Adsorption isotherms are reported for Na−mordenite and H−mordenite at several temperatures with a series of gas adsorptives above their critical temperature. The data sets are analyzed with the multiple equilibrium analysis (MEA) method [Drago, R. S.; et al. J. Am. Chem. Soc. 1998, 120, 538−547. Drago, R. S.; et al. J. Phys. Chem. B 1997, 101, 7548−7555], which produces equilibrium constants (K i ), capacities (n i ), and thermodynamic parameters (enthalpies, ΔH i , and entropies, ΔS i ) of adsorption for each process. The limited pore size distribution present in the zeolite mordenite presents an interesting comparison to the amorphous carbons studied previously by MEA [Drago, R. S.; et al. J. Phys. Chem. B 1997, 101, 7548−7555]. The results of the MEA description of the adsorption data gathered for the interaction of an adsorbate (particularly, N2, CO, and Xe) with Na−mordenite and H−mordenite are compared to other literature reports (including infrared spectroscopic studies and Monte Carlo simulations), and good agreement is found. In general, for adsorbates that can access the small channel (small adsorbates), three processes are required to describe adsorption. Two processes are required to describe adsorption for the larger adsorbates into the large (main) channel. The smaller total micropore volumes of Na− and H−mordenite for these adsorptives result in decreased capacity compared to that of the amorphous carbons. The process capacities from MEA (mol g-1) are converted to pore volumes using the calculated molar volume of the adsorbate, and the accessible surface area for a given process is converted with the excluded molecular area of the adsorbate. The results show that MEA provides a more detailed and accurate assessment of the interaction of admolecules with microporous solids, which addresses a matter of fundamental importance to researchers and practitionersthe interactions between gas-phase molecules and a surface of a condensed phase. This analysis leads to an increased understanding of this behavior in gas adsorption and catalysis.
This paper presents an exploratory study of the binding interactions of xenon with the surface of several different proteins in the solution and solid states using both conventional and hyperpolarized 129 Xe NMR. The generation of hyperpolarized 129 Xe by spin exchange optical pumping affords an enhancement by 3-4 orders of magnitude of its NMR signal. As a result, it is possible to observe Xe directly bound to the surface of micromolar quantities of lyophilized protein.The highly sensitive nature of the 129 Xe line shape and chemical shift are used as indicators for the conditions most likely to yield maximal dipolar contact between 129 Xe nuclei and nuclear spins situated on the protein. This is an intermediate step toward achieving the ultimate goal of NMR enhancement of the binding-site nuclei by polarization transfer from hyperpolarized 129 Xe. The hyperpolarized 129 Xe spectra resulting from exposure of four different proteins in the lyophilized, powdered form have been examined for evidence of binding. Each of the proteins, namely, metmyoglobin, methemoglobin, hen egg white lysozyme, and soybean lipoxygenase, yielded a distinctly different NMR line shape. With the exception of lysozyme, the proteins all possess a paramagnetic iron center which can be expected to rapidly relax the 129 Xe and produce a net shift in its resonance position if the noble gas atom occupies specific binding sites near the iron. At temperatures from 223 to 183 K, NMR signals were observed in the 0-40 ppm chemical shift range, relative to Xe in the gas phase. The signals broadened and shifted downfield as the temperature was reduced, indicating that Xe is exchanging between the gas phase and internal or external binding sites of the proteins. Additionally, conventional 129 Xe NMR studies of metmyoglobin and lipoxygenase in the solution state are presented. The temperature dependence of the chemical shift and line shape indicate exchange of Xe between adsorption sites on lipoxygenase and Xe in the solvent on the slow to intermediate exchange time scale. The NMR results are compared with N 2 , Xe, and CH 4 gas adsorption isotherms. It is found that lipoxygenase is unique among the proteins studied in possessing a relatively high affinity for gas molecules, and in addition, demonstrating the most clearly resolved adsorbed 129 Xe NMR peak in the lyophilized state.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.