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Ionic liquids (ILs) are gaining attention as protein stabilizers and refolding additives. However, varying degrees of success with this approach motivates the need to better understand fundamental IL-protein interactions. A combination of experiment and simulation is used to investigate the thermal unfolding of lysozyme in the presence of two imidazolium-based ILs (1-ethyl-3-methylimidazolium ethylsulfate, [EMIM][EtSO 4 ] and 1-ethyl-3-methylimidazolium diethylphosphate, [EMIM][Et 2 PO 4 ]). Both ILs reduce lysozyme melting temperature Tm , but more gradually than strong denaturants. [EMIM][Et 2 PO 4 ] lowers lysozyme Tm more readily than [EMIM][EtSO 4 ], as well as requiring less energy to unfold the protein, as determined by the calorimetric enthalpy ΔH. Intrinsic fluorescence measurements indicate that both ILs bind to tryptophan residues in a dynamic mode, and furthermore, molecular dynamics simulations show a high density of [EMIM] + near lysozyme’s Trp62 residue. For both ILs approximately half of the [EMIM] + cations near Trp62 show perfect alignment of their respective rings. The [EMIM] + cations, having a "local" effect in binding to tryptophan,likely perturb a critically important Arg-Trp-Arg bridge through favorable π − π and cation-π interactions. Simulations show that the anions, [EtSO 4 ] - and [Et 2 PO 4 ] - , interact in a "global" manner with lysozyme, due to this protein’s strong net positive charge. The anions also determine the local distribution of ions surrounding the protein. [Et 2 PO 4 ] - is found to have a closer first coordination shell around the protein and stronger Coulomb interactions with lysozyme than [EtSO 4 ] - , which could explain why the former anion is more destabilizing. Patching of ILs to the protein surface is also observed, suggesting there is no universal IL solvent for proteins, and highlighting the complexity of the IL-protein environment.
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Is the Chemisorbed CO 2 in Ionic Liquid Electrolytes Active for Electrochemical Utilization? A Case Study on Carboxylate and Carbamate Speciation
Abstract This study examines the activity of chemisorbed CO2 species in the microenvironment formed by bifunctional ionic liquids (ILs) in the reactive capture and conversion (RCC) of CO2 to CO on silver. Comparative electroanalytical measurements with imidazolium based ILs were performed to probe the impact of electrostatic interactions, anion and cation basicity, and hydrogen bonding on RCC. Particularly, ILs with 1-ethyl,3-methylimidazolium ([EMIM]+) and 1-ethyl, 2,3-methylimidazolium ([EMMIM]+) cations and aprotic heterocyclic anions of 2-cyanopyrrolide ([2-CNpyr]) and 1,2,4-triazolide ([1,2,4-Triz]) were examined for RCC. It was found that anion–CO2 carbamate complexes facilitate RCC at significantly lower overpotentials compared to cation–CO2 carboxylate complexes. Additionally, [EMIM]+ was found to better stabilize anion–CO2 complexes than [EMMIM]+. Furthermore, it was found that 2-CNpyrH that naturally forms in CO2 absorption competes for electrode surface adsorption with the anion–CO2 carbamate complex, thereby reducing the electrochemical activity of the anion–CO2 complex. These results highlight the importance of IL structure in tuning the interfacial interactions and suggest that ILs with anion-dominated CO2 chemisorption enhances CO2 utilization in RCC applications.
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- Award ID(s):
- 2045111
- PAR ID:
- 10596732
- Publisher / Repository:
- The Electrochemical Society
- Date Published:
- Journal Name:
- Journal of The Electrochemical Society
- ISSN:
- 0013-4651
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Ionic liquids (ILs) exhibit unique properties that have led to their development and widespread use for a variety of applications. Development efforts have generally focused on achieving desired macroscopic properties via tuning of the IL through variation of the cations and anions. Both the macroscopic and microscopic properties of an IL influence its tunability and thus feasibility of use for selected applications. Works geared toward a microscopic understanding of the nature and strength of the intrinsic cation-anion interactions of ILs have been limited to date. Specifically, the intrinsic strength of the cation-anion interactions in ILs is largely unknown. In previous work, we employed threshold collision-induced dissociation (TCID) approaches supported and enhanced by electronic structure calculations to determine the bond dissociation energies (BDEs) and characterize the nature of the cation-anion interactions in a series of four 2:1 clusters of 1-alkyl-3-methylimidazolium cations with the hexafluorophosphate anion, [2C n mim:PF 6 ] + . To examine the effects of the 1-alkyl chain on the structure and energetics of binding, the cation was varied over the series: 1-ethyl-3-methylimidazolium, [C 2 mim] + , 1-butyl-3-methylimidazolium, [C 4 mim] + , 1-hexyl-3-methylimidazolium, [C 6 mim] + , and 1-octyl-3-methylimidazolium, [C 8 mim] + . The variation in the strength of binding among these [2C n mim:PF 6 ] + clusters was found to be similar in magnitude to the average experimental uncertainty in the measurements. To definitively establish an absolute order of binding among these [2C n mim:PF 6 ] + clusters, we extend this work again using TCID and electronic structure theory approaches to include competitive binding studies of three mixed 2:1 clusters of 1-alkyl-3-methylimidazolium cations and the hexafluorophosphate anion, [C n-2 mim:PF 6 :C n mim] + for n = 4, 6, and 8. The absolute BDEs of these mixed [C n-2 mim:PF 6 :C n mim] + clusters as well as the absolute difference in the strength of the intrinsic binding interactions as a function of the cation are determined with significantly improved precision. By combining the thermochemical results of the previous independent and present competitive measurements, the BDEs of the [2C n mim:PF 6 ] + clusters are both more accurately and more precisely determined. Comparisons are made to results for the analogous [2C n mim:BF 4 ] + and [C n-2 mim:PF 6 :C n mim] + clusters previously examined to elucidate the effects of the [PF 6 ] - and [BF 4 ] - anions on the binding.more » « less
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