Phospholipid monolayers play a critical role in the structure and stabilization of biological interfaces including all membranes, the alveoli of the lung, fat droplets in adipose tissue, and lipoproteins. The behavior of phospholipids in bilayers and at an air-water interface is well understood. However, the study of phospholipids at oil-water interfaces is limited due to technical challenges. In this study, egg-phosphatidylcholine (EPC) was deposited from small unilamellar vesicles onto a bubble of either air or triolein (TO) formed in a low salt buffer. The surface tension (γ) was measured using a drop tensiometer. We observed that EPC binds irreversibly to both interfaces and at equilibrium exerts 12 and 15 mN/m of pressure (Π) at an air and TO interface, respectively. After EPC was bound to the interface, the unbound EPC was washed out of the cuvette and the surface was compressed to study the Π/area relationship. To determine the surface concentration (Γ), which cannot be measured directly, compression isotherms from a Langmuir trough and drop tensiometer were compared. The air-water interfaces had identical characteristics using both techniques, thus Γ on the bubble can be determined by overlaying the two isotherms. TO and EPC are both surface active so in a mixed TO/ EPC monolayer both molecules will be exposed to water. Since TO is less surface active than EPC, as Π increases the TO is progressively ejected. To understand the Π/area isotherm of EPC on a TO bubble, a variety of TO-EPC mixtures were spread at the air-water interface. The isotherms show an abrupt break in the curve caused by the ejection of TO from the monolayer into a new bulk phase. By overlaying the compression isotherm above the ejection point with a TO bubble compression isotherm, Γ can be estimated. This allows determination of Γ of EPC on a TO bubble as a function of Π.

No abstract

Apolipoprotein B (apoB) is one of a unique group of proteins that form and bind to fat droplets, stabilize the emulsified fat, and direct their metabolism. ApoB, secreted on lipoproteins (emulsions), remains bound during lipid metabolism yet exhibits conformational flexibility. It has amphipathic ␤-strand (A␤S)-rich domains and amphipathic ␣-helix (A␣H)-rich domains. We showed that two consensus A␤S peptides of apoB bound strongly to hydrophobic interfaces [triolein͞water (TO͞W) and dodecane͞ water], were elastic, and were not pushed off the interface when the surface was compressed. In contrast, an A␣H peptide modeling helical parts of apoB was forced off the TO͞W interface by compression and readsorbed when the interface was expanded. In this report, the surface behavior of apoB-100 was studied at the TO͞W interface. Solubilized apoB lowered the interfacial tension of TO͞W in a concentration-dependent fashion. At equilibrium tension, if the surface was compressed, part of apoB was pushed off but quickly readsorbed when the surface was expanded. Even when the surface area was compressed by Ϸ55%, part of the apoB molecule remained bound. The maximum surface pressure that apoB could withstand without being partially ejected was 13 mN͞m. ApoB showed high elasticity at the TO͞W interface. Based on studies of the consensus A␤S and A␣H peptides, we suggest that A␤Ss anchor apoB and are its nonexchangeable motif, whereas its conformational flexibility arises from both the elastic nature of the A␤S and the ability of A␣H domains of the molecule to desorb and readsorb rapidly in response to surface pressure changes.adiposomes ͉ oil bodies ͉ protein stabilization of fat droplets ͉ surface activity ͉ surface elasticity

This review focuses on some new techniques to study the behavior of peptides and proteins bound to oil droplets. We will show how model peptides e.g., amphipathic a helices (AaH) and amphipathic b strand (AbS) and some apolipoproteins adsorb to triacylglycerol (TAG) droplets and how they behave once adsorbed to the interface. While most of the studies described involve peptides and proteins at an oil/water interface, studies can also be carried out when the surface has been partially covered with phospholipids. This work is important because it examines biophysical changes that take place at lipid droplet interfaces and how this may relate to the metabolism of lipoproteins and lipid droplets.

However, above ϳ19 mN/m (⌸ OFF ) the entire apoA-I molecule desorbed into the water. ApoA-I was more flexible at the TO/W interface than CSP and showed more elasticity at oscillation periods 4 -128 s even at high compression, whereas CSP was elastic only at faster periods (4 and 8 s) and moderate compression. Flexibility and surface pressure-mediated desorption and re-adsorption of apoA-I probably provides lipoprotein stability during metabolic-remodeling reactions in plasma.