrocarbon radical to fumarate, and 3) hydrogen abstraction by the addition product to regenerate the radical enzyme. Since the biodegradation of hydrocarbon fuels via the fumarate addition mechanism has been linked to bio-corrosion, an improved understanding of this reaction is imperative to our efforts of predicting the susceptibility of proposed alternative fuels to biodegradation. An enhanced knowledge of the fuel biodegradation process also has the potential to develop new bioremediation strategies. Gas-phase electronic structure calculations on model aromatic (toluene) and aliphatic (butane) compounds have been used to evaluate the impact of hydrocarbon structure on the energetics and kinetics of the fumarate addition mechanism. The results indicate that the rate of toluene degradation is ~100 times faster than butane at 298K. It is also found that the first abstraction step is kinetically significant for both hydrocarbons and is consistent with deuterium isotope effect studies on toluene degradation. The detailed computations also show that kinetics play an important role in the predicted stereo-chemical preference of the succinate products for both toluene and butane. The computational and kinetic modeling work presented here also illustrates the importance of considering pre-reaction and product complexes in order to accurately treat gas phase systems that involve intra and inter-molecular non-covalent interactions.

Hydrocarbon structure impacts on the fumarate addition mechanism: Perspectives on the biodegradation susceptibility of fuels from electronic structure calculations<br />

249th National Meeting (2015)

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