Cooperative and reciprocal chiral structure formation of an alanine-based peptide confined at the surface of cationic surfactant membranes

The confinement of anionic oligoalanine peptides at the surface of cationic membranes can cooperatively reinforce peptide/peptide interactions and induce secondary-structure formation, and, reciprocally, induce chirality expression of the membrane at the mesoscopic level, thus leading to the formation of three-dimensional chiral fibrillar networks. Such a strong binding effect of peptides with cationic membranes and the resulting cooperative assembly behaviors are observed with two different types of cationic surfactant, namely, two-head two-tail gemini and one-head two-tail surfactants. The ensemble of assembly properties, such as critical micellar concentration (cmc), Krafft temperature (T(k) ), molecular area at the air/water interface, molecular organization (as studied by FTIR attenuated total reflectance (ATR) measurements and small-angle X-ray scattering), and morphology of the aggregates (as observed by optical and electron microscopy studies), are reported. The results clearly demonstrate that the molecular organization and mesoscopic supramolecular structures are controlled by a subtle balance between the peptide/peptide interactions, ionic interactions between the membranes and peptides, and the interactions the between surfactant molecules, which are governed by hydrophobicity and steric interactions. Investigation into such cooperative organization can shed light on the mechanism of supramolecular chirality expression in membrane systems and allow understanding of the structure of peptides in interactions with lipid bilayers.     

Introduction of curvature in amphipathic oligothiophenes for defined aggregate formation

In this study the possibility to control the size and shape of self-assembled structures through the local curvature of their molecular building blocks has been investigated. To this end a series of amphipathic conjugated oligothiophenes with a well-defined curvature of their backbone has been designed and synthesized. The molecular (local) curvature of these oligothiophenes resulted from a preference for cis instead of trans conformations at specific positions along the oligothiophene backbone, which can be controlled by the sequence of hydrophilic and hydrophobic groups, while their ratio was kept constant. The self-assembly of ter-, sexi-, and dodecathiophenes appeared to be a low-cooperative process, involving the formation of premicellar aggregates at sub-millimolar concentrations, which at concentrations in the millimolar regime transformed into micelles and cylindrical micelles. The aggregates display fine structures with dimensions reminiscent of the thiophene molecules. The structure-morphology relationship of the ter- and sexithiophenes could be described by conventional packing theory. However, with the dodecathiophene, the backbone curvature governed the formation of cylindrical aggregates with a well-defined diameter. These results demonstrate that it is possible to control the aggregation morphology of simple amphipathic oligothiophenes by implementation of an additional structural motif namely, the curvature.     

Correction to : Estimating Measurement Uncertainty on Stress-Strain Curves from SHPB

Experimental Mechanics - Equation (12) in the original article [1] gives the strain from the measured voltage on a full bridge configuration of the four strain gages. The full-bridge is intended to...     

Design and test of a friction damper to reduce engine vibrations on a space launcher

Space launchers are submitted to complex vibration environments and this can impact the payload it is carrying. Ensuring the protection of the payload therefore requires the addition of a secondary system. In this paper, a rapid design method for the dimensioning of a friction damper is developed, based on the equivalent energy dissipation with that of a viscous damper. A friction damper is designed and a prototype is built. The friction damper is first characterised alone and it is then mounted inside a scale model of a launcher last stage. The friction damper is adequately modelled by a spring in series with a friction element. The damper prototype proves to efficiently damp the rocket engine vibrations, and the design method used for dimensioning the friction damper gives a good approximation for the optimal sliding force of the damper.     

Understanding droplet coalescence and its use to estimate interfacial tension

The study of coalescence of polymer droplets is presented in the viscosity ratio range (p) going from 0.1 to 10. It is shown that the determination of the characteristic time of coalescence is a good way to estimate the interfacial tension. Polydimethylsiloxane (PDMS) is mixed with polyisobutylene (PIB) and the temperature change provides a way to modify the interfacial tension of the PDMS/PIB system significantly, as measured using a pendant drop apparatus. We obtain a dependence of the reduced coalescence time as a function of p^-1/2 which gives access to the interfacial tension. This technique can be an interesting choice for estimating interfacial tension without requiring sophisticated techniques. In a further attempt to correlate these observations with a theoretical model (Verdier C (2001) Polymer 42), the flow field inside and outside the droplets is investigated. PIV measurements are carried out where the evidence of elongational regimes is demonstrated. Such experiments are also interesting for future comparisons with numerical results.     

Mixing behavior of fluorinated and hydrogenated gemini surfactants at the air-water interface

Mixing behavior of hydrogenated and fluorinated cationic gemini surfactants was studied at the air-water interface by Brewster angle microscopy and pi-A isotherm curves. In the bulk, these two molecules did not mix and showed phase separation. At the air-water interface, if a monolayer was formed by separate deposition of the two solutions, they formed separate domains, and the compression occurred in two steps: first the domains with hydrogenated gemini surfactant were compressed until they showed collapse; then the domains with fluorinated gemini surfactant were compressed. If the two solutions were mixed before the deposition, they remained mixed upon compression; on the other hand, separate domains under separate deposition were shown to mix if the subphase was heated.     

New variational principle for the Vlasov-Maxwell equations

A new Eulerian variational principle is presented for the Vlasov-Maxwell equations. This principle uses constrained variations for the Vlasov distribution in eight-dimensional extended phase space. The standard energy-momentum conservation law is then derived explicitly by the Noether method. This new variational principle can be applied to various reduced Vlasov-Maxwell equations in which fast time scales have been asymptotically eliminated (e.g., low-frequency gyrokinetic theory).