School of Chemistry - Research Publications

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Author: O'Hair, Richard × Author: Williams, Spencer × Type: Journal Article ×

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    Differentiation of aminohydroxypropanesulfonic acid structural isomers using tandem mass spectrometry-based methods
    Anh Nguyen, LT ; Bowen, CJ ; Burchill, L ; Williams, SJ ; O’Hair, RAJ (Elsevier BV, 2023-09)
    D-Cysteinolic acid (D-CA) is an important metabolite within the biosulfur cycle. A structural isomer, (R)-3-amino-2-hydroxypropanesulfonate ((R)-AHPS), is less common in nature but potentially can be misidentified as D-CA due to their many shared physical properties. To support confident assignment of these two isomers by use of mass spectrometry alone, this study explores the fragmentation reactions of their [M + H]+ and [M - H]- ions using collision-induced dissociation (CID). Electrospray ionization mass spectrometry (ESI-MS) experiments were conducted on authentic standards using an ion trap mass spectrometer, while a triple-quadrupole (QqQ) mass spectrometer was used in the selective reaction monitoring (SRM) mode to record energy-resolved CID. Density-functional theory (DFT) calculations were carried out at the M06/6-31+G* level of theory to study gas-phase fragmentation mechanisms. The data generated revealed kinetically-controlled fragmentations involving participation of neighboring amino groups in the positive ion mode. Negative ion mode MS analysis could distinguish the structural isomers through different collision energy-resolved results for m/z 95 product ions, CH3SO3−. DFT calculations revealed an enthalpy (ΔH) (Gibbs energy (ΔG)) gap of 31.8 (31.4) kJ/mol between transition state barriers of a concerted mechanism for D-CA, and a more preferred stepwise mechanism for (R)-AHPS.
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    Gas-Phase Intercluster Thiyl-Radical Induced C-H Bond Homolysis Selectively Forms Sugar C2-Radical Cations of Methyl D-Glucopyranoside: Isotopic Labeling Studies and Cleavage Reactions
    Osburn, S ; Speciale, G ; Williams, SJ ; O'Hair, RAJ (SPRINGER, 2017-07)
    A suite of isotopologues of methyl D-glucopyranosides is used in conjunction with multistage mass spectrometry experiments to determine the radical site and cleavage reactions of sugar radical cations formed via a recently developed 'bio-inspired' method. In the first stage of CID (MS2), collision-induced dissociation (CID) of a protonated noncovalent complex between the sugar and S-nitrosocysteamine, [H3NCH2CH2SNO + M]+, unleashes a thiyl radical via bond homolysis to give the noncovalent radical cation, [H3NCH2CH2S• + M]+. CID (MS3) of this radical cation complex results in dissociation of the noncovalent complex to generate the sugar radical cation. Replacement of all exchangeable OH and NH protons with deuterons reveals that the sugar radical cation is formed in a process involving abstraction of a hydrogen atom from a C-H bond of the sugar coupled with proton transfer to the sugar, to form [M - H• + D+]. Investigation of this process using individual C-D labeled sugars reveals that the main site of H/D abstraction is the C2 position, since only the C2-deuterium labeled sugar yields a dominant [M - D• + H+] product ion. The fragmentation reactions of the distonic sugar radical cation, [M - H•+ H+], were studied by another stage of CID (MS4). 13C-labeling studies revealed that a series of three related fragment ions each contain the C1-C3 atoms; these arise from cross-ring cleavage reactions of the sugar. Graphical Abstract ᅟ.
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