Most aerobic bacteria secrete siderophores to facilitate iron acquisition. Two families of siderophores were isolated from strains belonging to two different genera of marine bacteria. The aquachelins, from Halomonas aquamarina strain DS40M3, and the marinobactins, from Marinobacter sp. strains DS40M6 and DS40M8, each contain a unique peptidic head group that coordinates iron(III) and an appendage of one of a series of fatty acid moieties. These siderophores have low critical micelle concentrations (CMCs). In the absence of iron, the marinobactins are present as micelles at concentrations exceeding their CMC; upon addition of iron(III), the micelles undergo a spontaneous phase change to form vesicles. These observations suggest that unique iron acquisition mechanisms may have evolved in marine bacteria.

In mixtures of cetyltrimethylammonium bromide (CTAB) and sodium perfluorooctanoate (FC 7) in aqueous solution, novel bilayer cylinders with hemispherical end caps and open, flat discs coexist with spherical unilamellar vesicles, apparently at equilibrium. Such equilibrium among bilayer cylinders, spheres, and discs is only possible for systems with a spontaneous curvature, R o, and a positive Gaussian curvature modulus, . We have measured the size distributions of the spherical vesicles, cylinders, and discs by using cryo-electron microscopy; a simple analysis of this length distribution allows us to independently determine that the mean curvature modulus, Ϸ 5 ؎ 1 kBT and Ϸ 2 ؎ 1 kBT. This is one of the few situations in which R o, , and can be determined from the same experiment. From a similar analysis of the disk size distribution, we find that the edges of the discs are likely stabilized by excess CTAB. The fraction of discs, spherical vesicles, and cylinders depends on the CTAB͞FC 7 mole ratio: increasing CTAB favors discs, while decreasing CTAB favors cylinders. This control over aggregate shape with surfactant concentration may be useful for the design of templates for polymerization, mesoporous silicates, etc.cryogenic transmission electron microscopy ͉ surfactants ͉ vesicles T he starting point for the description of bilayer organization in solution is the harmonic approximation to the bending free energy (1):R 1 and R 2 are the principle radii of curvature of the structures, R o is the spontaneous radius of curvature, and are the mean and Gaussian curvature elastic constants, respectively, and A is the area of the bilayer membrane. The harmonic approximation is appropriate when the membrane thickness (1-3) [here Ϸ3 nm (4)] and the Debye length for ionic surfactants (2, 5, 6) [also Ϸ1-3 nm (4)] are small compared with R 1 and R 2 (Ϸ20-30 nm, see Fig. 1). The differences of the bending free energy, F B , of different aggregate geometries can often be of the order of k B T, leading to the possibility of multiple structures in equilibrium.The two elastic constants, and , play very different roles in determining bilayer organization. The magnitude of reflects the energy needed to bend the bilayer away from its spontaneous radius of curvature, R o . For ϳ k B T, thermal fluctuations give rise to significant curvature fluctuations, which lead to a net repulsive interaction between bilayers at short distances. This steric repulsion can stabilize unilamellar vesicles over multilamellar liposomes (3,4,7,8). Larger values of (ӷk B T), combined with a spontaneous curvature that picks out a particular vesicle radius, lead to unilamellar vesicles as the curvature variations inherent to multilamellar structures are energetically prohibited (3,4,9). A spontaneous bilayer curvature (1͞R o 0) is only possible when nonideal surfactant mixing causes the interior and exterior monolayers of the vesicle bilayer to have different compositions or environments (3, 10, 11).influences only the topology (and hence the number) of t...

The boundaries between domains in single-layer graphene strongly influence its electronic properties. However, existing approaches for domain visualization, which are based on microscopy and spectroscopy, are only effective for domains that are less than a few micrometres in size. Here, we report a simple method for the visualization of arbitrarily large graphene domains by imaging the birefringence of a graphene surface covered with nematic liquid crystals. The method relies on a correspondence between the orientation of the liquid crystals and that of the underlying graphene, which we use to determine the boundaries of macroscopic domains.