Surface Diffusion in Phospholipid Foam Films: 1. Dependence of the Diffusion Coefficient on the Lipid Phase State, Molecular Length, and Charge

Abstract

The surface lateral diffusion properties of a range of pure phospholipid-stabilized black foam films (BFF) were studied by fluorescence recovery after photobleaching. The thin films were formed in a specially constructed chamber and fluorescent-labeled by inclusion of low levels of the anionic surfactant 5- N-(octadecanoyl)aminofluorescein. A systematic investigation of temperature dependence of the surface diffusion coefficient in the range 8-85°C was undertaken with phospholipid BFFs stabilized by DMPG, DMPC, DMPE, DLPE, DPPE, DOPE, and DPPA along with some mixtures (DMPC/DMPG and DLPE/DPPA). It was found that the diffusion coefficient was dependent on the lipid phase state. With the exception of negatively charged DMPG, surface mobility of all lipids in the gel state was due to flow rather than to diffusion. In contrast, above the liquid crystalline phase transition, diffusion predominated and increased with temperature from about 2 × 10 -8 to 9 × 10 -8 cm 2/s, with a different slope for the different lipids. Increases in the hydrophobic chain length of the phospholipids resulted in a decrease in the diffusion coefficient for values obtained at any given temperature above the phase transition, in the order DLPE > DMPE > DPPE. In the case of unsaturated Cl8:1 DOPE, measurable diffusion extended down to 15°C compared to 24°C for DMPC and 45°C for the other PEs. The relative importance of the effects of molecular chain length, unsaturation, and charge is discussed.