Survanta, a clinically used bovine lung surfactant extract, in contact with surfactant in the subphase, shows a coexistence of discrete monolayer islands of solid phase coexisting with continuous multilayer "reservoirs" of fluid phase adjacent to the air-water interface. Exchange between the monolayer, the multilayer reservoir, and the subphase determines surfactant mechanical properties such as the monolayer collapse pressure and surface viscosity by regulating solid-fluid coexistence. Grazing incidence x-ray diffraction shows that the solid phase domains consist of two-dimensional crystals similar to those formed by mixtures of dipalmitoylphosphatidylcholine and palmitic acid. The condensed domains grow as the surface pressure is increased until they coalesce, trapping protrusions of liquid matrix. At approximately 40 mN/m, a plateau exists in the isotherm at which the solid phase fraction increases from approximately 60 to 90%, at which the surface viscosity diverges. The viscosity is driven by the percolation of the solid phase domains, which depends on the solid phase area fraction of the monolayer. The high viscosity may lead to high monolayer collapse pressures, help prevent atelectasis, and minimize the flow of lung surfactant out of the alveoli due to surface tension gradients.

A class of lamellar biological hydrogels comprised of fluid membranes of lipids and surfactants with small amounts of low molecular weight poly(ethylene glycol)-derived polymer lipids (PEG-lipids) were studied by x-ray diffraction, polarized light microscopy, and rheometry. In contrast to isotropic hydrogels of polymer networks, these membrane-based birefringent liquid crystalline biogels, labeled L-alpha,g, form the gel phase when water is added to the liquid-like lamellar L-alpha phase, which reenters a liquid-like mixed phase upon further dilution. Furthermore, gels with larger water content require less PEG-lipid to remain stable. Although concentrated (approximately 50 weight percent) mixtures of free PEG (molecular weight, 5000) and water do not gel, gelation does occur in mixtures containing as little as 0.5 weight percent PEG-lipid. A defining signature of the L-alpha,g regime as it sets in from the fluid lamellar L-alpha phase is the proliferation of layer-dislocation-type defects, which are stabilized by the segregation of PEG-lipids to the defect regions of high membrane curvature that connect the membranes.

Endogenous lung surfactant, and lung surfactant replacements used to treat respiratory distress syndrome, can be inactivated during lung edema, most likely by serum proteins. Serum albumin shows a concentration-dependent surface pressure that can exceed the respreading pressure of collapsed monolayers in vitro. Under these conditions, the collapsed surfactant monolayer can not respread to cover the interface, leading to higher minimum surface tensions and alterations in isotherms and morphology. This is an unusual example of a blocked phase transition (collapsed to monolayer form) inhibiting bioactivity. The concentration-dependent surface activity of other common surfactant inhibitors including fibrinogen and lysolipids correlates well with their effectiveness as inhibitors. These results show that respreading pressure may be as important as the minimum surface tension in the design of replacement surfactants for respiratory distress syndrome.

No abstract

Over a range of conditions, lipid and surfactant monolayers exhibit coexistence of discrete solid domains in a continuous liquid. The surface shear viscosity, mu(s), of such monolayers collapses onto a single curve: mu(s)/mu(so) = [1-(A/A(c))](-1), in which mu(so) is the viscosity of the liquid phase, A is the area fraction of the solid phase measured by fluorescence microscopy, and A(c) is a critical solid phase fraction. This scaling relationship is directly analogous to that of three-dimensional dispersion of spheres in a solvent with long-range repulsive interactions, with area fraction replacing volume fraction.

Developing synthetic lung surfactants to replace animal extracts requires a fundamental understanding of the roles of the various lipids and proteins in native lung surfactant. We used Brewster angle microscopy (BAM), atomic force microscopy (AFM), and Langmuir isotherms to study the influence of palmitoyloleoylphosphatidylglycerol (POPG) in monolayers of dipalmitoylphosphatidylcholine and palmitic acid mixtures with or without dSP-B1-25, a peptide dimer based on the first 25 amino acids of surfactant protein B (SP-B). At surface pressures between 30 and 40 mN/m, only monolayers containing POPG and dSP-B1-25 showed plateaus in the isotherm similar to those in Survanta, a bovine extract replacement lung surfactant that contains native SP-B and SP-C proteins. BAM images show distinct morphological changes in the fluid phase during these plateaus, while AFM images of deposited monolayers show that multilayer structures, which we named "nanosilos", form in the fluid phase at the plateau. These nanosilos are from 50 to 300 nm in diameter and from 5 to 8 nm in height and are similar to those observed in deposited Survanta monolayers. We propose that POPG and SP-B interact to stabilize the monolayer composition by trapping POPG in three-dimensional surface-associated aggregates at high surface pressures, preventing the irreversible loss of POPG and SP-B to the subphase.

We report x-ray scattering, rheological, and freeze-fracture and polarizing microscopy studies of a liquid crystalline hydrogel called Lalpha,g. The hydrogel, found in DMPC, pentanol, water, and PEG-DMPE mixtures, differs from traditional hydrogels, which require high MW polymer, are disordered, and gel only at polymer concentrations exceeding an "overlap" concentration. In contrast, the Lalpha,g uses very low-molecular-weight polymer-lipids (1212, 2689, and 5817 g/mole), shows lamellar order, and requires a lower PEG-DMPE concentration to gel as water concentration increases. Significantly, the Lalpha,g contains fluid membranes, unlike Lbeta' gels, which gel via chain ordering. A recent model of gelation in Lalpha phases predicts that polymer-lipids both promote and stabilize defects; these defects, resisting shear in all directions, then produce elasticity. We compare our observations to this model, with particular attention to the dependence of gelation on the PEG MW used. We also use x-ray lineshape analysis of scattering from samples spanning the fluid-gel transition to obtain the elasticity coefficients kappa and B; this analysis demonstrates that although B in particular depends strongly on PEG-DMPE concentration, gelation is uncorrelated to changes in membrane elasticity.

We describe the facile two-step synthesis of nanotubes that form pure, well-defined, nanostructured materials. We have synthesized a secondary amine HBr salt as the headgroup of a single-chain diacetylenic lipid. This molecule can form a number of different self-assembled nanostructures in aqueous or organic solvents. In water, this lipid forms a monodisperse preparation of nanotubes at high yields. Partially dissolving a preparation of nanotubes dried from aqueous solution results in a remarkably organized structure that resembles a nanocarpet. Details of the nanotube structure were investigated by scanning electron microscopy, transmission electron microscopy, and small-angle X-ray spectroscopy. The aqueous nanotubes have a cross-sectional diameter of 89 nm. The walls of the tubes are an exquisitely uniform 27 nm thick and are shown to consist of five lipid bilayers with a repeat spacing of 57.8 A. The chemical structure of the material shows no chiral centers, but suspensions of the nanotubes in an aqueous medium show an unexpected circular dichroism signal. The versatility of this new material as a platform for nanostructure design and synthesis is enhanced by its biocidal activity. This antimicrobial activity along with the regularity the nanostructures will enhance the development of a range of applications from biosensors to artificial retinas.