Lanthanide(III) Complexes with Two Hexapeptides Incorporating Unnatural Chelating Amino Acids: Secondary Structure and Stability

Unnatural metal‐chelating amino acids bearing aminodiacetate side‐chains have been introduced into two hexapeptides to obtain efficient lanthanide‐binding peptides. The synthesis of the enantiopure Fmoc‐Ada_n(tBu)₂‐OH synthons is described with overall yields of 32 and 50 % for n=2 and n=3 side‐chain carbon atoms, respectively. The two peptides AcWAda_nPGAda_nGNH₂ ( P ⁿ ) were synthesized from the protected synthons by standard solid‐phase peptide synthesis. Studies of the lanthanide complexes of the two peptides P ⁿ by luminescence titrations, mass spectrometry, circular dichroism, and solution NMR spectroscopy demonstrate that the Ada_n chain length has a dramatic effect on the complexation properties. Indeed, the flexible compound P³ forms a mononuclear complex of moderate stability (β₁₁=10^9.9), which tends to transform into a binuclear species in the presence of excess of the metal ion. Interestingly, the more compact peptide P² provides stable Ln³⁺ complexes with the exclusive formation of the mononuclear Ln P² adduct. The stability constant of Tb P² is two orders of magnitude higher (β₁₁=10^12.1) than that measured for P³ . The 800 MHz NMR spectrum of the La³⁺ complex of P² evidences a well‐defined type II β‐turn as well as a hydrophobic Trp(indole)–Pro interaction. These interactions exemplify the non‐innocent character of the peptide spacer in the complex La P² as well as the role of a peptide secondary structure in the stabilization of metal complexes.     

Gaps in the differential forms spectrum on cyclic coverings

We are interested in the spectrum of the Hodge–de Rham operator on a -covering X over a compact manifold M of dimension n + 1. Let Σ be a hypersurface in M which does not disconnect M and such that M − Σ is a fundamental domain of the covering. If the cohomology group H ^n/2(Σ) is trivial, we can construct for each a metric g = g _N on M, such that the Hodge–de Rham operator on the covering (X, g) has at least N gaps in its (essential) spectrum. If , the same statement holds true for the Hodge–de Rham operators on p-forms provided .     

Darrieus–Landau instability of premixed flames enhanced by fuel droplets

Recent experiments on spray flames propagating in a Wilson cloud chamber have established that spray flames are much more sensitive to wrinkles or corrugations than single-phase flames. To propose certain elements of explanation, we numerically study the Darrieus–Landau (or hydrodynamic) instability (DL-instability) developing in premixtures that contain an array of fuel droplets. Two approaches are compared: numerical simulation starting from the general conservation laws in reactive media, and the numerical computation of Sivashinsky-type model equations for DL-instability. Both approaches provide us with results in deep agreement. It is first shown that the presence of droplets in fuel–air premixtures induces initial perturbations which are large enough to trigger the DL-instability. Second, the droplets are responsible for additional wrinkles when the DL-instability is developed. The latter wrinkles are of length scales shorter than those of the DL-instability, in such a way that the DL-unstable spray flames have a larger front surface and therefore propagate faster than the single-phase ones when subjected to the same instability.     

Retardation and relaxation behaviour of poly (ether-block-amide)s (PEBA) copolymers by thermally stimulated creep (TSCr) and current (TSCu)

The retardation and relaxation behaviour of poly(ether-block-amide)s PEBA copolymers was investigated as a function of hard polyamide sequence length by using thermally stimulated creep (TSCr) and Current (TSCu). Two primary TSCr and TSCu peaks, called ß_PE and ß_PA were observed in the temperature range 150–300 K. The “low temperature” peak ß_PE was characterized by a peak position T ß_PE which depends poorly upon hard segment content as the corresponding DSC glass transition temperature Tg_PE. The peak position Tß_PA of the “high temperature” relaxation showed a progressive shift towards higher temperatures as the hard segment average length was increased as it is observed in series of pure polyamide oligomers. So, complex spectra suggest that an amorphous phase separation occurs in PEBA copolymers. The TSCr ß_PE and ß_PA modes were analysed by thermal sampling method (TS). The elementary TSCr processes isolated in the ß_PE and ß_PA distributed retardation modes were characterized by retardation times following a compensation law. The comparison of activation and compensation parameters confirmed the existence of a biphasic amorphous phase in PEBA. In TSCr spectra, an additional peak, noted α, is observed at a temperature close to the DSC melting point T_mPE of the poly(tetramethylene glycol) segments.     

Laplaciens en interaction

We study here the asymptotic of the eigenfunctions of the Laplace operator in the situation of adding handles. We show that there is a possibility of resonance between the two limit manifolds only when the difference of their dimension is 1 or 2.     

L'Algorithme doubleQ · R avec ≪shift≫

Sans résumé     

Enhanced Fluorescence Sensing Using Sol-Gel Materials

The versatility of the sol-gel process with regard to the design of optimized films for fluorescence-based sensing is treated in detail. Sol-gel films are typically used to provide a microporous support matrix in which analyte-sensitive fluorophores are entrapped and into which smaller analyte species may diffuse and interact. The versatility of the process facilitates tailoring of the physicochemical film properties to optimize sensor performance. General principles of sensor optimization are presented. These include issues such as immobilization properties, refractive index control, and sensitivity enhancement. The particular advantages conferred by the use of ormosils are emphasized. In addition, the contribution made by tailored sol-gel films to recent advances in specific fluorescence-based sensor applications is emphasized. This work focuses mainly on results produced in the laboratory of the authors over the past decade.     

Hydrolysis of phosphodiesters by diiron complexes: design of nonequivalent iron sites in purple acid phosphatase models

New mu-oxo-diferric complexes have been designed for hydrolysis of phosphodiesters. To mimic the diiron active site of purple acid phosphatase, a combinatorial method has been used to select complexes containing two distinct iron coordination spheres. The introduction of a bidentate ligand, a substituted phenanthroline (L) into complex 1, [Fe2O(bipy)4(OH2)2](NO3)4, generates in solution the complex [Fe2O(bipy)3(L)(OH2)2](NO3)4 as shown by ESI/MS and 1H NMR studies. The latter complex was found to be 20-fold more active than complex 1. On the basis of kinetic studies, we demonstrated that the complex [Fe2O(bipy)3(L)(OH)(OH2)](NO3)3 was the active species and the reaction proceeded through the formation of a ternary complex in which one iron binds a hydroxide and the second, the substrate. At nonsaturating concentrations of the substrate, the increased activity with increased methyl substituents in L was due to an increased affinity of the complex for the substrate. The activity of [Fe2O(bipy)3(33'44'Me2-Phen)(OH2)2](NO3)4 [33'44'Me2Phen = 3,3',4,4'-dimethyl-1,10-phenanthroline] was found to be comparable to that reported for Co(III) or Ce(IV) complexes.     

The Photophysical Properties of Ruthenium Polypyridyl Complexes within a Sol-Gel Matrix

Tetraethoxysilane (TEOS) sol-gel monoliths impregnated with three different luminescent [Ru(L)₃]²⁺ complexes (where L = 2,2-bipyridine (bpy), 1,10-phenanthroline (phen), or 4,7-diphenyl,1,10-phenanthroline (dpp)) have been prepared. The photoluminescence properties of these complexes were followed as a function of time, as the sol-gel reactions proceeded. Clear differences were observed in the emission energies and the emission lifetimes of the complexes upon gel formation. These differences can be explained by the different extent of electrostatic interaction between the surface of the siloxane polymer and the immobilised complex.