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Thomas Zemb

Publications and source records attributed to Thomas Zemb.

12 recordsLinked to original sources
PubMed2024

Rabies Virus Phosphoprotein Exhibits Thermoresponsive Phase Separation with a Lower Critical Solution Temperature.

Rabies virus (RABV) generates membrane-less liquid organelles (Negri bodies) in the cytoplasm of its host cell, where genome transcription and replication and nucleocapsid assembly take place, but the mechanisms of their assembly and maturation remain to be explained. An essential component of the viral RNA synthesizing machine, the phosphoprotein (P), acts as a scaffold protein for the assembly of these condensates. This intrinsically disordered protein forms star-shaped dimers with N-terminal negatively charged flexible arms and C-terminal globular domains exhibiting a large dipole moment. Our study shows that in vitro self-association of RABV P drives a complex thermoresponsive phase separation with a lower critical solution temperature. Protein dimers assemble already below the saturation concentration, and condensation is driven by attractive conformation-specific interactions leading to reentrant liquid phase separation over a narrow range of salt concentration. We propose a minimal molecular model in which P can adopt three limit conformational states and the disordered N-terminal arms control the interactions between giant dipoles that is consistent with our observations.

Rabies virus↗
PubMed2006

Phase behavior, topology, and growth of neutral catanionic reverse micelles.

The ternary catanionic system octylammoniumoctanoate/octane/water is studied by combined SANS, light scattering, conductivity, and phase diagram approach in the water-poor microemulsion region. The sphere-to-cylinder growth and branching depends on the concentration, the water-to-surfactant ratio, and the temperature. The unidimensional growth leads to a network of interconnected wormlike micelles. Like most studied linear nonionic surfactants, in this true catanionic system at equimolarity of anionic and cationic surfactant, the curvature toward water increases with temperature, making connections between cylinders less frequent.

Anions↗
PubMed2006

Switching mechanics with chemistry: a model for the bending stiffness of amphiphilic bilayers with interacting headgroups in crystalline order.

Bilayer structures in catanionic systems experimentally showed peculiar mechanical behavior. The observed increase in the bending stiffness is supposedly connected to additional hydrogen bonds forming between anionic headgroups. With a simple model, we can explain the extreme sensitivity of the bending stiffness of the membrane on the molar ratio of the charged molecules. This effect is further amplified by the sandwichlike structure of the membrane, where the apolar core separating the headgroups acts via a kind of lever-arm principle. As a consequence of these combined effects, the model membrane changes from a soft behavior with bending rigidities on the order of 10k(B)T to an extremely stiff membrane with a bending stiffness more than 2 orders of magnitude larger where most of this change occurs within a molar ratio interval smaller than 0.1.

Computer Simulation↗
PubMed2006

Counter-ion activity and microstructure in polyelectrolyte complexes as determined by osmotic pressure measurements.

We have investigated the activity of counter-ions at 60 degrees C through the osmotic coefficient K in solutions of anionic and cationic polyelectrolyte complexes of variable compositions. For excess of polyanion in the complexes (molar fraction of polycation f < 0.5), K increases as the polyanion is neutralized by the polycation (f getting closer to 0.5). By contrast, for an excess of polycation (f > 0.5), K stays constant or even slightly decreases as the polycation is getting neutralized by the polyanion. This asymmetric behavior depending on the charge of the complexes indicates that the globally negatively charged complexes are homogeneous and can be treated as a single polyelectrolyte of reduced linear charge density. On the other hand, the positively charged complexes show a micro-phase separation between neutral fully compensated microdomains and domains where the excess polycation is locally segregated. These two different microstructures are reminiscent of the coacervation and segregation regimes observed at higher concentrations and salinities, and also of polyelectrolyte complexes with oppositely charged surfactants. This interpretation is supported by two simple predictive models.

Chemistry Techniques, Analytical↗
PubMed2006

Salt screening and specific ion adsorption determine neutral-lipid membrane interactions.

The simplest, single-component biological membrane challenges accepted models of macromolecular interactions: lipid lamellar phases swell when immersed in monovalent salt solutions. Moreover, typical of a Hofmeister series, Br salts swell multilayers more than Cl salts, offering an excellent opportunity to investigate long-standing questions of ionic specificity. In accord with earlier measurements of liposome mobilities in electric fields, we find an added electrostatic repulsion of membranes due to anion binding, with a much stronger Br binding compared with Cl. However, contrary to the expectation that electrostatic repulsion should vanish in high salinity, swelling of lipid multilayers is monotonic with increasing salt concentration for both Br and Cl salts. The apparent contradiction is resolved by recognizing that although the electrostatic repulsion is progressively screened by increasing salt concentration, so is the van der Waals (vdW) attraction. Negligible in low salt, weakening of vdW forces becomes significant by the time electrostatic forces vanish. The result is a smooth monotonic swelling curve with no apparent distinction between low and high salt concentration regimes. Furthermore, when compared with theoretical predictions, measured vdW forces decay much too slowly with added salt. However, by accounting for the recently measured salt deficit near lipid bilayers, the expected scaling with Debye screening length is recovered. The combination of ion-specific binding and nonspecific ionic screening of low-frequency fluctuations explains salt effects on lipid membrane interactions and, by extension, explains specific (Hofmeister) effects at macromolecular interfaces between low and high dielectric.

Adsorption↗
PubMed2006

Surface immobilization and mechanical properties of catanionic hollow faceted polyhedrons.

We report here for the first time on surface immobilization of hollow faceted polyhedrons formed from catanionic surfactant mixtures. We find that electrostatic interaction with the substrate dominates their adhesion behavior. Using polyelectrolyte coated surfaces with tailored charge densities, polyhedrons can thus be immobilized without complete spreading, which allows for further study of their mechanical properties using AFM force measurements. The elastic response of individual polyhedrons can be locally resolved, showing pronounced differences in stiffness between faces and vertexes of the structure, which makes these systems interesting as models for structurally similar colloidal scale objects such as viruses, where such effects are predicted but cannot be directly observed due to the smaller dimensions. Elastic constants of the wall material are estimated using shell and plate deformation models and are found to be a factor of 5 larger than those for neutral lipidic bilayers in the gel state. We discuss the molecular origins of this high stiffness.

Journal Article↗
PubMed2005

The role of counterions on the elasticity of highly charged lamellar phases: a small-angle x-ray and neutron-scattering determination.

The structure and fluctuations of the swollen L(alpha) lamellar phase of highly charged surfactant didodecyldimethylammonium halide fluid bilayers (DDA+X-) are studied using high-resolution small-angle x-ray scattering and medium-resolution, high-contrast small-angle neutron-scattering. The Caille parameter eta, as a function of the swelling (L(alpha) periodicity d), was determined from the full q-range fits of the measured scattering profiles for three different counterions (X- = Cl-, Br-, and NO3-). This parameter quantifies the amplitude of the membrane fluctuations within the Landau-de Gennes smectic-A linear elasticity theory. The different anions used gave strong specific effects at the maximum swelling of the L(alpha) phase, while at lower swellings a two-phase coexistence of swollen and collapsed lamellae (d approximately 30 and approximately 80 angstroms) was observed for bromide and nitrate ions. Over the intermediate dilution range for all three counterions, a single L(alpha) phase can be continuously swollen with pure water which is governed by an equation of state (i.e., osmotic pressure versus period) and thermally excited fluctuation amplitudes that can be well described by the same Poisson-Boltzmann calculation. The membranes were found to be slightly stiffer than predicted by purely electrostatic repulsions, and this is tentatively attributed to an extra bending rigidity contribution from the surfactant chains.

Journal Article↗
PubMed2004

Shape control through molecular segregation in giant surfactant aggregates.

Mixtures of cationic and anionic surfactants crystallized at various ratios in the absence of added salt form micrometer-sized colloids. Here, we propose and test a general mechanism explaining how this ratio controls the shape of the resulting colloidal structure, which can vary from nanodiscs to punctured planes; during cocrystallization, excess (nonstoichiometric) surfactant accumulates on edges or pores rather than being incorporated into crystalline bilayers. Molecular segregation then produces a sequence of shapes controlled by the initial mole ratio only. Using freeze-fracture electron microscopy, we identified three of these states and their corresponding coexistence regimes. Fluorescence confocal microscopy directly showed the segregation of anionic and cationic components within the aggregate. The observed shapes are consistently reproduced upon thermal cycling, demonstrating that the icosahedral shape corresponds to the existence of a local minimum of bending energy for facetted icosahedra when the optimal amount of excess segregated material is present.

Journal Article↗
PubMed2004

Determination of pore size of catanionic icosahedral aggregates.

We show that it is possible to measure the porosity of facetted micron-sized hollow icosahedra of catanionic solutions by performing fluorescence recovery after photobleaching measurements. The size of spontaneous permanent pores in bilayers formed via molecular segregation is compatible with what is observed by freeze-fracture electron microscopy and is discussed versus theoretical expressions of bending energy.

Journal Article↗
PubMed2004

Effect of recognized and unrecognized salt on the self-assembly of new thermosensitive metal-chelating surfactants.

New functional thermoreversible metal complexing surfactants consisting of a chelating amino acid residue grafted to the tip of a nonionic surfactant [alkyl poly(oxyethylene) CiEj] or in a branched position are studied. Nonionic surfactants are thermoreversible and exhibit a clouding phenomenon associated with phase separation of micelles. The functional molecules retain both the surface-active properties and the characteristic thermoreversible behavior. Because of the hydrophilic contribution of the chelating group (acetyl lysine), the cloud point and the area at the air-water interface are higher for functional surfactants than for nonionic precursors. These new surfactants have efficient complexing properties toward metal ions and are more efficient than the mixture of the corresponding nonionic surfactant and the acetyl lysine ligand solubilized in micelles. This reveals the synergistic effect obtained by the covalent link between the two functions. Addition of a bulky group on classical amphiphilic structures modifies markedly the packing constraints at the origin ofmicellar structures. Small-angle X-ray or neutron scattering results, modeled jointly on the absolute scale, demonstrate the influence of unrecognized lithium nitrate (LiNO3) as well as specifically recognized uranyl nitrate [UO2(NO3)2] salts on micellar structure and phase boundaries. The determination of the micellar shape variations induced by a recognized salt, that is, a decrease of the polar headgroup, allows the rationalization of uncommon synergistic effects on the cloud point variation: increase with lithium nitrate, no decrease in the presence of uranyl nitrate, and a very large decrease when these two salts are present together.

Lithium Compounds↗
PubMed2002

Swelling of a lecithin lamellar phase induced by small carbohydrate solutes.

In this paper, we consider the effect of adding small carbohydrate solutes (small sugars) to DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) L(alpha) dispersions and the consequences on the force balance at zero osmotic pressure (maximal swelling). We show the importance of long incubations required to obtain samples at thermodynamic equilibrium where molecular diffusion has been completed. The monotonic increase of maximal swelling versus sugar content occurs as a combined effect of the screening of the van der Waals contribution and fluctuations in the lamellar stacks. According to this new approach, it is shown that changes in dielectric properties result in a much less pronounced effect than entropic forces (undulations) generated by the softening of the membranes at high sugar content. However, this sugar-induced swelling cannot be explained quantitatively by adding an entropic contribution to molecular interactions. Quantitative disagreement between the proposed mechanism and our observations is due either to nonadditivity of molecular interactions with entropic forces or to the relation used to account for the entropic contribution.

Carbohydrates↗