The hydrophobic surfactant proteins SP-B and SP-C promote rapid adsorption of pulmonary surfactant to an air/water interface. Previous evidence suggests that they achieve this effect by facilitating the formation of a rate-limiting negatively curved stalk between the vesicular bilayer and the interface. To determine whether the proteins can alter the curvature of lipid leaflets, we used x-ray diffraction to investigate how the physiological mixture of these proteins affects structures formed by 1-palmitoyl-2-oleoyl phosphatidylethanolamine, which by itself undergoes the lamellar-to-inverse hexagonal phase transition at 71 degrees C. In amounts as low as 0.03% (w:w) and at temperatures as low as 57 degrees C, the proteins induce formation of bicontinuous inverse cubic phases. The proteins produce a dose-related shift of diffracted intensity to the cubic phases, with minimal evidence of other structures above 0.1% and 62 degrees C, but no change in the lattice-constants of the lamellar or cubic phases. The induction of the bicontinuous cubic phases, in which the individual lipid leaflets have the same saddle-shaped curvature as the hypothetical stalk-intermediate, supports the proposed model of how the surfactant proteins promote adsorption.

The first step required for the determination of surface tension from the shape of a captive bubble is the correct alignment of both the solid support against which the bubble floats and the camera used to record its profile. The solid support should be perpendicular to the gravitationally vertical axis. The camera used to visualize the bubble must be aligned to its axis of symmetry. Alignment of roll for both the camera and solid support is straightforward. For well-collimated light, yaw is unimportant. We show here how to align pitch, first adjusting the camera relative to the gravitational vertical, and then adjusting an agarose dome used as a ceiling above captive bubbles within the visual frame of reference.

Bubbles and droplets offer multiple advantages over Langmuir troughs for compressing interfacial films. Experiments, however, that manipulate films to maintain constant surface tension (γ) present problems because they require feedback. Measurements of bubbles and droplets calculate γ from the shape of the interface, and calculations in real time based on finding the Laplacian shape that best fits the interface can be difficult. Faster methods obtain γ from only the height and diameter, but the bubbles and droplets rest against a solid support, which obscures one section of the interface and complicates measurements of the height. The experiments here investigated a series of optical variables that affect the visualized location of the different surfaces for captive bubbles. The pitch of the support and camera as well as the collimation of illuminating light affected the accuracy of the measured dimensions. The wavelength of illumination altered the opacity of turbid subphases and hydrated gel used to form the solid support. The width of all visualized edges depended on the spectral width and collimation of the illuminating light. The intensity of illumination had little effect on the images as long as the grayscale remained within the dynamic range of the camera. With optimization of these optical factors, the width of all edges narrowed significantly. The surfaces away from the solid support approached the infinite sharpness of the physical interface. With these changes, the grayscale at the upper interface provided the basis for locating all surfaces, which improved real-time measurements based on the height and diameter.

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compensated, at least in part, by interactions with the channel wall and by interactions with the lipid headgroups at the channel mouth. Consequently, differences in single channel permeability (pf) measured for gramicidin A channels embedded into different lipids were interpreted in terms of differences in water dehydration costs. However, recent atomistic molecular dynamics simulations identified lipid headgroup interactions with the channel entrance leading to transient blocking of the channel. This observation suggests that the lipid environment affects the channel not only by changing the water energetics but also by mechanically blocking the entrance. To test this hypothesis we measured ion and water fluxes through acylated gramicidin-A derivatives, which were reconstituted into solvent free diphytanoyl-phosphatidyl-choline membranes. Ion conductance of channels with C9 and C10 acyl-chain anchors differed only by about 20 % from wild type gramicidin-A conductance. Similarly, the anchor had only a minor effect on dimer stability as indicated by a decrease in channel lifetime from 2.3 s to 2 s or 1.6 s for the C9 and C10 derivatives, respectively. As the gramicidin channels most of the time do not contain ions, the acyl-chain anchor affects water transport more efficiently. Two C9 anchors increased pf by a factor 2 or 3 depending on their position. In contrast, derivates with only one C9 or C10 acyl anchor showed no increase in pf. Taken together with data about the lipid dependency of pf, these results indicate that the lipid headgroups affect single file transport by both changing the solvation energy and by blocking the channel entrance.