Lanthanide-binding peptides with two pendant aminodiacetate arms: impact of the sequence on chelation

Lanthanide complexes with a series of hexapeptides-incorporating two unnatural chelating amino acids with aminodiacetate groups, Ada(1) and Ada(2)-have been examined in terms of their speciation, structure, stability and luminescence properties. Whereas Ada(2) acts as a tridentate donor in all cases, Ada(1) may act as a tetradentate donor thanks to the coordination of the amide carbonyl function assisted by the formation of a six-membered chelate ring. The position of the Ada(1) residue in the sequence is demonstrated to be critical for the lanthanide complex speciation and structure. Ada(1) promotes the coordination of the backbone amide function to afford a highly dehydrated Ln complex and an S-shape structure of the peptide backbone, only when found in position 2.     

Using immobilized enzymes to reduce RNase contamination in RNase mapping of transfer RNAs by mass spectrometry

RNase (ribonuclease) mapping by nucleobase-specific endonucleases combined with mass spectrometry (MS) is a powerful analytical method for characterizing ribonucleic acids such as transfer RNAs. Typical free solution enzymatic digestion of RNA samples results in a significant amount of RNase being present in the sample solution analyzed by MS. In some cases, the RNase can lead to contamination of the high performance liquid chromatography and MS instrumentation. Here we investigate and compare several different approaches for reducing or eliminating contaminating RNase from the digested RNA sample before LC-MS analysis. Approaches using immobilized RNases were found to be most effective, with no enzyme carryover into the digested sample detected. Among the various options for immobilized RNases, we show that carbodiimide-based reactions can be used to couple RNases to carboxylic acid-terminated magnetic beads. The immobilized enzymes retain biological activity, are re-usable, and do not interfere with subsequent LC-MS analysis of the expected RNase digestion products. The use of immobilized RNases provides a simple approach for eliminating enzyme contamination in mass spectrometry-based RNase mapping experiments.     

Model peptides based on the binding loop of the copper metallochaperone Atx1: selectivity of the consensus sequence MxCxxC for metal ions Hg(II), Cu(I), Cd(II), Pb(II), and Zn(II)

The amino acid sequence MxCxxC is conserved in many soft-metal transporters that are involved in the control of the intracellular concentration of ions such as Cu(I), Hg(II), Zn(II), Cd(II), and Pb(II). A relevant task is thus the selectivity of the motif MxCxxC for these different metal ions. To analyze the coordination properties and the selectivity of this consensus sequence, we have designed two model peptides that mimic the binding loop of the copper chaperone Atx1: the cyclic peptide P(C) c(GMTCSGCSRP) and its linear analogue P(L) (Ac-MTCSGCSRPG-NH2). By using complementary analytical and spectroscopic methods, we have demonstrated that 1:1 complexes are obtained with Cu(I) and Hg(II), whereas 1:1 and 1:2 (M:P) species are successively formed with Zn(II), Cd(II), and Pb(II). The complexation properties of the cyclic and linear peptides are very close, but the cyclic compound provides systematically higher affinity constants than its unstructured analogue. The introduction of a xPGx motif that forms a type II beta turn in P(C) induces a preorganization of the binding loop of the peptide that enhances the stabilities of the complexes (up to 2 orders of magnitude difference for the Hg complexes). The affinity constants were measured in the absence of any reducing agent that would compete with the peptides and range in the order Hg(II) > Cu(I) > Cd(II) > Pb(II) > Zn(II). This sequence is thus highly selective for Cu(I) compared to the essential ion Zn(II) that could compete in vivo or compared to the toxic ions Cd(II) and Pb(II). Only Hg(II) may be an efficient competitor of Cu(I) for binding to the MxCxxC motif in metalloproteins.     

Self-assembly of soluble anthracene, tetracene and pentacene derivatives

New soluble disubstituted acenes (tetracene, pentacene) have been designed in order to mimic 2,3-alkoxy derivatives of anthracene. They have been shown to self-assemble and gellify a large variety of organic solvents. This process involves the formation of nano-fibres through non-covalent molecular interactions (van der Waals, π-π stacking), weaving a three-dimensional supramolecular network. Efficient transfer of excitation energy through these nano-structures has been evidenced in an anthracene light-harvesting matrix doped with less than 1 mol% of a tetracene energy trap.     

Dielectric relaxations in PEEK by combined dynamic dielectric spectroscopy and thermally stimulated current

The molecular dynamics of a quenched poly(ether ether ketone) (PEEK) was studied over a broad frequency range from 10^?3 to 10⁶ Hz by combining dynamic dielectric spectroscopy (DDS) and thermo-stimulated current (TSC) analysis. The dielectric relaxation losses ε′′_KK has been determined from the real part ε′_T(ω) thanks to Kramers–Kronig transform. In this way, conduction and relaxation processes can be analyzed independently. Two secondary dipolar relaxations, the γ and the β modes, corresponding to non-cooperative localized molecular mobility have been pointed out. The main α relaxation appeared close to the glass transition temperature as determined by DSC; it has been attributed to the delocalized cooperative mobility of the free amorphous phase. The relaxation times of dielectric relaxations determined with TSC at low frequency converge with relaxation times extracted from DDS at high frequency. This correlation emphasized continuity of mobility kinetics between vitreous and liquid state. The dielectric spectroscopy exhibits the α_c relaxation, near 443 K, which has been associated with the rigid amorphous phase confined by crystallites. This present experiment demonstrates coherence of the dynamics of the PEEK heterogeneous amorphous phase between glassy and liquid state and significantly improve the knowledge of molecular/dynamic structure relationships.     

Environmental ageing of aerospace epoxy adhesive in bonded assembly configuration

Thermal analysis (TA) techniques were applied in order to predict the influence of thermal treatment, on the photocatalytic performance of TiO₂ materials prepared via sol–gel method in various temperatures between 250 and 600 °C in different alcohols (methanol/ethanol). Calorimetric results showed that the formation of TiO₂ is faster in methanol than in ethanol. TA patterns showed that slight differences observed in the thermal behavior of the material can affect both its textural and photocatalytic properties. The appearance of the endothermic peaks in the area of 250–450 °C refer to crystallization of amorphous to crystalline phases or to the transformation of the less active rutile to the more active anatase phase. The results obtained from TG/DSC are in accordance to XRD results and SEM images. Thermal treatment affects the photocatalytic properties of the materials. Samples prepared in methanol showed better photocatalytic behavior than those in ethanol while the increase in temperature decreases the effectiveness of the materials.     

Spectre du Laplacien agissant sur les p-formes différentielles et écrasement d'anses

Sans résumé     

Direct formation of fullerene monolayers using [4+2] Diels-Alder cycloaddition

The formation of covalent C(60) monolayers through [4+2] Diels-Alder cycloaddition between C(60) and anthracene monolayers grafted onto a silicon oxide surface was investigated by ellipsometry, fluorescence and by atomic force microscopy.