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1. Computational Investigation of the Thermochemistry of the CO₂ Capture Reaction by Ethylamine, Propylamine, and Butylamine in Aqueous Solution Considering the Full Conformational Space via Boltzmann Statistics

   https://doi.org/10.1021/acs.jpca.1c06294  

   Schell, Joseph; Yang, Kaidi; Glaser, Rainer (November 2021, The Journal of Physical Chemistry A)

   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 ΔG_BD ≈ ΔG_MSS. 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^WA_BD 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^WA_BD = −5.4 kcal/mol. The overall difference is ΔG_BD – Δ^WA_BD = 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. 

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2. 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**

   https://doi.org/10.1002/cphc.202300053  

   Yang, Kaidi; Schell, Joseph; Gallazzi, Fabio; Wycoff, Wei; Glaser, Rainer (April 2023, ChemPhysChem)

   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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3. Nuclear Magnetic Resonance Study of CO ₂ Capture by Fluoroalkylamines: Ammonium Ion p K _a Depression via Fluorine Modification and Thermochemistry of Carbamylation

   https://doi.org/10.1021/acs.joc.3c00701  

   Jameson, Brian; Knobbe, Kari; Glaser, Rainer (August 2023, The Journal of Organic Chemistry)

   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 pK_a(Xa). Two fluorination strategies aimed at ammonium ion pK_a 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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4. Challenges and Opportunities: Metal–Organic Frameworks for Direct Air Capture

   https://doi.org/10.1002/adfm.202307478  

   Bose, Saptasree; Sengupta, Debabrata; Rayder, Thomas_M; Wang, Xiaoliang; Kirlikovali, Kent_O; Sekizkardes, Ali_K; Islamoglu, Timur; Farha, Omar_K (October 2023, Advanced Functional Materials)

   Abstract Global reliance on fossil fuel combustion for energy production has contributed to the rising concentration of atmospheric CO₂, creating significant global climate challenges. In this regard, direct air capture (DAC) of CO₂from the atmosphere has emerged as one of the most promising strategies to counteract the harmful effects on the environment, and the further development and commercialization of this technology will play a pivotal role in achieving the goal of net‐zero emissions by 2050. Among various DAC adsorbents, metal–organic frameworks (MOFs) show great potential due to their high porosity and ability to reversibly adsorb CO₂at low concentrations. However, the adsorption efficiency and cost‐effectiveness of these materials must be improved to be widely deployed as DAC sorbents. To that end, this perspective provides a critical discussion on several types of benchmark MOFs that have demonstrated high CO₂capture capacities, including an assessment of their stability, CO₂capture mechanism, capture‐release cycling behavior, and scale‐up synthesis. It then concludes by highlighting limitations that must be addressed for these MOFs to go from the research laboratory to implementation in DAC devices on a global scale so they can effectively mitigate climate change. 

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5. Metrics for quantifying the efficiency of atmospheric CO ₂ reduction by marine carbon dioxide removal (mCDR)

   https://doi.org/10.1088/1748-9326/ad7477  

   Yamamoto, Kana; DeVries, Tim; Siegel, David_A (September 2024, Environmental Research Letters)

   Abstract Marine carbon dioxide removal (mCDR) is gaining interest as a tool to meet global climate goals. Because the response of the ocean–atmosphere system to mCDR takes years to centuries, modeling is required to assess the impact of mCDR on atmospheric CO₂reduction. Here, we use a coupled ocean–atmosphere model to quantify the atmospheric CO₂reduction in response to a CDR perturbation. We define two metrics to characterize the atmospheric CO₂response to both instantaneous ocean alkalinity enhancement (OAE) and direct air capture (DAC): the cumulative additionality (α) measures the reduction in atmospheric CO₂relative to the magnitude of the CDR perturbation, while the relative efficiency (ϵ) quantifies the cumulative additionality of mCDR relative to that of DAC. For DAC,αis 100% immediately following CDR deployment, but declines to roughly 50% by 100 years post-deployment as the ocean degasses CO₂in response to the removal of carbon from the atmosphere. For instantaneous OAE,αis zero initially and reaches a maximum of 40%–90% several years to decades later, depending on regional CO₂equilibration rates and ocean circulation processes. The global meanϵapproaches 100% after 40 years, showing that instantaneous OAE is nearly as effective as DAC after several decades. However, there are significant geographic variations, withϵapproaching 100% most rapidly in the low latitudes whileϵstays well under 100% for decades to centuries near deep and intermediate water formation sites. These metrics provide a quantitative framework for evaluating sequestration timescales and carbon market valuation that can be applied to any mCDR strategy. 

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