On the mechanism of proton transfer in imidazole

The force fields, in-plane vibrations, and relative intensities of Raman spectra have been calculated and analyzed for the N₁H and N₃H tautomers of imidazole, imidazolium cation, and their model structures. The results obtained for the isolated state of imidazole correspond to the intramolecular mechanism of proton transfer.     

Elucidating the Coordination Chemistry and Mechanism of Biological Nitrogen Fixation

How does the enzyme nitrogenase reduce the inert molecule N₂ to NH₃ under ambient conditions that are so different from the energy‐expensive conditions of the best industrial practices? This review focuses on recent theoretical investigations of the catalytic site, the iron–molybdenum cofactor FeMo‐co, and the way in which it is hydrogenated by protons and electrons and then binds N₂. Density functional calculations provide reaction profiles and activation energies for possible mechanistic steps. This establishes a conceptual framework and the principles for the coordination chemistry of FeMo‐co that are essential to the chemical mechanism of catalysis. The model advanced herein explains relevant experimental data.     

Stabilisation of Methylene Radicals by Cob(II)alamin in Coenzyme B12 Dependent Mutases

Coenzyme B₁₂ initiates radical chemistry in two types of enzymatic reactions, the irreversible eliminases (e.g., diol dehydratases) and the reversible mutases (e.g., methylmalonyl‐CoA mutase). Whereas eliminases that use radical generators other than coenzyme B₁₂ are known, no alternative coenzyme B₁₂ independent mutases have been detected for substrates in which a methyl group is reversibly converted to a methylene radical. We predict that such mutases do not exist. However, coenzyme B₁₂ independent pathways have been detected that circumvent the need for glutamate, β‐lysine or methylmalonyl‐CoA mutases by proceeding via different intermediates. In humans the methylcitrate cycle, which is ostensibly an alternative to the coenzyme B₁₂ dependent methylmalonyl‐CoA pathway for propionate oxidation, is not used because it would interfere with the Krebs cycle and thereby compromise the high‐energy requirement of the nervous system. In the diol dehydratases the 5′‐deoxyadenosyl radical generated by homolysis of the carbon–cobalt bond of coenzyme B₁₂ moves about 10 Å away from the cobalt atom in cob(II )alamin. The substrate and product radicals are generated at a similar distance from cob(II )alamin, which acts solely as spectator of the catalysis. In glutamate and methylmalonyl‐CoA mutases the 5′‐deoxyadenosyl radical remains within 3–4 Å of the cobalt atom, with the substrate and product radicals approximately 3 Å further away. It is suggested that cob(II )alamin acts as a conductor by stabilising both the 5′‐deoxyadenosyl radical and the product‐related methylene radicals.     

Formation of Tertiary 3-Nitro Allylic Alcohol by Condensation of 2-Chloroiso- butyrophenone with Nitromethanide Anion: Estimation of Gibbs Free Enthalpies of Reaction in Solution for the Reaction Mechanism [1]

Summary. The reaction of 2-chloroisobutyrophenones and nitromethanide anion gives stereoselectively (E)-3-nitro allylic alcohols. The Gibbs free enthalpies of reaction in DMSO for carbanion addition, epoxide formation, and rearrangement to 3-nitro allylic alcohol, as elementary steps for the reaction, were estimated from corresponding neutral gas reactions and using a thermodynamical approach to the transfer of gaseous compounds to DMSO. A criterion for assigning the sign of affinity of liquid compounds to DMSO was developed on the basis of the Gibbs enthalpies of liquefaction. The information obtained on reaction rate and thermodynamic viability of the steps indicates that carbanion addition is the rate-determining step.In memory of Prof. Dr. M. Ballester, deceased on April 6, 2005     

用WAXD和SAXS研究辐照聚酰胺1010结构

用ＷＡＸＤ和ＳＡＸＳ方法研究了聚酰胺１０１０的聚集态结构和结晶结构的辐射损伤过程和机理。辐射交联与裂解主要在非晶区和结晶表面或中间相发生；辐射损伤并从这里开始，随辐射剂量增大，非晶化逐渐向晶区伸展，有氢键的（０１０）晶面损伤几率高于（１００）晶面；辐射后效应导致新的交联与裂解生成，主要影响（０１０）晶面的形成与完善，使Ｌ１００及Ｗｃ变小。     

Antioxidative effects of flavonols and their glycosides against the free-radical-induced peroxidation of linoleic acid in solution and in micelles

The antioxidative effect of flavonols and their glycosides against the peroxidation of linoleic acid has been studied in homogeneous solution (tBuOH/H(2)O, 3:2) and in sodium dodecyl sulfate and cetyl trimethylammonium bromide micelles. The peroxidation was initiated thermally by the water-soluble initiator 2,2'-azobis(2-methylpropionamidine) dihydrochloride, and the reaction kinetics were studied by monitoring the formation of linoleic acid hydroperoxides. The synergistic antioxidant effect of the flavonols with alpha-tocopherol (vitamin E) was also studied by following the decay kinetics of alpha-tocopherol and the alpha-tocopheroxyl radical. Kinetic analysis of the antioxidative process demonstrates that the flavonols are effective antioxidants in solution and in micelles, either alone or in combination with alpha-tocopherol. The antioxidative action involves trapping the initiating radicals in solution or in the bulk-water phase of the micelles, trapping the propagating lipid peroxyl radicals on the surface of the micelles, and regenerating alpha-tocopherol by reducing the alpha-tocopheroxyl radical. It was found that the antioxidant activity of the flavonols and their glycosides depends significantly on the position and number of the hydroxy groups, the oxidation potential of the molecule, and the reaction medium. The flavonols bearing ortho-dihydroxy groups possess significantly higher antioxidative activity than those without such functionalities, and the glycosides are less active than their parent aglycones. The activity of the flavonols is higher in micelles than in solution, while the activity of alpha-tocopherol is lower in micelles than in solution. This is because the predominant factor for controlling the activity is the hydrogen-bonding interaction of the antioxidant with the micellar surface in the case of hydrophilic flavonols, while it is the inter- and intramicellar diffusion in the case of lipophilic alpha-tocopherol.     

Ring-closing olefin metathesis on ruthenium carbene complexes: model DFT study of stereochemistry

Ring-closing metathesis (RCM) is the key step in a recently reported synthesis of salicylihalamide and related model compounds. Experimentally, the stereochemistry of the resulting cycloolefin (cis/trans) depends strongly on the substituents that are present in the diene substrate. To gain insight into the factors that govern the observed stereochemistry, density functional theory (DFT) calculations have been carried out for a simplified dichloro(2-propylidene)(imidazole-2-ylidene)ruthenium catalyst I, as well as for the real catalyst II with two mesityl substituents on the imidazole ring. Four model substrates are considered, which are closely related to the systems studied experimentally, and in each case, two pathways A and B are possible since the RCM reaction can be initiated by coordination of either of the two diene double bonds to the metal center. The first metathesis yields a carbene intermediate, which can then undergo a second metathesis by ring closure, metallacycle formation, and metallacycle cleavage to give the final cycloolefin complex. According to the DFT calculations, the stereochemistry is always determined in the second metathesis reaction, but the rate-determining step may be different for different catalysts, substrates, and pathways. The ancillary N-heterocyclic carbene ligand lies in the Ru-Cl-Cl plane in the simplified catalyst I, but is perpendicular to it in the real catalyst II, and this affects the relative energies of the relevant intermediates and transition states. Likewise, the introduction of methyl substituents in the diene substrates influences these relative energies appreciably. Good agreement with the experimentally observed stereochemistry is only found when using the real catalyst II and the largest model substrates in the DFT calculations.     

Hydrothermal synthesis, structure, Raman spectroscopy, and self-irradiation studies of Cm(IO3)3

The study of curium iodate, Cm(IO₃)₃, was undertaken as part of a systematic investigation of the 4f- and 5f-elements’ iodates. The reaction of ²⁴⁸CmCl₃ with aqueous H₅IO₆ under mild hydrothermal conditions results in the reduction of IO₆⁵⁻ to IO₃⁻ anions, and the subsequent formation of Cm(IO₃)₃ single crystals. Crystallographic data are: (193 K, MoKα, ): monoclinic, space group P2₁/c, , , , β=100.142(2)°, V=811.76(14), Z=4, R(F)=2.11%, for 119 parameters with 1917 reflections with I>2σ(I). The structure consists of Cm³⁺ cations bound by iodate anions to form [Cm(IO₃)₈] units, where the local coordination environment around the curium centers can be described as a distorted dodecahedron. There are three crystallographically unique iodate anions within the structure; two iodates bridge between three Cm centers, and one iodate bridges between two Cm centers and has a terminal oxygen atom. The bridging of the curium centers by the iodate anions creates a three-dimensional structure. Three strong Raman bands with comparable intensities were observed at 846, 804, and 760 cm⁻¹ and correspond to the I-O symmetric stretching of the three crystallographically distinct iodate ions. The Raman profile suggests a lack of inter-ionic vibrational coupling of the I-O stretching, while intra-ionic coupling provides symmetric and asymmetric components that correspond to each iodate site. Repeated collection of X-ray diffraction data for a crystal of Cm(IO₃)₃ over a period of time revealed a gradual expansion of the unit cell from self-irradiation. After 71 days, the new parameters were: , , , β=100.021(2)°, V=818.3(2).     

Mechanisms and kinetics of polymerization of thermoplastic polyimides. II. Study of “bridged” dianhydride/aromatic amine systems

The aim of the present work was to identify and follow the main and side reactions involved in the ring dehydration of amic acid prepared from “bridged” dianhydrides whose central substituent is an electron acceptor or donor, and an aromatic diamine. Several isomeric structures may appear as a result of the opening reactivity and selectivity of anhydride groups towards the aromatic amine. Reaction mechanisms and kinetics were thus studied in solvent phase with HPLC and ¹³C-NMR and in solid molten phase by FTIR and solid ¹³C-NMR. The experimental conditions (liquid and solid) and the structure of the products (type of central substituent) affecting the mechanisms and kinetics of the reactions were noted. © 1993 John Wiley & Sons, Inc.     

UV spectra and excitation delocalization in DNA: influence of the spectral width.

The singlet excited states of the model DNA duplex (dA)10.(dT)10 are studied. Calculations are performed in the exciton theory framework. Molecular dynamics calculations provide the duplex geometry. The dipolar coupling is determined using atomic transition charges. The monomer transition energies are simulated by Gaussian functions resembling the absorption bands of nucleosides in aqueous solutions. Most of the excited states are found to be delocalized over at least two bases and result from the mixing of different monomer states. Their properties are only weakly affected by conformational changes of the double helix. On average, the highest oscillator strength is carried by the upper eigenstates. The duplex absorption spectra are shifted a few nanometers to higher energies with respect to the spectra of noninteracting monomers. The states with larger spatial extent are located close to the maximum of the absorption spectrum.