A fluorescence study with polarised incident light of the compression of phospholipid monolayers spread at the air/water interface: orientation processes in the glycerol region.

Monolayers of phospholipids spread at the air/water interface were studied by means of fluorescence measurements. Using linearly polarised incident light and following the behaviour of a fluorescent covalently-labelled phospholipid (dansylphosphatidylethanolamine) embedded in the monolayer, it was possible to obtain information about the orientation changes at the glycerol level of the phospholipid. When using dipalmitoylphosphatidylcholine as phospholipid, the main orientation of the probe appears unchanged during the phase transition process. On the other hand, the standard deviation of the distribution function of orientation is larger in the liquid-expanded state relative to the condensed state. When using phosphatidic acid or a mixture of dipalmitoylphosphatidylcholine and phosphatidic acid as phospholipids, the same orientation of the probe is observed as with pure dipalmitoylphosphatidylcholine. This orientation stays the same whatever the ionic content of the subphase, i.e. the ionisation state of the polar head. The glycerol region of a phospholipid spread in monolayer at the air/water interface appears unaffected structurally either by the nature of the polar moiety, by its ionisation state, or by the physical state of the hydrocarbon chains.