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  • 1
    Electronic Resource
    Electronic Resource
    Novel keto acid formate-lyase and propionate kinase enzymes are components of an anaerobic pathway in Escherichia coli that degrades L-threonine to propionate (1998)
    Oxford BSL : Blackwell Science Ltd
    Molecular microbiology 27 (1998), S. 0 
    Details
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: An immunological analysis of an Escherichia coli strain unable to synthesize the main pyruvate formate-lyase enzyme Pfl revealed the existence of a weak, cross-reacting 85 kDa polypeptide that exhibited the characteristic oxygen-dependent fragmentation typical of a glycyl radical enzyme. Polypeptide fragmentation of this cross-reacting species was shown to be dependent on Pfl activase. Cloning and sequence analysis of the gene encoding this protein revealed that it coded for a new enzyme, termed TdcE, which has 82% identity with Pfl. On the basis of RNA analyses, the tdcE gene was shown to be part of a large operon that included the tdcABC genes, encoding an anaerobic threonine dehydratase, tdcD, coding for a propionate kinase, tdcF, the function of which is unknown, and the tdcG gene, which encodes a L-serine dehydratase. Expression of the tdcABCDEFG operon was strongly catabolite repressed. Enzyme studies showed that TdcE has both pyruvate formate-lyase and 2-ketobutyrate formate-lyase activity, whereas the TdcD protein is a new propionate/acetate kinase. By monitoring culture supernatants from various mutants using 1H nuclear magnetic resonance (NMR), we followed the anaerobic conversion of L-threonine to propionate. These studies confirmed that 2-ketobutyrate, the product of threonine deamination, is converted in vivo by TdcE to propionyl-CoA. These studies also revealed that Pfl and an as yet unidentified thiamine pyrophosphate-dependent enzyme(s) can perform this reaction. Double null mutants deficient in phosphotransacetylase (Pta) and acetate kinase (AckA) or AckA and TdcD were unable to metabolize threonine to propionate, indicating that propionyl-CoA and propionyl-phosphate are intermediates in the pathway and that ATP is generated during the conversion of propionyl-P to propionate by AckA or TdcD.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2958.1998.00696.x
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  • 2
    Electronic Resource
    Electronic Resource
    Transfer of a quadridentate N2S2 ligand from M2+ (M=Cr, Mn, Fe, Co or Ni) to Cu2+: Unexpected reactivities of bimetallic intermediates (1998)
    Springer
    Transition metal chemistry 23 (1998), S. 701-713 
    Details
    ISSN: 1572-901X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A series of complexes of stoichiometry [MX2(dipyS)] {dipyS = bis(2–picolyl)-1,3–dithiopropane); M=Cr, Mn, Fe, Co, Ni, X=Cl; M=Ni or Cu, X=NO3} and [VOCl(dipyS)]Cl have been prepared and characterised, including the X-ray crystal structure of [Ni(ONO2)2(dipyS)]. The kinetics of the transfer of dipyS from these complexes (M=Cr, Mn, Fe, Co or Ni) to Cu2+, to form [Cu(dipyS)]2+, have been studied in MeOH. For M=Ni, the kinetics are consistent with a mechanism involving rate-limiting dissociation of the initial pyridyl—M bond. Subsequent binding of Cu2+ to the pendant pyridyl-residue (or binding Cl− to the vacant site on M) is followed by the complete transfer of dipyS from M to Cu. For M=Cr, Mn or Co, the same mechanism is believed to operate, but in these cases intermediates in the dipyS transfer to Cu2+ have been detected spectroscopically. Evidence is presented that these intermediates have Cu2+ bound to a pendant pyridyl-group on [MCl2(dipyS)] and that the subsequent complete transfer of dipyS involves rate-limiting dissociation of a M—S bond. For M=Fe, e.p.r. spectroscopy shows that the complex is a dimer in solution. However, the transfer reaction with Cu2+ involves an analogous intermediate to that with M=Cr, Mn or Co, but only at high concentrations of Cu2+. Unexpectedly, the binding of Cu2+ inhibits the transfer of dipyS from Fe to Cu. The electronic factors which give rise to this behaviour are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006983218095
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  • 3
    Electronic Resource
    Electronic Resource
    An ESR study of the polymerization of tetrahydrofuran with simple inorganic cations containing sulphur and nitrogen (1985)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 23 (1985), S. 828-831 
    Details
    ISSN: 0749-1581
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cations of NS2+·AsF6-, N(SCI)2+·AsF6- and NS+·AsF6- have been shown to be efficient initiators for the polymerization of tetrahydrofuran. The former two salts were found, by electron spin resonance spectroscopy, to produce remarkably high concentrations of persistent free radicals which have been identified as being entirely inorganic, namely N2S3+· and, more tentatively, N2S2+·. A mechanism for generation of the radicals is proposed.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260231008
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  • 4
    Electronic Resource
    Electronic Resource
    ESR investigation of the dimerization of some dithiazolidinyl radicals (1987)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 25 (1987), S. 60-64 
    Details
    ISSN: 0749-1581
    Keywords: ESR ; Energetics of dimerization ; dithiazolidiny radicals ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Four types of dithiazolidin-2-yl radical were selected for dimerization measurements in solution. The radicals had different types of substituents and/or different amounts of strain; ESR measurements for a new dithiazolidinyl radical prepared from trans-cyclooctene are presented. Toluene, diethyl ether and chloroform were used as solvents. Radical concentration measurements incorporated corrections for the variation of the Q of the spectrometer cavity with solvent and temperature. It was found that ΔH⊖ for the dimerization process is dependent only on the radical structure.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260250115
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  • 5
    Electronic Resource
    Electronic Resource
    ENDOR spectra of vanadyl complexes in asphaltenes (1987)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 25 (1987), S. 829-830 
    Details
    ISSN: 0749-1581
    Keywords: ESR ; ENDOR ; Asphaltenes ; Vanadyl complexes ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It has been suggested, on the basis of electron spin resonance measurements, that the coordination of vanadyl complexed in asphaltenes is porphyrin-like. We have examined a number of asphaltenes using electron nuclear double resonance (ENDOR) spectroscopy and obtained spectra from 14N and 1H nuclei. Comparison of the spectra with those of vanadyl octaethyl- and tetraphenyl-porphyrins confirms that the porphyrin structure does indeed occur in the asphaltenes.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260250917
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  • 6
    Electronic Resource
    Electronic Resource
    ESR spectroscopic investigation of some 4-substituted 1,3,2-dithiazolyl and 2,3,1-dithiazolyl radicals (1992)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 30 (1992), S. 666-672 
    Details
    ISSN: 0749-1581
    Keywords: Electron spin resonance spectroscopy ; Sulphur-nitrogen-containing radicals ; Dithiadiazolyl radicals ; Dithiadiazolyl radicals ; Dithiazophospholyl radicals ; Phosphaalkynes ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Further information has been obtained on the electronic structure of 1,3,2-dithiazol-2-yl radicals. The structure has been well characterized previously except for positions 4 and 5 in the ring. We have obtained solution and powder ESR spectra of radicals substituted with 13C in both positions and of radicals substituted with nitrogen or phosphorus in the 4-position. It is probably the first time that a compound has been prepared containing the ring. Two especially interesting facts emerged from the ESR spectra of these 1,3,2-dithiazolyl radicals: the 13C-substituted radical has a considerable amount of p spin density in the plane of the ring and the phosphorussubstituted radical has a very low unpaired spin density at the phosphorus atom. It is known that 1,3,2,4-dithiadiazolyl radicals isomerize readily by photolysis to the symmetrical 2,3,1,4-dithiadiazolyl radicals; we attempted the same reaction with But and a new radical was produced that we have been unable to identify.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260300717
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  • 7
    Electronic Resource
    Electronic Resource
    Electron spin resonance spectroscopy of free radicals (1993)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 31 (1993), S. 1027-1030 
    Details
    ISSN: 0749-1581
    Keywords: ESR ; Isotropic spectra ; Powder spectra ; Dithiadiazolyl radicals ; Dimerization ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Solution and powder ESR spectra of the 4-trifluoromethyl-1,2,3,5-dithiadiazolyl and 4-pentafluorophenyl-1,2,3,5-dithiadiazolyl free radicals were analysed. Comparison of the ESR parameters with those obtained for the protonated equivalents shows that fluorination produces a drift of unpaired electron spin density away from the nitrogen 2px orbitals towards sulphur 3px orbitals (x is the direction perpendicular to the heterocyclic ring), consistent with an increase in electronegativity of the substituent. The fluorinated radicals dimerize more readily than their protonated counterparts.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260311114
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  • 8
    Electronic Resource
    Electronic Resource
    ESR spectroscopy of some dicyano-, pyrazino- and quinoxalino-1,3,2-dithiazolyl radicals (1992)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 30 (1992), S. 774-778 
    Details
    ISSN: 0749-1581
    Keywords: ESR ; 1,3,2-Dithiazol-2-yl radicals ; Pyrazine ; Quinoxaline ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several symmetrical 1,3,2-dithiazol-2-yl radicals have been examined that have in common nitrogen-containing substituents at the 4- and 5-positions, namely 4,5-dicyano-1,3,2-dithiazol-2-yl, [15N2]-1,3,2-dithiazolo [4,5-b] pyrazin-2-yl and 1,3,2-dithiazoleo [4,5-b] quinoxalin-2-yl. The ESR spectrum of the 4,5-dicyano radical does not reveal any 14N splittings from the cyanide groups; however, the pyrazine and the quinoxaline derivatives do have multi-lined well resolved spectra arising from substantial hyperfine interactions with these nuclei. The interactions are sufficiently large to be observed in the powder spectra of the latter. The presence of proton hyperfine interactions shows that unpaired-electron spin density is delocalized over all the rings. It has been found that the radicals attached to pyrazine and quinoxaline can be incorporated into single crystals of naphthalene and phenazine, respectively.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260300814
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