Solvation Induction of Free Energy Barriers of Decarboxylation Reactions in Aqueous Solution from Dual-Level QM/MM Simulations.

Carbon dioxide capture, corresponding to the recombination process of decarboxylation reactions of organic acids, is typically barrierless in the gas phase and has a relatively low barrier in aprotic solvents. However, these processes often encounter significant solvent-reorganization-induced barriers in aqueous solution if the decarboxylation product is not immediately protonated. Both the intrinsic stereoelectronic effects and solute-solvent interactions play critical roles in determining the overall decarboxylation equilibrium and free energy barrier. An understanding of the interplay of these factors is important for designing novel materials applied to greenhouse gas capture and storage as well as for unraveling the catalytic mechanisms of a range of carboxy lyases in biological CO production. A range of decarboxylation reactions of organic acids with rates spanning nearly 30 orders of magnitude have been examined through dual-level combined quantum mechanical and molecular mechanical simulations to help elucidate the origin of solvation-induced free energy barriers for decarboxylation and the reverse carboxylation reactions in water.