(54f) Understanding Thermodynamics of the Cytochrome P450 Enzymatic Cycle for Ionic Liquid Biodegradation

Ionic liquids (ILs) have been considered highly efficient solvents, co-solvents, or agents for applications in the chemical industry. They present numerous advantages over traditional solvents: low volatility, high thermal and chemical stability, tunable properties, etc. These unique properties enable them to be used in industrial processes such as ISOALKYL process for alkylation, mercury removal, battery electrolytes, lignocellulose dissolution, gas absorption, drug delivery, and organic catalysis, etc. Though ILs are termed as "green solvents" their persistence in the environment could be a potential problem. Experimental investigations have shown that ionic liquids can be recalcitrant, partially biodegradable or fully biodegradable. The potential for biodegradation has been shown to be higher when the alkyl chain contains six or more carbon atoms. Various studies have hypothesized that iron porphyrin, an active site of bacterial cytochrome P450 protein, is responsible for the oxidative transformation of the alkyl side chain which initiates the ionic liquid biodegradation. The hydroxylation reaction by iron porphyrin occurs via a number of intermediate steps. This work presents a detailed thermodynamic analysis for each of the intermediate steps in the protein medium by adopting quantum mechanical calculations for 1-n-alkyl-3-methylimidazolium chloride (n = 2, 4, 6, 8, and 10) to understand the chain length-dependent biodegradation. Equilibrium geometries for each of the intermediate steps will be presented. Important geometrical criteria such as imidazolium ring position, distance between imidazolium ring and iron porphyrin ring, and cation-anion distances in protein medium for each intermediate steps will be discussed. Rate-limiting steps for ionic liquid hydroxylation reaction by iron porphyrin will be identified. This work also sheds light on the oxygen binding mechanism in the presence of ILs into the binding pocket. We will discuss the evolution of byproduct water to understand the regeneration of the initial state of the catalyst.