A Fluorescence Microscopic Study Concerning the Phase Diagram of Phospholipids

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Abstract
The phase diagram of phospholipid monolayers is investigated using fluorescence microscopic and thermodynamic techniques. The images recorded using low light level camera tubes show patterns of inhomogeneities in the condensed as well as in the expanded states. These are ascribed to different dye solubilities and different molecular densities, respectively, in coexisting phases. – For pressures above that corresponding to a sharp break in surface pressure versus area isotherms at least two phases coexist. The monolayer is macroscopically fluid with an almost constant diffusion coefficient throughout the so‐called liquid expanded, liquid condensed and the intermediate phase. For dipalmitoylphosphatidylcholine (DPPC) at molecular areas as small as 55 Å2/molecule a drastic decrease in diffusion by about two orders of magnitude is observed. The gas/liquid transition is observed from a formation of characteristic, symmetrical lipid domains on the water surface.