Translational diffusion in phospholipid monolayers measured by fluorescence microphotolysis.

A method is described that eliminates surface flow in monolayers at the air-water interface and makes possible diffusion measurements by fluorescence microphotolysis ("photobleaching"). In contrast to previous studies that did not account for surface flow, lipid probe diffusion has been found to be similar in densely packed monolayers and in related bilayers. Furthermore, it seems that lipid diffusion is based on the same molecular mechanism in monolayers, bilayers, and potentially also cell membranes. In monolayers of L-alpha-dilauroylphosphatidylcholine (Lau2-PtdCho) the translational diffusion coefficient D of the fluorescent lipid probe N-4-nitrobenzo-2-oxa-1,3 diazole egg phosphatidylethanolamine decreased from 110 microns2/s at a surface pressure II = 1 mN/m to 15 microns2/s at II = 38 mN/m (T = 21-22 degrees C). Data could be fitted by the "free volume model." In monolayers of L-alpha-dipalmitoylphosphatidylcholine (Pam2-PtdCho) D decreased by greater than 3 orders of magnitude upon increasing II at constant temperature, thus indicating a fluid-to-crystalline phase transition. In Lau2-PtdCho/Pam2-PtdCho monolayers phase separation has been visualized in the fluorescence microscope and the effect on D measured. These results suggest that monolayers are a promising model system for studying the molecular mobility of lipids and other cell membrane components.