INFLUENCE OF FLUORESCENT LIPID PROBES ON THE PACKING OF THEIR ENVIRONMENT - A MONOLAYER STUDY

In ord- to evaluate the disturbance of lipid molecular organization by fluorescent lipid probes we studied the influence of lipids carrYing various fluorophores in one of the hydrocarbon chains on the behavior at the air-water interface of monolayers made from pure phospholipids, gangliosides and/or their mixtures. With fluorescent labeled phosphatidylcholines (PC) the perturbance in packing of the host lipid induced by the probe increased in the series of fluorophores: 9-anthrylvinyl (AV) < NBD < 1-pyrenyl < 3-perylenoyl. Typical values for the increase in mean area per molecule lay between 5 and 11 angstrom2 at T = 20-degrees-C and a surface pressure of 10 mN/m. The relative perturbance also depended strongly on the depth of insertions of the fluorophore into the monolayer. When the fluorophore was located near to the methyl end groups of the host lipids the perturbance induced by the probe was much smaller than when the fluorophore was nearer to the glycerol backbone. The influence of AV-labeled lipids showing minimal perturbance was investigated in more detail. The character of the pressure-area isotherms for monolayers made from individual dipalmitoyl-PC or gangliosides was almost unchanged by incorporation of 1% of the corresponding AV-labeled lipid with the same head group and did not depend much on the packing density of the host lipid. In binary mixtures of different head group species of lipids the disturbance caused by a given AV-labeled lipid probe was even smaller than in a monolayer of its individual prototype. With dipalmitoyl-PC monolayers, the ratio of the slopes of the liquid-expanded to the liquid-condensed transition isotherms did not change on addition of AV-labeled lipids and did not depend on the polar head group of the probe. This permits to 'factor out' the perturbances induced by the fluorophore by comparing the fluorescent parameters of two or more AV-labeled lipids differing in their polar head groups.