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We are developing energy-efficient and reversible carbon capture and release (CCR) systems that mimic the Lys201 carbamylation reaction in the active site of ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO). The multiequilibria scenario ammonium ion Xa ⇌ amine Xb ⇌ carbamic acid Xc ⇌ carbamate Xd requires the presence of both free amine and CO2 for carbamylation and is affected by the pKa(Xa). Two fluorination strategies aimed at ammonium ion pKa depression and low pH carbamylation were analyzed with (2,2,2-trifluoroethyl)butylamine 2b and 2,2-difluoropropylamine 3b and compared to butylamine 1b. The determination of K1 and ΔG1 of the carbamylation reactions requires the solution of multiequilibria systems of equations based on initial conditions, 1H NMR measurements of carbamylation yields over a wide pH range, and knowledge of K2– K5 values. K2 and K3 describe carbonic acid acidity, and ammonium ion acidities K4 were measured experimentally. We calibrated carbamic acid acidities K5 based on the measured value K6 of aminocarbamic acid using isodesmic reactions. The proton exchange reactions were evaluated with ab initio computations at the APFD/6-311+G* level in combination with continuum solvation models and explicit solvation. The utilities of 1–3 will be discussed as they pertain to the development of fluorine-modified RuBisCO-mimetic reversible CCR systems.
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NMR Study of CO 2 Capture by Butylamine and Oligopeptide KDDE in Aqueous Solution: Capture Efficiency and Gibbs Free Energy of the Capture Reaction as a Function of pH**
Abstract We have been interested in the development of rubisco‐based biomimetic systems for reversible CO2capture from air. Our design of the chemical CO2capture and release (CCR) system is informed by the understanding of the binding of the activator CO2(ACO2) 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 CO2capture reaction. We are studying the structural chemistry and the thermodynamics of CO2capture based on the tetrapeptide CH3CO−KDDE−NH2(“KDDE”) in aqueous solution to develop rubisco mimetic CCR systems. Here, we report the results of1H NMR and13C NMR analyses of CO2capture 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 of1H NMR signals as a function of pH in the range 8≤pH≤11. The determination of ΔG1(R) for the reaction R−NH2+CO2R−NH−COOH requires the solution of a multi‐equilibrium equation system, which accounts for the dissociation constantsK2andK3controlling carbonate and bicarbonate concentrations, the acid dissociation constantK4of the conjugated acid of the amine, and the acid dissociation constantK5of the alkylcarbamic acid. We show how the multi‐equilibrium equation system can be solved with the measurements of the daughter/parent ratioX, the knowledge of the pH values, and the initial concentrations [HCO3−]0and [R‐NH2]0. For the reaction energies of the carbamylations of butylamine and KDDE, our best values are ΔG1(Bu)=−1.57 kcal/mol and ΔG1(KDDE)=−1.17 kcal/mol. Both CO2capture 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.
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- Award ID(s):
- 2153206
- PAR ID:
- 10418420
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemPhysChem
- Volume:
- 24
- Issue:
- 11
- ISSN:
- 1439-4235
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Rubisco is the enzyme responsible for CO2 fixation in nature, and it is activated by CO2 addition to the amine group of its lysine 201 side chain. We are designing rubisco-based biomimetic systems for reversible CO2 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 CO2 capture reaction, we report here the results of computational studies of the thermodynamics of CO2 capture by small alkylamines in aqueous solution. We studied CO2 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 ΔGBD ≈ ΔGMSS. 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 ΔWABD 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 ΔWABD = −5.4 kcal/mol. The overall difference is ΔGBD – ΔWABD = 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.more » « less
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