Covalent modification of calcium hydroxyapatite surface by grafting phenyl phosphonate moieties

The reaction between phenyl phosphonic dichloride (C₆H₅P(O)Cl₂) and synthetic calcium hydroxy- and fluorapatite has been investigated. The presence of mono- or polymeric (C₆H₅PO) fragment bound to hydroxyapatite was evidenced by IR, and solid-state ³¹P NMR spectroscopy. X-ray powder analysis has shown that the apatitic structure remains unchanged during the reaction. In contrast, no reaction was found using fluorapatite. According to the results found for these two different apatites a mechanism was proposed for the formation of covalent P-O-P bonds as the result of a reaction between the C₆H₅P(O)Cl₂ organic reagent and (HPO₄)⁻ and/or OH⁻ ions of the hydroxyapatite.     

Photodissociation of heme distal methionine in ferrous cytochrome C revealed by subpicosecond time-resolved resonance Raman spectroscopy

Cytochrome c (cyt c) is an electron-transfer heme protein that also binds nitric oxide (NO). In resting cyt c, two endogenous ligands of the heme iron are histidine-18 (His) and methionine-80 (Met) side chains, and NO binding requires the cleavage of one of the axial bonds. Previous femtosecond transient absorption studies suggested the photolysis of either Fe-His or Fe-Met bonds. We aimed at unequivocally identifying the internal side chain that is photodissociated in ferrous cyt c and at monitoring heme structural dynamics, by means of time-resolved resonance Raman (TR3) spectroscopy with approximately 0.6 ps time resolution. The Fe-His stretching mode at 216 cm-1 has been observed in photoproduct TR3 spectra for the first time for a c-type heme. The same transient mode was observed for a model ferrous cyt c N-fragment (residues 1-56) ligated with two His in the resting state. Our TR3 data reveal that upon ferrous cyt c photoexcitation, (i) distal Met side chain is instantly released, producing a five-coordinated domed heme structure, (ii) proximal His side chain, coupled to the heme, exhibits distortion due to strain exerted by the protein, and (iii) alteration in heme-cysteine coupling takes place along with the relaxation of the protein-induced deformations of the heme macrocycle.     

An approach to the tricyclic core of madangamines based upon a biogenetic scheme

[reaction: see text] A short synthesis of intermediates possessing the tricyclic core of natural madangamines, bioactive alkaloids found in marine sponges, is described. The key reaction entails the condensation of the sodium salt of diethylacetonedicarboxylate with a dihydropyridinium salt derivative. This new approach is modeled on a biogenetic proposal linking madangamines to ircinals, related alkaloids occurring in sponges of the same order.     

Mechanism for resonant O electron stimulated desorption from condensed O2. Non-adiabatic transitions and dynamics

Absolute kinetic energy distributions and yields associated with ground state ³P and excited state ¹D oxygen atoms have been obtained for O⁻ anion electron stimulated desorption from condensed O₂ in the electron energy range 6–15 eV. The observed yields are understood as resulting essentially from dissociative electron attachment reactions via the two lowest ²Σ⁺_g O⁻₂ resonance states through adiabatic and non-adiabatic transitions to the limits O⁻(²P) + O(³P) and O⁻(²P) + O(¹D). The kinetic energy distributions show the prominent role of electron multiple collision processes and post-dissociation interactions of the O⁻ anions in the condensed phase.     

Structure cristalline de SmZrF7. Relations structurales avec le type ReO3

The crystal structure of the fluorozirconate SmZrF₇ has been determined from single crystal X-ray data; the final R value is 0.028. In this compound, isotypic with all the fluorozirconates LnZrF₇ (Ln = rare earth), the symmetry of the cell is monoclinic (space group: P₂₁). The Zr atoms are surrounded by six fluorine atoms forming an almost perfect octahedron: the average distance ZrF is 2.006 Å. The Sm atoms are surrounded by a 8 fluorine atoms polyhedron, the distances SmF lie from 2.221 to 2.411 Å. The association of these two polyhedra by sharing corner forms slabs with an approximately ReO₃ type arrangement: the thickness of the slabs is two octahedra.These slabs with the composition M₂X₇ are held together so that the top fluorine atoms at the surface of a slab increase the coordination of the Sm atom of an adjacent slab. This MX_3.5 phase corresponds to a new structural type between MX₃ (ReO₃ type) and MX₄ (SnF₄ type).     

Monomeric (dialkylamino)boranes: a new and efficient boron source in palladium catalyzed C-B bond formation with aryl halides

Thermal decomposition of hindered amine-borane adducts leads in high yields to monomeric (dialkylamino)boranes R1R2N-BH2 (R1 and R2 = alkyl) that are new and efficient boron-sources in the Pd0 catalyzed borylation reaction affording monomeric aryl(dialkylamino)boranes R1R2N-BHR3 (R3 = aryl).     

The 1,3-benzotellurazole : A new heterocyclic system

We describe the synthesis of a new heterocyclic system : the 1,3-benzotellurazole, and some of its substituted derivatives.     

Effect of conformational changes on a one-electron reduction process: evidence of a one-electron P-P bond formation in a bis(phosphinine)

EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P-P bond (3.479 A), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P-P distance in neutral 1 is large (5.8 A). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P-P single bond (P-P 2.389 A). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C-Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P-P distance. The ability of the phosphinine radical anion to reversibly form weak P-P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P-P bond.     

Fermentation of a skim milk concentrate with Streptococcus thermophilus and chymosin: structure, viscoelasticity and syneresis of gels

Fermentation of a reconstituted skim milk concentrate (8% protein) was investigated to elucidate the effects of various fermentation parameters on the structural, rheological and visual (wheying-off) properties of the resulting gels (pH 4.60). Fermentation trials were performed with non-exocellular polysaccharide-producing strains of Streptococcus thermophilus at various fermentation temperatures and at various chymosin levels. Oscillatory vane rheometry carried out on the intact gels (at 4 °C) showed that the level of chymosin had a significant impact on the gel strength (storage modulus G′). This can be explained by the arrangement of casein micelles into more compact aggregates and the enhanced fusion of aggregated casein micelles as revealed by transmission electron microscopy for the gels fermented with chymosin. Wheying-off of the stirred gels as measured by a centrifugation test (at 4 °C) and pore size of the intact gel structures investigated by scanning electron microscopy both increased with increasing level of chymosin and increasing fermentation temperature (resulting in an increase in acidification rate). A higher level of syneresis (wheying-off) is explained by the larger pore size, since larger pores present a lower resistance to the outflow of whey from the gel.