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1. Resolving identities of emerging per and polyfluoroalkyl substances isomers based on COSMO-RS derived retention factor and mass fragmentation patterns

   Simpson, Scott ; Guardian, M.G. ; Aga, D.S. ( December 2021 , Pacifichem)

   Chromatographic retention times and mass spectrometral fragmentation of per- and polyfluoroalkyl substances (PFASs) standards were determined using the optimized parameters obtained for liquid chromatography with tandem high-resolution mass spectrometry (LC-HRMS) analysis. Characteristic fragment ions obtained at various collision energies (MS2 fragmentation) were used for structural elucidation to predict the identities of newly discovered (emerging) PFASs detected in environmental samples. Moreover, the COnductor-like Screening MOdel for Realistic Solvents (COSMO-RS) was used to calculate the octanol-water partition coefficients (Kow) and mean isotropic polarizabilities of known PFASs, and the values were plotted against their chromatographic retention factors (k) to obtain a multivariable regression model that can be used to predict k values of unknown PFASs. Retention factor values of different structural isomers of the unknown PFASs were calculated and compared to the experimental k. For all the unknown PFASs, the predicted k value for the isomer that matches the corresponding MS2 fragmentation was found to be within 5% of the experimentally measured k value. This study demonstrates the applicability of a simple approach that combines the use of computationally-derived log Kow and polarizabilities, experimentally-determined k values, together with observed MS2 fragmentation patterns, in assigning the structures of emerging PFASs at environmentally relevant conditionsmore »when no reference standards are available.« less
2. Quantitative Structure Retention Relationships to Identify Per-/Poly-Fluoroalkyl Substances and Other Contaminants of Emerging Concern

   Simpson, S. ; Antle, J.l Halwatura ; LaRock, M. ; Dickman, R. ; Aga, D. ( May 2022 , Annual meeting abstracts)

   Nontarget analysis using liquid chromatography–high resolution mass spectrometry (LC–HRMS) is a valuable approach in characterizing for contaminants of emerging concern (CECs) in the environment. However, identification of these analytes can be quite costly or taxing without proper analytical standards. To circumvent this problem we utilize Quantitative structure-retention relationships (QSRR) models to predict elution order and retention times. Properties calculated from density functional theory (DFT) and the conductor-like screening model for real solvents (COSMO-RS) theory are used to produce our QSRR models, which can be calculated for virtually any analyte. We show that this methodology has been successful in identification of per- /poly-fluoroalkyl substances (PFAS) and other contaminants. Nontarget analysis using liquid chromatography– high resolution mass spectrometry (LC–HRMS) is a valuable approach in characterizing for contaminants of emerging concern (CECs) in the environment. However, identification of these analytes can be quite costly or taxing without proper analytical standards. To circumvent this problem we utilize Quantitative structureretention relationships (QSRR) models to predict elution order and retention times. Properties calculated from density functional theory (DFT) and the conductor-like screening model for real solvents (COSMO-RS) theory are used to produce our QSRR models, which can be calculated for virtually any analyte. We show thatmore »this methodology has been« less
3. LC–HRMS and Chemical Derivatization Strategies for the Structure Elucidation of Caribbean Ciguatoxins: Identification of C-CTX-3 and -4

   https://doi.org/10.3390/md18040182  

   Kryuchkov, Fedor ; Robertson, Alison ; Miles, Christopher O. ; Mudge, Elizabeth M. ; Uhlig, Silvio ( April 2020 , Marine Drugs)

   Ciguatera poisoning is linked to the ingestion of seafood that is contaminated with ciguatoxins (CTXs). The structural variability of these polyether toxins in nature remains poorly understood due to the low concentrations present even in highly toxic fish, which makes isolation and chemical characterization difficult. We studied the mass spectrometric fragmentation of Caribbean CTXs, i.e., the epimers C-CTX-1 and -2 (1 and 2), using a sensitive UHPLC–HRMS/MS approach in order to identify product ions of diagnostic value. We found that the fragmentation of the ladder-frame backbone follows a characteristic pattern and propose a generalized nomenclature for the ions formed. These data were applied to the structural characterization of a pair of so far poorly characterized isomers, C-CTX-3 and -4 (3 and 4), which we found to be reduced at C-56 relative to 1 and 2. Furthermore, we tested and applied reduction and oxidation reactions, monitored by LC–HRMS, in order to confirm the structures of 3 and 4. Reduction of 1 and 2 with NaBH4 afforded 3 and 4, thereby unambiguously confirming the identities of 3 and 4. In summary, this work provides a foundation for mass spectrometry-based characterization of new C-CTXs, including a suite of simple chemical reactions to assistmore »the examination of structural modifications.« less
4. Differentiating aspartic acid isomers and epimers with charge transfer dissociation mass spectrometry (CTD-MS)

   https://doi.org/10.1039/D1AN02279B  

   Edwards, Halle M. ; Wu, Hoi-Ting ; Julian, Ryan R. ; Jackson, Glen P. ( March 2022 , The Analyst)

   The ability to understand the function of a protein often relies on knowledge about its detailed structure. Sometimes, seemingly insignificant changes in the primary structure of a protein, like an amino acid substitution, can completely disrupt a protein's function. Long-lived proteins (LLPs), which can be found in critical areas of the human body, like the brain and eye, are especially susceptible to primary sequence alterations in the form of isomerization and epimerization. Because long-lived proteins do not have the corrective regeneration capabilities of most other proteins, points of isomerism and epimerization that accumulate within the proteins can severely hamper their functions and can lead to serious diseases like Alzheimer's disease, cancer and cataracts. Whereas tandem mass spectrometry (MS/MS) in the form of collision-induced dissociation (CID) generally excels at peptide characterization, MS/MS often struggles to pinpoint modifications within LLPs, especially when the differences are only isomeric or epimeric in nature. One of the most prevalent and difficult-to-identify modifications is that of aspartic acid between its four isomeric forms: l -Asp, l -isoAsp, d -Asp, and d -isoAsp. In this study, peptides containing isomers of Asp were analyzed by charge transfer dissociation (CTD) mass spectrometry to identify spectral features that could discriminatemore »between the different isomers. For the four isomers of Asp in three model peptides, CTD produced diagnostic ions of the form c n +57 on the N-terminal side of iso-Asp residues, but not on the N-terminal side of Asp residues. Using CTD, the l - and d forms of Asp and isoAsp could also be differentiated based on the relative abundance of y - and z ions on the C-terminal side of Asp residues. Differentiation was accomplished through a chiral discrimination factor, R , which compares an ion ratio in a spectrum of one epimer or isomer to the same ion ratio in the spectrum of a different epimer or isomer. The R values obtained using CTD are as robust and statistically significant as other fragmentation techniques, like radical directed dissociation (RDD). In summary, the extent of backbone and side-chain fragments produced by CTD enabled the differentiation of isomers and epimers of Asp in a variety of peptides.« less
5. Carbon Capture from Biogas by Deep Eutectic Solvents: A COSMO Study to Evaluate the Effect of Impurities on Solubility and Selectivity

   https://doi.org/10.3390/cleantechnol3020029  

   Quaid, Thomas ; Reza, M. Toufiq ( June 2021 , Clean Technologies)

   Deep eutectic solvents (DES) are compounds of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) that contain a depressed melting point compared to their individual constituents. DES have been studied for their use as carbon capture media and biogas upgrading. However, contaminants’ presence in biogas might affect the carbon capture by DES. In this study, conductor-like screening model for real solvents (COSMO-RS) was used to determine the effect of temperature, pressure, and selective contaminants on five DES’ namely, choline chloride-urea, choline chloride-ethylene glycol, tetra butyl ammonium chloride-ethylene glycol, tetra butyl ammonium bromide-decanoic acid, and tetra octyl ammonium chloride-decanoic acid. Impurities studied in this paper are hydrogen sulfide, ammonia, water, nitrogen, octamethyltrisiloxane, and decamethylcyclopentasiloxane. At infinite dilution, CO2 solubility dependence upon temperature in each DES was examined by means of Henry’s Law constants. Next, the systems were modeled from infinite dilution to equilibrium using the modified Raoults’ Law, where CO2 solubility dependence upon pressure was examined. Finally, solubility of CO2 and CH4 in the various DES were explored with the presence of varying mole percent of selective contaminants. Among the parameters studied, it was found that the HBD of the solvent is the most determinant factor for themore »effectiveness of CO2 solubility. Other factors affecting the solubility are alkyl chain length of the HBA, the associated halogen, and the resulting polarity of the DES. It was also found that choline chloride-urea is the most selective to CO2, but has the lowest CO2 solubility, and is the most polar among other solvents. On the other hand, tetraoctylammonium chloride-decanoic acid is the least selective, has the highest maximum CO2 solubility, is the least polar, and is the least affected by its environment.« less