The main transition of dipalmitoylphosphatidylcholine monolayers: a liquid expanded to solid condensed high order transformation.

We have investigated the orientation processes of a fluorescent probe, 12-(9-anthroyloxy)stearic acid (12-9-AS), embedded in a dipalmytoylphosphatidylcholine (DPPC) monolayer spread at the air/water interface. These orientation processes are represented by an orientation parameter defined as the ratio of two dimerization constants obtained from fluorescence experiments using two perpendicular linearly polarized incident light beams. The orientation parameter shows three particular domains of orientation during the compression of the DPPC film, these domains being separated by two drastic changes in the orientation parameter, one located at the LC/SC transition point (0.48 nm2.molecule-1) and the other at the end of the LE/LC transition region (0.58 nm2.molecule-1). Attribution of the phases implicated in the bidimensional liquid/solid transformation was thus revisited. Therefore, from the present results together with results gathered in the literature, we show that the main monolayer phase transition of DPPC implies a transformation from a molecular state having a characteristic area ALE (phase LE) to a molecular state of characteristic area ASC (phase SC) with an intermediate molecular state of characteristic area ALC (phase LC). During the main monolayer transformation (the LE/SC phase transition) the highly condensed domain with a molecular area ASC (0.48 nm2.molecule-1) are thus surrounded by a region of interfacial lipid of molecular area ALC (0.58 nm2.molecule-1) distributed in a fluid phase of characteristic area ALE (0.74 nm2.molecule-1).