R. Ober

Correlation length of hydrophobic polyelectrolyte solutions

The combination of two techniques (Small Angle X-ray Scattering and Atomic Force Microscopy) has allowed us to measure in reciprocal and real space the correlation length ξ of salt-free aqueous solutions of a highly charged hydrophobic polyelectrolyte as a function of the polymer concentration Cp, charge fraction f and chain length N. Contrary to the classical behaviour of hydrophilic polyelectrolytes in the strong-coupling limit, ξ is strongly dependent on f. In particular, a continuous transition has been observed from ξ ~ Cp−1/2 to ξ ~ Cp−1/3 when f decreased from 100% to 35%. We interpret this unusual behaviour as the consequence of the two features characterising the hydrophobic polyelectrolytes: the pearl necklace conformation of the chains and the anomalously strong reduction of the effective charge fraction.

Study of the conformational rigidity of polyelectrolytes by elastic neutron scattering: 2. Molecular dimensions and conformation of poly(methacrylic acid)

Abstract Elastic neutron scattering at small angles allows the radius of gyration of a PMA sample of molecular weight M v ∼ 13000 to be measured in highly dilute solution; the variation of this parameter with concentration and charge density to be followed, and the viscometric results to be confirmed. The scattered intensity is proportional to q −2 ( q = momentum transfer) for the highest concentrations studied, which correspond to a Gaussian distribution function of the sub units of the chain, and to q −1 at the lowest concentrations, of which it is concluded that the polyion is then a zig-zag with only a few subunits.

Binary separation in very thin nematic films: thickness and phase coexistence.

The behavior as a function of temperature of very thin films (10 to 200 nm) of pentylcyanobiphenyl on silicon substrates is reported. In the vicinity of the nematic-isotropic transition we observe a coexistence of two regions of different thicknesses: thick regions are in the nematic state while thin ones are in the isotropic state. Moreover, the transition temperature is shifted downward following a 1/h(2) law ( h is the film thickness). Microscope observations and small-angle x-ray scattering allowed us to draw a phase diagram which is explained in terms of a binary first-order phase transition where thickness plays the role of an order parameter.

Pressure—area curves of single and mixed monolayers of phospholipids and the possible relevance to properties of lung surfactant☆

Abstract This paper presents a study of the dynamic surface pressure-area (II- A ) behavior of surface film made with the major lipidic components of the pulmonary surfactant namely l -α-dipalmitoylphosphatidylcholine (DPPC) and phosphatidylglycerol (PG). A Langmuir trough is used under temperature and dynamical conditions very similar to the physiological state as a model system. Several mixtures (DPPC-PG 9:1, 8:2, 7:3, and 6:4) were used and the effect of temperature was studied particularly in the gel-liquid crystalline phase transition region of the mixtures as determined by DSC. DPPC alone permits the obtainment of high surface pressures in dynamic conditions, PG itself having good respreading properties. In the transition range of the mixtures, we observed new effects; there were two pressure plateaux at compression corresponding to two collapses and one plateau at expansion. After the first cycle, a permanent cycling regime settles down. This suggests an interesting effect concerning the exogenous replacement therapy in neonatal Respiratory Distress Syndrome (RDS). This new effect is also very important in the understanding of the mechanism of action of PG and its role.

Self diffusion and spectral modifications of a membrane protein, the Rubrivivax gelatinosus LH2 complex, incorporated into a monoolein cubic phase.

The light-harvesting complex LH2 from a purple bacterium, Rubrivivax gelatinosus, has been incorporated into the Q230 cubic phase of monoolein. We measured the self-diffusion of LH2 in detergent solution and in the cubic phase by fluorescence recovery after photobleaching. We investigated also the absorption and fluorescence properties of this oligomeric membrane protein in the cubic phase, in comparison with its beta-octyl glucoside solution. In these experiments, native LH2 and LH2 labeled by a fluorescent marker were used. The results indicate that the inclusion of LH2 into the cubic phase induced modifications in the carotenoid and B800 binding sites. Despite these significant perturbations, the protein seems to keep an oligomeric structure. The relevance of these observations for the possible crystallization of this protein in the cubic phase is discussed.

Unbinding-binding transition induced by molecular snaps in model membranes.

We have used a lamellar phase made of a nonionic surfactant, dodecane and water, as a model membrane to investigate its interactions with macromolecular inclusions bringing together two membranes, i.e., acting as macromolecular snaps. In systems devoid of inclusions, the interlamellar distance depends on the total volume fraction of membranes Phi. We show that, in presence of a transmembrane protein, or of several de novo designed peptides of different length and composition, the lamellar phase undergoes a binding transition. Under such conditions, the interlamellar distance is no longer proportional to Phi(-1), but rather to the surface concentration of snaps within the membrane. It also appears that, in the presence of the hydrophobic segment of peptide snaps, the length of the inclusions must be at least equal to the hydrophobic length of the membrane to be active. Experimental results have been precisely fitted to a model of thermally stabilized membranes, decorated with snaps. However, in the presence of inclusions, the parameter describing the interactions between membranes, has to take into account the length of the inclusion to preserve good predictive capabilities.

Structure of concentrated winsor microemulsions by SANS

We have performed contrast variation SANS experiments in Winsor microemulsions. We discuss the results in terms of film curvature, shown to be related to the film-water partial structure factor experimentally measured. We thus evidence that the Winsor microemulsion studied are random and bicontinuous as predicted by recent theories.

A small angle X-ray scattering investigation of the structure of a ternary water-in-oil microemulsion

Abstract A ternary water-in-oil microemulsion composed of xylene, water and a surface-active extractant, sodium bis-2-ethylhexyl phosphate, was investigated by small angle X-ray scattering (SAXS) and conductimetry. In order to decide between two possible interpretations of the SAXS data, i.e. evidence for the occurrence of highly polydisperse, independent droplets or of slightly polydisperse and interacting ones, the effects of temperature, water content, dilution and type of solvent were investigated. The scattering profiles were simulated using a log—normal size distribution and the mean spherical approximation (MSA) for the interactions. This led us to proposes model of slightly polydisperse and aggregated particles with a mean radius of about 1.4 nm. The addition of water swells the micelles while concentration and aggregation number remain constant. This then generates an aggregation phenomenon which was confirmed by conductimetry measurements. This investigation makes it possible to optimize the conditions for producing a microemulsion such as is often used in liquid—liquid extraction of metals.

Pressure—area curves of phospholipid monolayers in relation to pulmonary surfactant

Abstract In this paper is presented a study of the dynamic surface pressure—area (Π— A ) curves of monolayers at the air—water interface. These were made with the major lipidic components of the alveolar surfactant. The synthetic phospholipids are L-α-dipalmitoylphosphatidylcholine (DPPC) and phosphatidylglycerol (PG). The experiments consist of successive compression—expansion cycles of pure and mixed phospholipid surface films on a modified Langmuir balance, used under temperature and dynamical conditions very similar to those of the physiological pulmonary state. The studies were carried out at different temperatures between 22 and 37°C. The effect of temperature was particularly studied in the gel—liquid crystalline phase-transition region of the DPPC—PG mixtures, as determined by differential scanning calorimetry (DSC). From our results, we can see that pure DPPC permits the obtention of high surface pressure under dynamic conditions, whereas pure PG presents good re-spreading properties. In the transition range of the mixtures, the shape of the Π— A curves is quite different; we observe one intermediate pressure plateau on compression, and a slightly different one on expansion. In this temperature region, from the second cycle, the cycling behaviour settles down to a stable regime under the present conditions. This new effect is very interesting with regard to the mechanism of action of PG in the pulmonary surfactant and suggests a favourable effect for an exogenous replacement therapy in neonatal respiratory distress syndrome (RDS).

Study of the conformation of polyelectrolytes in dilute solutions by elastic neutron scattering

Abstract Experiments using small angle neutron scattering have been performed on dilute solutions of polyelectrolytes without added salt; they allow to show that this method is very suitable for measuring the radius of gyration and determining the conformation of polyions in solution.