Structures of tetrasilylmethane derivatives (XMe2Si)2C(SiMe3)2 (X = H, Cl, Br) in the gas phase, and their dynamic structures in solution

The structures of the molecules (XMe2Si)2C(SiMe3)2, where X = H, Cl, Br, have been determined by gas electron diffraction (GED) using the SARACEN method of restraints, with all analogues existing in the gas phase as mixtures of C1- and C2-symmetric conformers. Variable temperature 1H and 29Si solution-phase NMR studies, as well as 13C NMR and 1H/29Si NMR shift correlation and 1H NMR saturation transfer experiments for the chlorine and bromine analogues, are reported. At low temperatures in solution there appear to be two C1 conformers and two C2 conformers, agreeing with the isolated-molecule calculations used to guide the electron diffraction refinements. For (HMe2Si)2C(SiMe3)2 the calculations indicated six conformers close in energy, and these were modeled in the GED refinement

Crystal, Solution and In silico Structural Studies of Dihydrodipicolinate Synthase from the Common Grapevine

Dihydrodipicolinate synthase (DHDPS) catalyzes the rate limiting step in lysine biosynthesis in bacteria and plants. The structure of DHDPS has been determined from several bacterial species and shown in most cases to form a homotetramer or dimer of dimers. However, only one plant DHDPS structure has been determined to date from the wild tobacco species, Nicotiana sylvestris (Blickling et al. (1997) J. Mol. Biol. 274, 608–621). Whilst N. sylvestris DHDPS also forms a homotetramer, the plant enzyme adopts a ‘back-to-back’ dimer of dimers compared to the ‘head-to-head’ architecture observed for bacterial DHDPS tetramers. This raises the question of whether the alternative quaternary architecture observed for N. sylvestris DHDPS is common to all plant DHDPS enzymes. Here, we describe the structure of DHDPS from the grapevine plant, Vitis vinifera, and show using analytical ultracentrifugation, small-angle X-ray scattering and X-ray crystallography that V. vinifera DHDPS forms a ‘back-to-back’ homotetramer, consistent with N. sylvestris DHDPS. This study is the first to demonstrate using both crystal and solution state measurements that DHDPS from the grapevine plant adopts an alternative tetrameric architecture to the bacterial form, which is important for optimizing protein dynamics as suggested by molecular dynamics simulations reported in this study

Spectroscopic Analyses of Aromatic-hydrocarbons Extracted From Naturally and Artificially Matured Coals

Experimental simulation of Mahakam delta (Indonesia) coals maturation has been carried out in a confined system under pressure. Chloroformic extracts have been fractionated by liquid chromatography. Aromatic hydrocarbons have been concentrated and analyzed by FTIR spectroscopy (Nicolet 20 SX B), high-resolution proton NMR (Brucker AM 500 MHz), and synchronous excitation-emission UV fluorescence (Perkin-Elmer LS 50). Band assignment was made in order to take into account the evolution of the proton NMR, FTIR, and synchronous UV fluorescence spectra with respect to bibliographic data and spectral resolution. Results have been compared to those of a natural homogeneous series of coals of increasing rank. The corrected organic carbon content (% COC = % C x 100/(% C + % H + % O + % N)) was chosen as a common maturity index for both natural and artificial series. The progressive removal of aliphatic chains, the increase in the proton aromaticity, and the structural rearrangement are similar for both series. However, some significant discrepancies are noticed between the artificial and natural series. They mostly concern the rate of aromatization, the intensity of ring condensation, and the amount of extractable hydrocarbons

How do diamonds grow in metal melt together with silicate minerals? An experimental study of diamond morphology

The origin and evolution of metal melts in the Earth's mantle and their role in the formation of diamond are the subject of active discussion. It is widely accepted that portions of metal melts in the form of pockets can be a suitable medium for diamond growth. This raises questions about the role of silicate minerals that form the walls of these pockets and are present in the volume of the metal melt during the growth of diamonds. The aim of the present work was to study the crystallization of diamond in a complex heterogeneous system: metal-melt–basalt–carbon. The experiments were performed using a multianvil high-pressure apparatus of split-sphere type (BARS) at a pressure of 5.5&thinsp;GPa and a temperature of 1500&thinsp;∘C. The results demonstrated crystallization of diamond in metal melt together with garnet and clinopyroxene, whose chemical compositions are similar to those of eclogitic inclusions in natural diamond. We show that the presence of silicates in the crystallization medium does not reduce the chemical ability of metal melts to catalyze the conversion of graphite into diamond, and, morphologically, diamond crystallizes mainly in the form of a cuboctahedron. When the content of the silicate material in the system exceeds 5&thinsp;wt&thinsp;%, diamond forms parallel-growth aggregates, but 15&thinsp;wt&thinsp;% of silicate phases block the crystallization chamber, preventing the penetration of metallic melt into them, thus interrupting the growth of diamond. We infer that the studied mechanism of diamond crystallization can occur at lower-mantle conditions but could also have taken place in the ancient continental mantle of the Earth, under reducing conditions that allowed the stability of Fe–Ni melts.</p

Configuration and C-13 Nmr-spectra of Alkylcyclopropane Derivatives

Eight alkylcyclopropane derivatives were prepared in a conventional manner and analysed by 13C NMR spectroscopy. Additivity parameters were calculated from the chemical shifts of the endocyclic carbons, and the configurational structures derived for these compounds are confirmed by the internal coherence found for this set of parameters

Analyse conformationnelle et spectres RMN

Dans un domaine de température compris entre — 60 et + 60°C, les diméthyl-3,4 méthoxycarbonyl-3 pyrazoline-Δ 1 et diphényl- 3,4 méthoxycarbonyl-3 pyrazoline-Δ1 subissent une variation de la constante de couplage interprotonique J4,5 trans directement significative d'un équilibre entre deux conformations enveloppe. Pour ces molécules et d'autres pyrazolines-1 différemment substituées, nous avons calculé les populations de conformères suivant un schéma qui comporte les étapes suivantes : recherche des coefficients d'une équation de Karplus sur base de constantes de couplage correspondant à des angles dièdres mesurés par radiocristallographie . calcul, à l'aide de cette équation, de l'angle d'enveloppe relié à la constante de couplage J4,5 cis et des constantes de couplage J4,5 trans propres à chaque conformère ; détermination de la position de l’équilibre conformationnel à partir de la constante de couplage J4,5 trans expérimentale. Les angles d'enveloppe et la répartition des conformères varient d’une pyrazoline à l'autre, en fonction de la nature et de la configuration des substituants

Comparative NMR study in nematic phases of the bicyclic compounds: 1,4-naphthoquinone, 2,3-dichloro-1,4-naphthoquinone and naphthalene

An NMR study of 1,4-naphthoquinone, 2,3-dichloro-1,4-naphthoquinone dissolved in EBBA is reported. Comparison of the structural results with those previously obtained (see ibid., vol.17, p.225 (1973)) for 1,4-naphthoquinone in /b p/,/b p/'-n-hexyloxyazoxybenzene and naphthalene in EBBA is given. It is concluded that important solute-solvent interactions exist, which affect the molecular structure and the orientation of the dissolved molecule.Anglai