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    Gas-Phase Models for the Nickel- and Palladium-Catalyzed Deoxygenation of Fatty Acids
    Parker, K ; Weragoda, GK ; Pho, V ; Canty, AJ ; Polyzos, A ; O'Hair, RAJ ; Ryzhov, V (WILEY-V C H VERLAG GMBH, 2020-11-05)
    Using fatty acids as renewable sources of biofuels requires deoxygenation. While a number of promising catalysts have been developed to achieve this, their operating mechanisms are poorly understood. Here, model molecular systems are studied in the gas phase using mass spectrometry experiments and DFT calculations. The coordinated metal complexes [(phen)M(O2CR)]+ (where phen=1,10‐phenanthroline; M=Ni or Pd; R=CnH2n+1, n≥2) are formed via electrospray ionization. Their collision‐induced dissociation (CID) initiates deoxygenation via loss of CO2 and [C,H2,O2]. The CID spectrum of the stearate complexes (R=C17H35) also shows a series of cations [(phen)M(R’)]+ (where R’ < C17) separated by 14 Da (CH2) corresponding to losses of C2H4‐C16H32 (cracking products). Sequential CID of [(phen)M(R’)]+ ultimately leads to [(phen)M(H)]+ and [(phen)M(CH3)]+, both of which react with volatile carboxylic acids, RCO2H, (acetic, propionic, and butyric) to reform the coordinated carboxylate complexes [(phen)M(O2CR)]+. In contrast, cracking products with longer carbon chains, [(phen)M(R)]+ (R>C2), were unreactive towards these carboxylic acids. DFT calculations are consistent with these results and reveal that the approach of the carboxylic acid to the “free” coordination site is blocked by agostic interactions for R > CH3.
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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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    Photoexcited Pd(ii) auxiliaries enable light-induced control in C(sp3)-H bond functionalisation
    Czyz, ML ; Weragoda, GK ; Horngren, TH ; Connell, TU ; Gomez, D ; O'Hair, RAJ ; Polyzos, A (ROYAL SOC CHEMISTRY, 2020-03-07)
    Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C-H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp3)-H halogenation products rather than C-C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C-X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp3)-H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C-H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.
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    Type IX Secretion System Cargo Proteins Are Glycosylated at the C Terminus with a Novel Linking Sugar of the Wbp/Vim Pathway
    Veith, PD ; Shoji, M ; O'Hair, RAJ ; Leeming, MG ; Nie, S ; Glew, MD ; Reid, GE ; Nakayama, K ; Reynolds, EC ; Trent, MS (AMER SOC MICROBIOLOGY, 2020-09-01)
    Porphyromonas gingivalis and Tannerella forsythia use the type IX secretion system to secrete cargo proteins to the cell surface where they are anchored via glycolipids. In P. gingivalis, the glycolipid is anionic lipopolysaccharide (A-LPS), of partially known structure. Modified cargo proteins were deglycosylated using trifluoromethanesulfonic acid and digested with trypsin or proteinase K. The residual modifications were then extensively analyzed by tandem mass spectrometry. The C terminus of each cargo protein was amide-bonded to a linking sugar whose structure was deduced to be 2-N-seryl, 3-N-acetylglucuronamide in P. gingivalis and 2-N-glycyl, 3-N-acetylmannuronic acid in T. forsythia The structures indicated the involvement of the Wbp pathway to produce 2,3-di-N-acetylglucuronic acid and a WbpS amidotransferase to produce the uronamide form of this sugar in P. gingivalis The wbpS gene was identified as PGN_1234 as its deletion resulted in the inability to produce the uronamide. In addition, the P. gingivalisvimA mutant which lacks A-LPS was successfully complemented by the T. forsythiavimA gene; however, the linking sugar was altered to include glycine rather than serine. After removal of the acetyl group at C-2 by the putative deacetylase, VimE, VimA presumably transfers the amino acid to complete the biosynthesis. The data explain all the enzyme activities required for the biosynthesis of the linking sugar accounting for six A-LPS-specific genes. The linking sugar is therefore the key compound that enables the attachment of cargo proteins in P. gingivalis and T. forsythia We propose to designate this novel linking sugar biosynthetic pathway the Wbp/Vim pathway.IMPORTANCEPorphyromonas gingivalis and Tannerella forsythia, two pathogens associated with severe gum disease, use the type IX secretion system (T9SS) to secrete and attach toxic arrays of virulence factor proteins to their cell surfaces. The proteins are tethered to the outer membrane via glycolipid anchors that have remained unidentified for more than 2 decades. In this study, the first sugar molecules (linking sugars) in these anchors are identified and found to be novel compounds. The novel biosynthetic pathway of these linking sugars is also elucidated. A diverse range of bacteria that do not have the T9SS were found to have the genes for this pathway, suggesting that they may synthesize similar linking sugars for utilization in different systems. Since the cell surface attachment of virulence factors is essential for virulence, these findings reveal new targets for the development of novel therapies.