A theoretical study of radical-only and combined radical/carbocationic mechanisms of arachidonic acid cyclooxygenation by prostaglandin H synthase

Prostaglandin H synthase catalyzes the oxygenation of arachidonic acid into the cyclic endoperoxide, prostaglandin G₂ (PGG₂), and the subsequent reduction of PGG₂ to the corresponding alcohol, prostaglandin H₂ (PGH₂), the precursor of all prostaglandins and thromboxanes. Both radical abstraction by a neighboring tyrosyl radical and combined radical/carbocationic models have been proposed to explain the cyclooxygenase part of this reaction. We have used density functional theory calculations to study the mechanism of the formation of the cyclooxygenated product PGG₂. We found an activation free energy for the initial hydrogen abstraction by the tyrosine radical of 15.6 kcal/mol, and of 14.5 kcal/mol for peroxo bridge formation, in remarkable agreement with the experimental value of 15.0 kcal/mol. Subsequent steps of the radical-based mechanism were found to happen with smaller barriers. A combined radical/carbocation mechanism proceeding through a sigmatropic hydrogen shift was ruled out, owing to its much larger activation free energy of 36.5 kcal/mol. Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00214-003-0476-9. Electronic Supplementary MaterialSupplementary material is available in the online version of this article at Electronic Supplementary Material: Supplementary material is available in the online version of this article at     

The Reological Diagram

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The Reological Diagram

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The relationship between properties of fluorinated graphite intercalates and matrix composition

Inclusion compounds: intercalates of fluorinated graphite matrix with acetonitrile (C₂F_xBr_z·yCH₃CN, x=0.92, 0.87, 0.69 and 0.49, z≈0.01) were synthesized. The kinetics of the thermal decomposition (the 1^st stage of filling→the 2^nd stage of filling) was studied under isothermal conditions. The relationship between intercalates properties and composition and structure of the matrix is discussed. The online version of the original article can be found at     

Excited states of conjugated hydrocarbon radicals using the molecular mechanics – valence bond (MMVB) method

By using a parameterised Heisenberg Hamiltonian coupled to a molecular mechanics force field, excited-state geometries were optimised for three conjugated hydrocarbon radicals: cyclopentadienyl, phenalenyl (perinaphthenyl), and triphenylmethyl. The results are compared with ab initio calculations, and with recent spectroscopic measurements. Electronic Supplementary Material is available in the online version of this article at http://dx.doi.org/10.1007/s00214-003-0461-3     

Errata

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Bücherbesprechungen

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Viscositsmessung an wässerigen Zucker- und Natriumphosphatlösungen

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Notizen Berichtigung

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Ueber die Struktur der von Millikan und Matauch untersuchten Oel- und Quecksilbertropfen

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Adsorptionsversuehe usw.

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Ueber Ionenadsorption an der Wasseroberfl?che

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R?ntgenographische Beobachtungen an einer technischen Probe der Azetylzellulose

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Bestimmung der spezifischen OberflÄche durch Adsorption

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