Chemical derivatization of classes of metabolites in biological systems for detection by NMR with enhanced sensitivity and resolution

NMR spectroscopy is one of the two most important analytical tools used in metabolic profiling of biological systems due to its reproducibility and ability to provide spectroscopic information on a broad range of metabolites. 1H NMR analysis of complex biological samples combined with multivariate data analysis provides a well-established approach for a wide range of applications in metabolomics. In this dissertation, nuclei such as 13C and 31P are explored for improved NMR-based metabolic profiling. 13C NMR is not highly utilized in metabolomics due to issues of poor sensitivity and low natural abundance. The wide chemical shift range and reduced J-coupling are attractive features despite the low sensitivity. In a novel approach, this study demonstrates that chemical derivatization using 13C labeled reagents used to enhance the sensitivity and resolution of selected classes of metabolites in complex biological fluids. Amine containing metabolites were selectively detected from a complex mixture by derivatizing the metabolites with 13C labeled acetic anhydride. 1-D 13C and 2-D HSQC spectra of biological samples after reaction with 13C enriched acetic anhydride can be obtained in less than 10 min, and these spectra show dramatically improved sensitivity (∼100 fold) and resolution. This method was used in analyzing trace amounts of amino acids in serum and urine from patients diagnosed with inborn errors of metabolism and in tissue extracts. The 31P nucleus has a higher natural abundance and wider chemical shift range than the 13C NMR. Chemical derivatization of labile hydrogens in hydroxyl and carboxylic acid functionalities with 2-chloro-4,4,5,5-tetramethyldioxaphospholane ensured a good spectral separation upon derivatization as the products appeared in a different chemical environment and thus provided higher sensitivity and resolution. The derivatization procedure was fast, required minimal sample preparation, and was reproducible and cost effective. 31P labeling was then successfully utilized for analyzing lipophilic compounds with labile hydrogens in human serum. In sum, application of enhanced detection of metabolites in pathological tissue and biofluids by appropriate chemical derivatization methodologies provides a convenient and sensitive method for the qualitative and quantitative analysis of various classes of metabolites that may have good diagnostic utility and for the study of pathophysiology in various diseases

Physical Properties and CO₂ Reaction Pathway of 1‑Ethyl-3-Methylimidazolium Ionic Liquids with Aprotic Heterocyclic Anions

Ionic liquids (ILs) with aprotic heterocyclic anions (AHA) are attractive candidates for CO₂ capture technologies. In this study, a series of AHA ILs with 1-ethyl-3-methylimidazolium ([emim]⁺) cations were synthesized, and their physical properties (density, viscosity, and ionic conductivity) were measured. In addition, CO₂ solubility in each IL was determined at room temperature using a volumetric method at pressures between 0 and 1 bar. The AHAs are basic anions that are capable of reacting stoichiometrically with CO₂ to form carbamate species. An interesting CO₂ uptake isotherm behavior was observed, and this may be attributed to a parallel, equilibrium proton exchange process between the imidazolium cation and the basic AHA in the presence of CO₂, followed by the formation of “transient” carbene species that react rapidly with CO₂. The presence of the imidazolium-carboxylate species and carbamate anion species was verified using ¹H and ¹³C NMR spectroscopy. While the reaction between CO₂ and the proposed transient carbene resulted in cation-CO₂ binding that is stronger than the anion-CO₂ reaction, the reactions of the imidazolium AHA ILs were fully reversible upon regeneration at 80 °C with nitrogen purging. The presence of water decreased the CO₂ uptake due to the inhibiting effect of the neutral species (protonated form of AHA) that is formed

Solid–Liquid Equilibria Measurements of Mixtures of Lithium Bis(trifluoromethanesulfonyl)imide with Varying Alkyl Chain Length Ammonium Bis(trifluoromethanesulfonyl)imide Ionic Liquids

Ionic liquid (IL)–metal mixtures are potential solvents for a variety of applications, including metal electrodeposition, electrolytes for batteries, catalysis, and for separations processes involving liquid–liquid extraction. The solubility of a metal in an IL is fundamentally important to selecting an appropriate IL for a potential process; however, relatively few measurements have been reported in the literature. The solid–liquid equilibria of binary mixtures of lithium bis­(trifluoromethanesulfonyl)­imide and three ammonium bis­(trifluoromethanesulfonyl)­imide ILs are investigated. Measurements are made through two different methods. A visual method allows direct observation of the phase behavior between room temperature and 373 K and a differential calorimetry method provides solid–liquid equilibria information up to 623 K. The activity coefficients of the solid in the liquid are calculated from the measured phase equilibria and the pure component physical properties. The mutual solubilities of lithium bis­(trifluoromethanesulfonyl)­imide and hexadecyl-trimethylammonium bis­(trifluoromethanesulfonyl)­imide are found to be higher than expected given the long alkyl chain length of the IL cation

Effect of Cation on Physical Properties and CO₂ Solubility for Phosphonium-Based Ionic Liquids with 2‑Cyanopyrrolide Anions

A series of tetraalkylphosphonium 2-cyanopyrrolide ([P_<i>nnnn</i>]­[2-CNPyr]) ionic liquids (ILs) were prepared to investigate the effect of cation size on physical properties and CO₂ solubility. Each IL was synthesized in our laboratory and characterized by NMR spectroscopy. Their physical properties, including density, viscosity, and ionic conductivity, were determined as a function of temperature and fit to empirical equations. The density gradually increased with decreasing cation size, while the viscosity decreased noticeably. In addition, the [P_<i>nnnn</i>]­[2-CNPyr] ILs with large cations exhibited relatively low degrees of ionicity based on analysis of the Walden plots. This implies the presence of extensive ion pairing or formation of aggregates resulting from van der Waals interactions between the long hydrocarbon substituents. The CO₂ solubility in each IL was measured at 22 °C using a volumetric method. While the anion is typically known to be predominantly responsible for the CO₂ capture reaction, the [P_<i>nnnn</i>]­[2-CNPyr] ILs with shorter alkyl chains on the cations exhibited slightly stronger CO₂ binding ability than the ILs with longer alkyl chains. We attribute this to the difference in entropy of reaction, as well as the variation in the relative degree of ionicity

Phase-Change Ionic Liquids for Postcombustion CO₂ Capture

Phase-change ionic liquids, or PCILs, are salts that are solids at normal flue gas processing temperatures (e.g., 40–80 °C) and that react stoichiometrically and reversibly with CO₂ (one mole of CO₂ for every mole of salt at typical postcombustion flue gas conditions) to form a liquid. Thus, the melting point of the PCIL–CO₂ complex is below that of the pure PCIL. A new concept for CO₂ separation technology that uses this key property of PCILs offers the potential to significantly reduce parasitic energy losses incurred from postcombustion CO₂ capture by utilizing the heat of fusion (Δ<i>H</i>_fus) to provide part of the heat needed to release CO₂ from the absorbent. In addition, the phase transition yields almost a step-change absorption isotherm, so only a small pressure or temperature swing is required between the absorber and the stripper. Utilizing aprotic heterocyclic anions (AHAs), the enthalpy of reaction with CO₂ can be readily tuned, and the physical properties, such as melting point, can be adjusted by modifying the alkyl chain length of the tetra-alkylphosphonium cation. Here, we present data for four tetrabutylphosphonium salts that exhibit PCIL behavior, as well as detailed measurements of the CO₂ solubility, physical properties, phase transition behavior, and water uptake for tetraethylphosphonium benzimidazolide ([P₂₂₂₂]­[BnIm]). The process based on [P₂₂₂₂]­[BnIm] has the potential to reduce the amount of energy required for the CO₂ capture process substantially compared to the current technology that employs aqueous monoethanolamine (MEA) solvents