NMR Study of CO₂ Capture by Butylamine and Oligopeptide KDDE in Aqueous Solution: Capture Efficiency and Gibbs Free Energy of the Capture Reaction as a Function of pH

We have been interested in the development of rubisco-based biomimetic systems for reversible CO2 capture from air. Our design of the chemical CO2 capture and release (CCR) system is informed by the understanding of the binding of the activator CO2 ((CO2)-C-A) in rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase). The active site consists of the tetrapeptide sequence Lys-Asp-Asp-Glu (or KDDE) and the Lys sidechain amine is responsible for the CO2 capture reaction. We are studying the structural chemistry and the thermodynamics of CO2 capture based on the tetrapeptide CH3CO-KDDE-NH2 ("KDDE") in aqueous solution to develop rubisco mimetic CCR systems. Here, we report the results of H-1 NMR and C-13 NMR analyses of CO2 capture by butylamine and by KDDE. The carbamylation of butylamine was studied to develop the NMR method and with the protocol established, we were able to quantify the oligopeptide carbamylation at much lower concentration. We performed a pH profile in the multi equilibrium system and measured amine species and carbamic acid/carbamate species by the integration of H-1 NMR signals as a function of pH in the range 8 <= pH <= 11. The determination of Delta G(1)(R) for the reaction R-NH2+CO2 <--> ${ \mathbin{{\stackrel{\textstyle\rightarrow} { {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}} } }} }$ R-NH-COOH requires the solution of a multi-equilibrium equation system, which accounts for the dissociation constants K-2 and K-3 controlling carbonate and bicarbonate concentrations, the acid dissociation constant K-4 of the conjugated acid of the amine, and the acid dissociation constant K-5 of the alkylcarbamic acid. We show how the multi-equilibrium equation system can be solved with the measurements of the daughter/parent ratio X, the knowledge of the pH values, and the initial concentrations [HCO3-](0) and [R-NH2](0). For the reaction energies of the carbamylations of butylamine and KDDE, our best values are Delta G(1)(Bu)=-1.57 kcal/mol and Delta G(1)(KDDE)=-1.17 kcal/mol. Both CO2 capture reactions are modestly exergonic and thereby ensure reversibility in an energy-efficient manner. These results validate the hypothesis that KDDE-type oligopeptides may serve as reversible CCR systems in aqueous solution and guide designs for their improvement.