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Christophe Tribet

Publications and source records attributed to Christophe Tribet.

6 recordsLinked to original sources
PubMed2006

Well-defined nanoparticles formed by hydrophobic assembly of a short and polydisperse random terpolymer, amphipol A8-35.

Amphipols are short amphilic polymers designed for applications in membrane biochemistry and biophysics and used, in particular, to stabilize membrane proteins in aqueous solutions. Amphipol A8-35 was obtained by modification of a short-chain parent polymer (poly(acrylic acid); PAA) with octyl- and isopropylamine, to yield an amphiphilic product with an average molar mass of 9-10 kg x mol(-1) (sodium salt form) and a polydispersity index of 2.0 to 3.1, depending on the source of PAA. The behavior of A8-35 in aqueous buffers was studied by size exclusion chromatography, static and dynamic light scattering, equilibrium and sedimentation velocity analytical ultracentrifugation, and small angle neutron scattering. Despite the variable length of the chains and the random distribution of hydrophobic groups along them, A8-35 self-organizes into well-defined assemblies. The data are best compatible with most of the polymer forming compact assemblies (particles) with a molar mass of approximately 40 kg x mol(-1), a radius of gyration of approximately 2.4 nm, and a Stokes radius of approximately 3.15 nm. Each particle contains, on average, four A8-35 macromolecules and 75-80 octyl chains. Neutron scattering reveals a sharp interface between the particles and water. A minor (approximately 0.1%) mass fraction of the material forms much larger aggregates, whose proportion may increase under certain conditions of preparation or handling, such as low pH. They can be removed by gel filtration.

Chromatography, Gel↗
PubMed2005

Association of octyl-modified poly(acrylic acid) onto unilamellar vesicles of lipids and kinetics of vesicle disruption.

Water-soluble polymers containing a few hydrophobic anchors are known to bind onto lipid vesicles and are used as stabilizers of liposome-based formulas. In contrast, polymers with high hydrophobicity destabilize the lipid bilayers. With macromolecules of intermediate hydrophobic/hydrophilic balance, a gradual sweep of the stabilization-destabilization capacity can be achieved and is considered as promising triggered systems for drug release, although the mechanism of permeabilization and membrane breakage using polymers is essentially conjectural to date. As a model system, we used short octyl-modified poly(acrylic acid)s (MW 8000 g/mol) sensitive to pH, temperature, and ionic strength in conjunction with small unilamellar vesicles mainly comprised of DPPC or egg-PC. Kinetics of vesicle fragmentation was followed using static and dynamic light scattering. Polymer adsorption was studied by nonradiative energy transfer between pyrene-labeled lipids and a naphthalene-modified polymer. The permeability of the vesicles was characterized by calcein leakage experiments. The key findings were (i) the lack of coupling between the density of bound polymer and the rate of disruption and (ii) the qualitative difference depending on whether the polymer contains or not isopropyl side groups. Point i relates to the increase of the rate of polymer adsorption with increasing bulk polymer concentration, while the breakage is essentially unaffected. Point ii relates to the stabilization of large membrane fragments (Stokes radius ca. 40 nm) in the presence of a polymer with no isopropyl side groups, while micelle-like assemblies (Stokes radius 8 nm) containing the lipids are obtained with an isopropyl-containing polymer of similar hydrophobicity. Both polymers display similar efficiency at disrupting small vesicles. The mechanism of polymer-induced disruption appears to differ markedly from the disruption steps now recognized for conventional (molecular) surfactant and is discussed on the basis of data obtained with different membrane fluidity, polymer structure, concentration, and hydrophilicity.

Journal Article↗
PubMed2004

Partial specific volume and solvent interactions of amphipol A8-35.

Amphipols are small amphiphilic polymers that can stabilize and keep soluble membrane proteins in aqueous solutions in the absence of detergent. A prerequisite to solution studies of membrane protein/amphipol complexes is the determination of the partial specific volume v2 and effective charge z of the polymer. The ratio (R) of the buoyant molar masses of particles in D2O and H2O solutions, obtained from sedimentation velocity (sH/sD method) and sedimentation equilibrium experiments, and their contrast match point (CMP), determined in small-angle neutron scattering experiments, depend on v2 and z. When z is known, v2 can be estimated from R with a good accuracy as long as v2 is close to 1. The effects of labile H/D exchange and of polyelectrolyte counter-ion dissociation in general cannot be neglected. The accuracy, advantages, and limits of the sH/sD method have been studied in details using model macromolecules (DNA, protein, and polysaccharide). The sH/sD method appears particularly advantageous for the study of heterogeneous samples. Measurements of density, sD/sH buoyant molar masses in H2O, D2O, and D2(18)O, and CMP of hydrogenated and partially deuterated A8-35, a polyacrylate-based amphipol containing 35 underivatized carboxylates per 100 monomers, led to a consistent description of its buoyancy and charge properties.

Carbon↗
PubMed2002

Allosteric transitions of Torpedo acetylcholine receptor in lipids, detergent and amphipols: molecular interactions vs. physical constraints.

The binding of a fluorescent agonist to the acetycholine receptor from Torpedo electric organ has been studied by time-resolved spectroscopy in three different environments: in native membrane fragments, in the detergent CHAPS, and after complexation by amphipathic polymers ('amphipols'). Binding kinetics was similar in the membrane and in amphipols, demonstrating that the receptor can display unaltered allosteric transitions outside its natural lipid environment. In contrast, allosteric equilibria were strongly shifted towards the desensitized state in CHAPS. Therefore, the effect of CHAPS likely results from molecular interactions rather than from the loss of bulk physical properties of the membrane environment.

Allosteric Regulation↗
PubMedDate not supplied

Ionic strength dependence of protein-polyelectrolyte interactions.

The effect of univalent electrolyte concentration on protein-polyelectrolyte complex formation has been measured by frontal analysis continuous capillary electrophoresis (FACCE) and turbidimetry for the interaction of bovine serum albumin (BSA) with a synthetic hydrophobically modified polyacid, for BSA with (porcine mucosal) heparin (Hp), a highly charged polyanion, and for Hp and insulin. All three highly diverse systems display maxima or plateaus in complex formation in the range of ionic strength 5 < I < 30 mM, confirmed in the case of BSA-Hp by multiple techniques. Similar maxima are reported in the literature, but with little discussion, for BSA-poly(dimethyldiallylammonium chloride), lysozyme-hyaluronic acid, and lysozyme-chondroitin sulfate, always in the I range 5-30 mM. While inversion of salt effect has been discussed specifically for the interaction of gelatin and sodium polystyrenesulfonate with gelatin(28) and with beta-lactoglobulin,(10) the general nature of this phenomenon, regardless of polyelectrolyte origin, molecular weight, and charge sign has not been recognized. The position of the maxima and their occurrence when protein and polyelectrolyte have the same net charge imply that they arise when Debye lengths extend, at low I, beyond half the protein diameter so that addition of salt screens repulsions, as well as attractions. This appears to be a general effect caused by electrostatic repulsions that can coexist simultaneously with hydrophobic interactions. Modeling of protein electrostatics via Delphi is used to visualize this effect for BSA, lysozyme, insulin, and beta-lactoglobulin.

Electrolytes↗