Un nouveau préconditionneur pour les problèmes elliptiques à coefficients variablesA new preconditonner for general elleptic problems

In this Note we describe a preconditionner for iteratively solving the linear system arising from the discretization of a general nonseparable elliptic problem by spectral element method. This preconditionner is constructed from approximating the original problem with the closest (in some sense) separable elliptic problem. A direct method is then used to invert the preconditionner. To cite this article: M. Azaïez et al., C. R. Mecanique 331 (2003).     

Effets d'une modulation en phase de température à la frontière sur l'instabilité convective d'une couche liquide viscoélastiqueEffects of a temperature modulation in phase at the frontier on the convective instability of a viscoelastic layer

In this Note we study the effects of the temperature modulation, applied at the horizontal boundaries, on the onset of convection of a horizontal liquid Maxwellian layer. It is assumed that the temperature imposed features a steady component and a time dependent component. To analyse the effect of the temperature modulation, the study is restricted to a linear stability analysis. Thus the Floquet theory and a technique of converting a boundary value problem to an initial value problem are used to solve the system of equations corresponding to the onset of convection. Results obtained may be used to characterize the influence of modulation effects and that of the viscoelastic nature of liquid on the critical Rayleigh number. To cite this article: B. Oukada et al., C. R. Mecanique 334 (2006).     

Intermolecular condensation products formed during the pyrolysis of peptides

Mass Spectrometry (MS) analysis of pyrolysis products of simple peptides has revealed several non-volatile thermal degradation products at masses lower than the precursor peptide. In addition to these products, many other signals were also observed at higher masses than the precursor peptide, and their characterization is the focus of this study. Here we report on the observation of homo and hetero condensation peptide products formed during the pyrolysis of peptides. The observed peptide condensation products are formed between two, three or even four peptides. Tandem MS (MS/MS) analyses of these products showed that C-terminal to N-terminal intermolecular bonding is preferred during pyrolysis when combining two peptides, rather than involving crosslinking between basic and acidic side chain groups like arginine and aspartic acid. These observations are rationalized by steric hindrance effect and known pK_a values of the peptide C- and N-termini and amino acid side groups like aspartic acid and arginine. Pyrolysis of a standard N-acetylated peptide showed no detectable condensation and/or crosslinked products, even in peptides with basic side groups, providing further evidence for the C-terminus to N-terminus intermolecular bonding between peptides under pyrolytic conditions.     

Rapid isocratic HPLC investigation of radiochemical purity for <Superscript>90</Superscript>Y-DOTATATE

DOTA-conjugated peptides, such as [DOTA⁰, Tyr³]ocreotide (DOTATOC) and [DOTA⁰, Tyr³]octreotate (DOTATATE) can be labeled with radionuclides such as ⁹⁰Y, ¹⁷⁷Lu and ¹¹¹In at high specific activities. These radiolabelled somatostatin analogues are used for peptide receptor radionuclide therapy (PRRT). Currently, radio-high performance liquid chromatography (radio-HPLC) and radio-thin layer chromatography (radio-TLC) are the methods of choice for the analysis of the labeled compounds. In literature, radiochemical purity of the radiolabelled DOTATATE was investigated using gradient reversed-phase radio-HPLC. However, these studies indicate long retention time of the radiolabelled compound of 14.52 min. In our study, a new simple and rapid reversed-phase isocratic system enables the radiochemical purity of the radiolabelled DOTATATE within a few minutes.     

HPLC investigation of the radiochemical purity of <Superscript>99m</Superscript>Tc-MIBI

Technetium-99m-sestamibi (^99mTc-MIBI) is a small, lipophilic and cationic compound used for myocardial perfusion imaging. In addition, ^99mTc-MIBI has been shown to be useful in identifying several types of tumors, such as breast, lung and thyroid cancers. The high quality of this radiopharmaceutical and its uniformity are very important facts for application of this preparation in clinical practice. The monograph for ^99mTc-MIBI recommends at least 90% radiochemical purity (RCP) for clinical use. Various factors may influence the RCP of certain reagent kits. Some of these include the amount of activity added to the reagent kit, heating time and the age of the formulated kit. The effect of these factors on RCP of ^99mTc-MIBI has been investigated using high performance liquid chromatography (HPLC) and instant thin layer chromatography.     

Comparative studies of nontoxic and toxic amyloids interacting with membrane models at the air-water interface

Many in vitro studies have pointed out the interaction between amyloids and membranes, and their potential involvement in amyloid toxicity. In a previous study, we generated a yeast toxic mutant (M8) of the harmless model amyloid protein HET-s((218-289)). In this study, we compared the self-assembling process of the nontoxic wild-type (WT) and toxic (M8) protein at the air-water interface and in interaction with various phospholipid monolayers (DOPE, DOPC, DOPI, DOPS and DOPG). We first demonstrate using ellipsometry measurements and polarization-modulated infrared reflection absorption spectroscopy (PMIRRAS) that the air-water interface promotes and modifies the assembly of WT since an amyloid-like film was instantaneously formed at the interface with an antiparallel β-sheet structuration instead of the parallel β-sheet commonly observed for amyloid fibers generated in solution. The toxic mutant (M8) behaves in a similar manner at the air-water interface or in bulk, with a fast self-assembling and an antiparallel β-sheet organization. The transmission electron microscopy (TEM) images established the fibrillous morphology of the protein films formed at the air-water interface. Second, we demonstrate for the first time that the main driving force between this particular fungus amyloid and membrane interaction is based on electrostatic interactions with negatively charged phospholipids (DOPG, DOPI, DOPS). Interestingly, the toxic mutant (M8) clearly induces perturbations of the negatively charged phospholipid monolayers, leading to a massive surface aggregation, whereas the nontoxic (WT) exhibits a slight effect on the membrane models. This study allows concluding that the toxicity of the M8 mutant could be due to its high propensity to interact with membranes.     

Characterization and Identification of a Chymotryptic Hydrolysate of Alpha-Lactalbumin Stimulating Cholecystokinin Release in STC-1 Cells

Alpha-lactalbumin hydrolysate is of significant interest, due to its potential application as a source of bioactive peptides in nutraceutical and pharmaceutical domains. This study was focused on the cholecystokinin (CCK) family compounds which are small peptides involved in the satiety control. The action of chymotryptic hydrolysate of alpha-lactalbumin on cholecystokinin release from intestinal endocrine STC-1 cells was investigated. We demonstrated for the first time that a chymotryptic hydrolysate of alpha-lactalbumin was able to highly stimulate CCK-releasing activity from STC-1 cells. The peptidic hydrolysate was characterized by LC/MS and MS/MS, thus highlighting the presence of 11 fractions containing 21 peptides, each potentially having the desired activity.     

A peptidomic approach for monitoring and characterising peptide cyanotoxins produced in Italian lakes by matrix-assisted laser desorption/ionisation and quadrupole time-of-flight mass spectrometry

In recent years, the occurrence of cyanobacterial blooms in eutrophic freshwaters has been described all over the world, including most European countries. Blooms of cyanobacteria may produce mixtures of toxic secondary metabolites, called cyanotoxins. Among these, the most studied are microcystins, a group of cyclic heptapeptides, because of their potent hepatotoxicity and activity as tumour promoters. Other peptide cyanotoxins have been described whose structure and toxicity have not been thoroughly studied. Herein we present a peptidomic approach aimed to characterise and quantify the peptide cyanotoxins produced in two Italian lakes, Averno and Albano. The procedure was based on matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry mass spectrometry (MALDI-TOF-MS) analysis for rapid detection and profiling of the peptide mixture complexity, combined with liquid chromatography/electrospray ionisation quadrupole time-of- flight tandem mass spectrometry (LC/ESI-Q-TOF-MS/MS) which provided unambiguous structural identification of the main compounds, as well as accurate quantitative analysis of microcystins. In the case of Lake Averno, a novel variant of microcystin-RR and two novel anabaenopeptin variants (Anabaenopeptins B(1) and Anabaenopeptin F(1)), presenting homoarginine in place of the commonly found arginine, were detected and characterised. In Lake Albano, the peculiar peptide patterns in different years were compared, as an example of the potentiality of the peptidomic approach for fast screening analysis, prior to fine structural analysis and determination of cyanotoxins, which included six novel aeruginosin variants. This approach allows for wide range monitoring of cyanobacteria blooms, and to collect data for evaluating possible health risks to consumers, through the panel of the compounds produced along different years.     

Selective adsorption of proteins on single-wall carbon nanotubes by using a protective surfactant

The dispersion of highly hydrophobic carbon materials such as carbon nanotubes in biological media is a challenging issue. Indeed, the nonspecific adsorption of proteins occurs readily when the nanotubes are introduced in biological media; therefore, a methodology to control adsorption is in high demand. To address this issue, we developed a bifunctional linker derived from pyrene that selectively enables or prevents the adsorption of proteins on single-wall carbon nanotubes (SWNTs). We demonstrated that it is possible to decrease or completely suppress the adsorption of proteins on the nanotube sidewall by using proper functionalization (either covalent or noncovalent). By subsequently activating the functional groups on the nanotube derivatives, protein adsorption can be recovered and, therefore, controlled. Our approach is simple, straightforward, and potentially suitable for other biomolecules that contain thio or amino groups available for coupling.