We have investigated the effect of the insertion of a triblock peptide (hydrophobic−hydrophilic−hydrophobic) in a nonionic lamellar phase composed of C₁₂E₄, decane, and water, stabilized by bilayer thermal fluctuations. Circular dichroism shows the peptide to be unordered in water, whereas its hydrophilic part is rigid and organized in an α-helix in the presence of surfactant bilayers. Surface tension measurements prove that the peptide is located at the hydrophobic−hydrophilic interface. Together with spectrofluorometry, these experiments suggest that the peptide lies on the bilayer surface. The Caillé parameter, η, of the lamellar phase, obtained by SAXS experiments, decreases with peptide concentration. This decrease has been interpreted as an increase of the bilayer effective thickness induced by the peptide and is well fitted by a recent model. The bilayer bending rigidity κ increases linearly with peptide concentration, up to two times the rigidity of a bare bilayer with mole ratio of peptide to surfactant as low as 5.2 × 10^-4. The smectic compressibility modulus, B̄, decreases, implying that the peptide presence softens interactions between bilayers.

We have investigated the distribution of the transmembrane myelin proteolipid protein when inserted into an oil-swollen lamellar phase, stabilized by steric interactions. When the hydrophobic membrane thickness, D, is larger than the hydrophobic length of the protein, d_π, only repulsive interactions are found between proteins. The repulsive forces are of electrostatic nature, arising from charges carried by the protein. The interaction potential between proteins, deduced from digitized freeze-fracture micrographs, is well fitted when the classical screened electrostatic model is used. When D is smaller than d_π, an attractive force is observed in addition to the repulsive electrostatic interactions. The attractive force originates from the membranes fluctuations. The model of membrane-mediated interactions due to the membrane thermal undulations permits us to describe our results when used in combination with the electrostatic potential.

The behavior of a reverse lamellar phase has been studied by small-angle X-ray scattering upon the insertion of triblock molecules. A decrease of the lamellar spacing and membrane thickness was observed, whereas the Caillé exponent remained constant whatever the concentration of triblock molecules. A similar behavior was observed when surfactant molecules were added to the lamellar phase instead of triblock molecules. We demonstrate that our experimental results are consistent with the assumption that triblock molecules only participate to increase the membrane surface area. Moreover, we emphasize the fact that the surfactant molecular area varies with the membrane thickness. Indeed, this property appears to be the main effect causing the observed dramatic decrease of the lamellar spacing.

We present a statistical mechanical treatment relating the macroscopic adhesion energy of two surfaces, which can be obtained by micropipette aspiration studies, to the microscopic adhesion energy between individual bonds. The treatment deals with the case of weak reversible bonds, so that the equilibrium partition function has significance. This description is coherent with previous theories. Experiment and theory are compared to probe the nature of weak bonds in membranes, where local equilibria can be obtained. The case of a bead and a vesicle decorated by nucleosides was considered.

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 β-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.