Atom-Condensed Fukui Function in Condensed Phases and Biological Systems and Its Application to Enzymatic Fixation of Carbon Dioxide

Local reactivity descriptors such as atom-condensed Fukui functions are promising computational tools to study chemical reactivity at specific sites within a molecule. Their applications have been mainly focused on isolated molecules in their most stable conformation without considering the effects of the surroundings. Here we propose to combine quantum mechanics/molecular mechanics Born–Oppenheimer molecular dynamics simulations to obtain the microstates (configurations) of a molecular system using different representations of the molecular environment and calculate Boltzmann-weighted atom-condensed local reactivity descriptors based on conceptual density functional theory. Our approach takes the conformational fluctuations of the molecular system and the polarization of its electron density by the environment into account, allowing us to analyze the effect of the molecular environment on reactivity. In this contribution, we apply the method mentioned above to the catalytic fixation of carbon dioxide by crotonyl-CoA carboxylase/reductase and study if the enzyme alters the reactivity of its substrate compared with an aqueous solution. Our main result is that the protein environment activates the substrate by the elimination of solute–solvent hydrogen bonds from aqueous solution in the two elementary steps of the reaction mechanism: the nucleophilic attack of a hydride anion from NADPH on the α,β-unsaturated thioester and the electrophilic attack of carbon dioxide on the formed enolate species.