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  • 1
    Electronic Resource
    Electronic Resource
    Ab initio molecular orbital studies of sigmatropic rearrangements (1978)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 14 (1978), S. 767-777 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio molecular-orbital theory with the STO-3G and 4-31 G basis sets has been used to study the 1,3-sigmatropic hydrogen rearrangements: propene→propene, formic acid→formic acid, and vinyl alcohol→acetaldehyde, and the1,5-shifts:1,3-pentadiene→ 1,3-pentadiene and β-hydroxyacrolein→ β-hydroxyacrolein. Transition states have been determined using gradient procedures. Improved descriptions of the energies of the reactions have been obtained using 3 × 3 configuration interaction. In accord with expectations based on orbital-symmetry considerations, the calculated barriers are considerably greater for 1,3- than for 1,5-shifts. The forbidden pathway for the degenerate 1,3-shift in propene is predicted to require less activation energy than the allowed pathway, a result that can be rationalized in terms of interactions with subjacent and superjacent orbitals.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560140609
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  • 2
    Electronic Resource
    Electronic Resource
    Ab initio studies of 1,3-sigmatropic rearrangements: Effect of basis set and electron correlation (1980)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 18 (1980), S. 107-116 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio molecular-orbital theory has been used to study the 1,3-sigmatropic hydrogen rearrangements: propene → propene, formic acic → formic acid, and vinyl alcohol → acetaldehyde. Fully optimized structures of stable molecules and transition states have been determined using gradient procedures and the 4-31G basis set. Improved energies have been obtained using a variety of techniques with basis sets up to the size of double-ζ plus polarization (DZP) and electron correlation up to the CEPA/DZP level. Although both polarization functions and electron correlation lead to a lowering of the calculated barriers, the values remain substantial for all three rearrangements.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560180117
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  • 3
    Electronic Resource
    Electronic Resource
    Negative chemical ionization mass spectrometry of explosivesPresented in part at the 11th Meeting of the British Mass Spectroscopy Group, Canterbury (1980). (1981)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 16 (1981), S. 331-335 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The negative chemical ionization mass spectra of nitrobenzene, ethylene glycol dinitrate and nitroglycerine have been obtained using various reagent ions. For nitrobenzene, [OH]- gives the [M - H]-, together with [M]-· ions formed by electron capture, but other reagent ions gave relatively low intensity adduct peaks. Ethylene glycol dinitrate and nitroglycerine gave abundant [M + X]- ions (X = NO2, NO3, Cl, Br, I), together with ions arising from the thermal decomposition of the samples in the heated inlet system. The rate of anion attachment to these compounds is much greater than that to related compounds having only one functional group, and it is suggested that this is due to the participation of the adjacent groups in the bonding between the substrate and anion.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210160802
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  • 4
    Electronic Resource
    Electronic Resource
    On the nature of the ‘methoxy’ cationPresented in part at the Seventh Biennial Conference of the Australian and New Zealand Society for Mass Spectrometry. Sydney, Australia (1981). (1982)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 17 (1982), S. 315-317 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence-electron correlation have been used to investigate the structure of the [COH3]+ ion formed at threshold from fragmentation of dimethyl ether in the mass spectrometer. The results indicate that this ion is the hydroxymethyl cation, [CH2OH]+, formed with excess energy in a two-step rearrangement-dissociation process involving an initial 1,2-hydrogen shift in the dimethyl ether radical cation to yield an oxonium ion intermediate, [CH2OHCH3]+·, followed by C—O bond cleavage. The intermediate oxonium ion is predicted to lie slightly lower in energy than the dimethyl ether radical cation and represents a stable, previously unreported [C2H6O]+· isomer. It is suggested that at high energies, decomposition of the dimethyl ether radical cation involves dissociation followed by rearrangement.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210170705
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  • 5
    Electronic Resource
    Electronic Resource
    The methylamine radical cation [CH3NH2]+· and its stable isomer the methylenammonium radical cation [CH2NH3]+· (1983)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 18 (1983), S. 12-15 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The potential energy surface for the [CH5N]+· system has been investigated using ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence-electron correlation. Two [CH5N]+· isomers can be distinguished: the well known methylamine radical cation, [CH3NH2]+·, and the less familiar methylenammonium radical cation, [CH2NH3]+·. The latter is calculated to lie 8 kJ mol-1 lower in energy. A substantial barrier (176 kJ mol-1) is predicted for rearrangement of [CH2NH3]+· to [CH3NH2]+·. In addition, a large barrier (202 kJ mol-1) is found for loss of a hydrogen radical from [CH2NH3]+· via direct N - H bond cleavage to give the aminomethyl cation [CH2NH2]+. These results are consistent with the existence of the methylenammonium ion [CH2NH3]+· as a stable observable species. The barrier to loss of a hydrogen radical from [CH3NH2]+· is calculated to be 140 kJ mol-1.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210180104
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  • 6
    Electronic Resource
    Electronic Resource
    The potential energy surface for the [C2H2O]+· system: The ketene radical cation [CH2=C=O]+· and its isomers (1984)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 19 (1984), S. 610-616 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence electron correlation have been employed to examine the [C2H2O]+· potential energy surface. Four [C2H2O]+· isomers have been identified as potentially stable, observable ions. These are the experimentally well-known ketene radical cation, [CH2=C=O]+· (a), and the presently unknown ethynol radical cation, [CH2≡C—OH]+· (b), the oxirene radical cation (c) and an ion resembling a complex of CO with [CH2]+·, (d). The calculated energies of b, c and d relative to a are 189, 257 and 259 kJ mol-1, respectively. Dissociation of ions a and d is found to occur without reverse activation energy.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210191205
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  • 7
    Electronic Resource
    Electronic Resource
    A revised mechanism for the structure-specific gas phase enol cation-cyclobutanol reaction (1981)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 16 (1981), S. 301-302 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ion cyclotron resonance studies using deuterium labelled substrates have revealed a more complex mechanism than previously proposed for the structure-specific ion/molecule reaction of the [C2H4O]+· and [C3H6O]+· enol ions with neutral cyclobutanol. This requires the intermediacy of a species in which all of the methylene groups present in both the reactant ion and neutral can become equivalent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210160706
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  • 8
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    Protein C Inhibitor Acts as a Procoagulant by Inhibiting the Thrombomodulin-Induced Activation of Protein C in Human Plasma (1998)
    American Society of Hematology
    Details
    Publication Date: 1998-03-01
    Description: Protein C inhibitor (PCI), which was originally identified as an inhibitor of activated protein C, also efficiently inhibits coagulation factors such as factor Xa and thrombin. Recently it was found, using purified proteins, that the anticoagulant thrombin-thrombomodulin complex was also inhibited by PCI. The paradoxical inhibitory effect of PCI on both coagulant and anticoagulant proteases raised questions about the role of PCI in plasma. We studied the role of thrombomodulin (TM)-dependent inhibition of thrombin by PCI in a plasma system. Clotting was induced by addition of tissue factor to recalcified plasma in the absence or presence of TM, and clot formation was monitored using turbidimetry. In the absence of TM, PCI-deficient plasma showed a slightly shorter coagulation time compared with normal plasma. Reconstitution with a physiologic amount of PCI gave normal clotting times. Addition of PCI to normal plasma and protein C–deficient plasma resulted in a minor prolongation of the clotting time. This suggested that PCI can act as a weak coagulation inhibitor in the absence of TM. TM caused a strong anticoagulant effect in normal plasma due to thrombin scavenging and activation of the protein C anticoagulant pathway. This effect was less pronounced when protein C–deficient plasma was used, but could be restored by reconstitution with protein C. When PCI was added to protein C–deficient plasma in the presence of TM, a strong anticoagulant effect of PCI was observed. This anticoagulant effect was most likely caused by the TM-dependent thrombin inhibition by PCI. However, when PCI was added to normal plasma containing TM, a strong procoagulant effect of PCI was observed, due to the inhibition of protein C activation. PCI-deficient plasma was less coagulant in the presence of TM. A concentration-dependent increase in clotting time was observed when PCI-deficient plasma was reconstituted with PCI. The combination of these results suggest that the major function of PCI in plasma during coagulation is the inhibition of thrombin. A decreased generation of activated protein C is a procoagulant consequence of the TM-dependent thrombin inhibition by PCI. We conclude that TM alters PCI from an anticoagulant into a procoagulant during tissue factor-induced coagulation.
    Print ISSN: 0006-4971
    Electronic ISSN: 1528-0020
    Topics: Biology , Medicine
    Published by American Society of Hematology
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  • 9
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    Kinetics of factor VIII-von Willebrand factor association (1996)
    American Society of Hematology
    Details
    Publication Date: 1996-03-01
    Description: The binding of factor VIII to von Willebrand factor (vWF) is essential for the protection of factor VIII against proteolytic degradation in plasma. We have characterized the binding kinetics of human factor VIII with vWF using a centrifugation binding assay. Purified or plasma vWF was immobilized with a monoclonal antibody (MoAb RU1) covalently linked to Sepharose (Pharmacia LKB Biotechnology, Uppsala, Sweden). Factor VIII was incubated with vWF-RU1-Sepharose and unbound factor VIII was separated from bound factor VIII by centrifugation. The amount of bound factor VIII was determined from the decrease of factor VIII activity in the supernatant. Factor VIII binding to vWF-RU1-Sepharose conformed to the Langmuir model for independent binding sites with a Kd of 0.46 +/- 0.12 nmol/L, and a stoichiometry of 1.3 factor VIII molecules per vWF monomer at saturation, suggesting that each vWF subunit contains a binding site for factor VIII. Competition experiments were performed with a recombinant vWF (deltaA2-rvWF), lacking residues 730 to 910 which contain the epitope for MoAB RU1. DeltaA2-rvWF effectively displaced previously bound factor VIII, confirming that factor VIII binding to vWF-RU1-Sepharose was reversible. To determine the association rate constant (k(on)) and the dissociation rate constant (k(off)), factor VIII was incubated with vWF-RU1-Sepharose for various time intervals. The observed association kinetics conformed to a simple bimolecular association reaction with k(on) = 5.9 +/- 1.9 x 10(6) M(-1) s(-1) and k(off) = 1.6 +/- 1.2 x 10(-3) s(-1) (mean +/- SD). Similar values were obtained from the dissociation kinetics measured after dilution of preformed factor VIII-vWF-RU1-Sepharose complexes. Identical rate constants were obtained for factor VIII binding to vWF from normal pooled plasma and to vWF from plasma of patients with hemophilia A. The kinetic parameters in this report allow estimation of the time needed for complex formation in vivo in healthy individuals and in patients with hemophilia A, in which monoclonally purified or recombinant factor VIII associates with endogenous vWF. Using the plasma concentration of vWF (50 nmol/L in monomers) and the obtained values for K(on) and K(off), the time needed to bind 50% of factor VIII is approximately 2 seconds.
    Print ISSN: 0006-4971
    Electronic ISSN: 1528-0020
    Topics: Biology , Medicine
    Published by American Society of Hematology
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  • 10
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    Inhibition of the intrinsic factor X activating complex by protein S: evidence for a specific binding of protein S to factor VIII (1995)
    American Society of Hematology
    Details
    Publication Date: 1995-08-01
    Description: Protein S is a vitamin K-dependent nonenzymatic anticoagulant protein that acts as a cofactor to activated protein C. Recently it was shown that protein S inhibits the prothrombinase reaction independent of activated protein C. In this study, we show that protein S can also inhibit the intrinsic factor X activation via a specific interaction with factor VIII. In the presence of endothelial cells, the intrinsic activation of factor X was inhibited by protein S with an IC50 value of 0.28 +/- 0.04 mumol/L corresponding to the plasma concentration of protein S. This inhibitory effect was even more pronounced when the intrinsic factor X activation was studied in the presence of activated platelets (IC50 = 0.15 +/- 0.02 mumol/L). When a nonlimiting concentration of phospholipid vesicles was used, the plasma concentration of protein S (300 nmol/L) inhibited the intrinsic factor X activation by 40%. Thrombin-cleaved protein S inhibited the endothelial cell-mediated factor X activation with an IC50 similar to that of native protein S (0.26 +/- 0.02 mumol/L). Protein S in complex with C4b-binding protein inhibited the endothelial cell-mediated factor X activation more potently than protein S alone (IC50 = 0.19 +/- 0.03 mumol/L). Using thrombin activated factor VIII, IC50 values of 0.53 +/- 0.09 mumol/L and 0.46 +/- 0.10 mumol/L were found for native protein S and thrombin-cleaved protein S, respectively. The possible interactions of protein S with factor IXa, phospholipids, and factor VIII were investigated. The enzymatic activity of factor IXa was not affected by protein S, and interaction of protein S with the phospholipid surface could not fully explain the inhibitory effect of protein S on the factor X activation. Using a solid-phase binding assay, we showed a specific, saturable, and reversible binding of protein S to factor VIII with a high affinity. The concentration of protein S where half-maximal binding was reached (B1/2max) was 0.41 +/- 0.06 mumol/L. A similar affinity was found for the interaction of thrombin-cleaved protein S with factor VIII (B1/2max = 0.40 +/- 0.04 mumol/L). The affinity of the complex protein S with C4B-binding protein appeared to be five times higher (B1/2max = 0.07 +/- 0.03 mumol/L). Because the affinities of the interaction of the different forms of protein S with factor VIII correspond to the IC50 values observed for the intrinsic factor X activating complex, the interaction of protein S with factor VIII may explain the inhibitory effect of protein S on the intrinsic factor X activating complex.(ABSTRACT TRUNCATED AT 400 WORDS)
    Print ISSN: 0006-4971
    Electronic ISSN: 1528-0020
    Topics: Biology , Medicine
    Published by American Society of Hematology
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