Main phase transitions in supported lipid single-bilayer

Abstract

We have studied the phase transitions of a phospholipidic single-bilayer supported on a mica substrate by real-time temperature-controlled atomic force microscopy. We show the existence of two phase transitions in this bilayer that we attribute to two gel (L(beta))/fluid (L(alpha)) transitions, corresponding to the independent melting of each leaflet of the bilayer. The ratio of each phase with temperature and the large broadening of the transitions' widths have been interpreted through a basic thermodynamic framework in which the surface tension varies during the transitions. The experimental data can be fit with such a model using known thermodynamic parameters.

FIGURE 1

AFM images (8 × 2.5 μm) of the lipid bilayer formation in 10⁻² M NaCl. Between each image, 2.5 μl of DMPC at 10 mg/ml has been added and stirred in the solution. The vesicle fusion is self-limited after the first bilayer.

FIGURE 2

AFM images (3 × 3 μm) of transitions 1 and 2 of the DMPC supported single-bilayer at different temperatures. The darkest areas are holes in the bilayer. The sample is continuously heated under the AFM tip at a rate of 0.1°C/min. The temperature indicated in each image corresponds to the temperature in the middle of the image.

FIGURE 3

Experimental curves of transition 1 (open triangles) and evolution of holes (solid triangles) at increasing and decreasing temperatures.

FIGURE 4

Calculated curve of the percentage of gel phase versus temperature. In this calculation we consider a simple transition at constant surface and constant matter.

FIGURE 5

Experimental and modeled curves of the two transitions. The experimental curves are the average of three different measurements.