Computational Investigation of the Thermochemistry of the CO Capture Reaction by Ethylamine, Propylamine, and Butylamine in Aqueous Solution Considering the Full Conformational Space via Boltzmann Statistics.

Rubisco is the enzyme responsible for CO fixation in nature, and it is activated by CO addition to the amine group of its lysine 201 side chain. We are designing rubisco-based biomimetic systems for reversible CO capture from ambient air. The oligopeptide biomimetic capture systems are employed in aqueous solution. To provide a solid foundation for the experimental solution-phase studies of the CO capture reaction, we report here the results of computational studies of the thermodynamics of CO capture by small alkylamines in aqueous solution. We studied CO addition to methyl-, ethyl-, propyl-, and butylamine with the consideration of the full conformational space for the amine and the corresponding carbamic acids and with the application of an accurate solvation model for the potential energy surface analyses. The reaction energies of the carbamylation reactions were determined based on just the most stable structures (MSS) and based on the ensemble energies computed with the Boltzmann distribution (BD), and it is found that Δ ≈ Δ. The effect of the proper accounting for the molecular translational entropies in solution with the Wertz approach are much more significant, and the free energy of the capture reactions Δ is more negative by 2.9 kcal/mol. Further accounting for volume effects in solution results in our best estimates for the reaction energies of the carbamylation reactions of Δ = -5.4 kcal/mol. The overall difference is Δ - Δ = 2.4 kcal/mol for butylamine carbamylation. The full conformational space analyses inform about the conformational isomerizations of carbamic acids, and we determined the relevant rotational profiles and their transition-state structures. Our detailed studies emphasize that, more generally, solution-phase reaction energies should be evaluated with the Helmholtz free energy and can be affected substantially by solution effects on translational entropies.