A Fluorescence Approach of the Determination of Translational Diffusion Coefficients of Lipids in Phospholipid Monolayer at the Air-Water Interface

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Abstract
The photobleaching technique as well as experimentation based on fluorescence recovery after bleaching can be extended to monolayers spread at the air-water interface. A mathematical model is derived which allows the determination of translational diffusion coefficients of species diffusing in such a system. Using 12-(9-anthroyl)stearic acid (anthroylstearate) as a fluorescent probe, dispersed either in dipalmitoylphosphatidylcholine or in dipalmitoylphosphatidylglycerol in various conditions of subphase ionic composition and surface pressure of the monolayer, including phase transition domains, the following was shown: Anthroylstearate molecules seem to aggregate in microdomains where their fluorescence properties remain unchanged regardless of the compression states of the host monolayer. In any case, a break in the diffusion constants appears on compressing films of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol. In particular, this break coincides with the liquid expanded to gel phase transition of these lipids when it occurs. Diffusion of anthroylstearate in dipalmitoylphosphatidylglycerol depends strongly on the subphase ionic strength and on the nature of cations: Na+, Mg2+ and Ca2+.