School of Chemistry - Research Publications

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    Environmental Polymer Degradation: Using the Distonic Radical Ion Approach to Study the Gas-Phase Reactions of Model Polyester Radicals
    Taggert, BI ; O'Hair, RAJ ; Wille, U (AMER CHEMICAL SOC, 2017-07-20)
    A novel precursor to the distonic O- and C-centered radical cations Oxo+O• and Oxo+C• was designed and synthesized, which represents model systems for radicals produced during polyester degradation. The precursor is equipped with a nitrate functional group, which serves as a masked site for these alkoxyl and carbon radicals that are unleashed through collision-induced dissociation (CID). Oxo+O• and Oxo+C• feature a cyclic carboxonium ion as permanent charge tag to enable monitoring their ion-molecule reactions on the millisecond to second time scale in the ion trap of the mass spectrometer. The reactions of Oxo+O• and Oxo+C• with cyclohexene, cyclohexadiene, ethyl acetate, 1,1-dimethoxyethane, and 1,2-dimethoxyethane, which exhibit structural features present in both intact and defective polyesters, were explored through product and kinetic studies to identify "hot spots" for radical-induced damage in polyesters. All reactions with Oxo+O• were extremely fast and proceeded predominantly through HAT. Oxo+C• was about two orders of magnitude less reactive and did not noticeably damage aliphatic ester moieties through hydrogen abstraction on the time scale of our experiments. Radical addition to alkene π systems was identified as an important pathway for C-radicals, which needs to be included in polymer degradation mechanisms.
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    Reaction of Distonic Aryl and Alkyl Radical Cations with Amines: The Role of Charge and Spin Revealed by Mass Spectrometry, Kinetic Studies, and DFT Calculations
    Andrikopoulos, B ; Sidhu, PK ; Taggert, B ; Nathanael, JG ; O'Hair, RAJ ; Wille, U (Wiley, 2020-01-01)
    Gas‐phase reaction of the aromatic distonic radical cations 4‐Pyr+. and 3‐Pyr+. with amines led to formation of the corresponding amino pyridinium ions 4‐Pyr+NR2 and 3‐Pyr+NR2 through amine addition at the site of the radical, followed by homolytic β‐fragmentation. The reaction efficiencies range from 66–100 % for 4‐Pyr+. and 57–86 % for 3‐Pyr+., respectively, indicating practically collision‐controlled processes in some cases. Computational studies revealed that the combination of positive charge and spin makes nucleophilic attack by the amine at the site of the radical barrierless and strongly exothermic by about 175±15 kJ mol−1, thereby rendering ‘conventional’ radical pathways through hydrogen abstraction or addition onto π systems less important. Exemplary studies with 4‐Pyr+. showed that this reaction can be reproduced in solution. A similar addition/radical fragmentation sequence occurs also in the gas‐phase reaction of amines with the aliphatic distonic radical cation Oxo+C., showing that the observed charge‐spin synergism is not limited to aromatic systems.
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    The role of peroxyl radicals in polyester degradation - a mass spectrometric product and kinetic study using the distonic radical ion approach
    Gervasoni, BD ; Khairallah, GN ; O'Hair, RAJ ; Wille, U (ROYAL SOC CHEMISTRY, 2015-01-01)
    Mass spectrometric techniques were used to obtain detailed insight into the reactions of peroxyl radicals with model systems of (damaged) polyesters. Using a distonic radical ion approach, it was shown that N-methylpyridinium peroxyl radical cations, Pyr(+)OO˙, do not react with non-activated C-H bonds typically present in polyesters that resist degradation. Structural damage in the polymer, for example small amounts of alkene moieties formed during the manufacturing process, is required to enable reaction with Pyr(+)OO˙, which proceeds with high preference through addition to the π system rather than via allylic hydrogen atom abstraction (kadd/kHAT > 20 for internal alkenes). This is due to the very fast and strongly exothermic subsequent fragmentation of the peroxyl-alkene radical adduct to epoxides and highly reactive Pyr(+)O˙, which both could promote further degradation of the polymer through non-radical and radical pathways. This work provides essential experimental support that the basic autoxidation mechanism is a too simplistic model to rationalize radical mediated degradation of polymers under ambient conditions.
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    What Are the Potential Sites of Protein Arylation by N-Acetyl-p-benzoquinone Imine (NAPQI)?
    Leeming, MG ; Gamon, LF ; Wille, U ; Donald, WA ; O'Hair, RAJ (AMER CHEMICAL SOC, 2015-11-01)
    Acetaminophen (paracetamol, APAP) is a safe and widely used analgesic medication when taken at therapeutic doses. However, APAP can cause potentially fatal hepatotoxicity when taken in overdose or in patients with metabolic irregularities. The production of the electrophilic and putatively toxic compound N-acetyl-p-benzoquinone imine (NAPQI), which cannot be efficiently detoxicated at high doses, is implicated in APAP toxicity. Numerous studies have identified that excess NAPQI can form covalent linkages to the thiol side chains of cysteine residues in proteins; however, the reactivity of NAPQI toward other amino acid side chains is largely unexplored. Here, we report a survey of the reactivity of NAPQI toward 11 N-acetyl amino acid methyl esters and four peptides. (1)H NMR analysis reveals that NAPQI forms covalent bonds to the side-chain functional groups of cysteine, methionine, tyrosine, and tryptophan residues. Analogous reaction products were observed when NAPQI was reacted with synthetic model peptides GAIL-X-GAILR for X = Cys, Met, Tyr, and Trp. Tandem mass spectrometry peptide sequencing showed that the NAPQI modification sites are located on the "X" residue in each case. However, when APAP and the GAIL-X-GAILR peptide were incubated with rat liver microsomes that contain many metabolic enzymes, NAPQI formed by oxidative metabolism reacted with GAIL-C-GAILR exclusively. For the peptides where X = Met, Tyr, and Trp, competing reactions between NAPQI and alternative nucleophiles precluded arylation of the target peptide by NAPQI. Although Cys residues are favorably targeted under these conditions, these data suggest that NAPQI can, in principle, also damage proteins at Met, Tyr, and Trp residues.