Photodimerization and lateral diffusion of 12-(9-anthroyloxy) stearic acid in dipalmitoylphosphoglycerocholine monolayers

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).

The liquid condensed diffusional transition of dipalmitoylphosphoglycerocholine in monolayers

The fine details of the phase transition of dipalmitoylphosphoglycerocholine (DPPC) monolayers at air/NaCl solution interfaces were investigated at 21 +/- 1 degrees C by using the fluorescence after photobleaching technique employing 12-(9-anthroyloxy)stearic acid as fluorescent probe. The mode of compression of the monolayer (i.e., continuous compression or successive additions of the lipid at fixed area) together with the ionic strength of the subphase (0.1 or 1.0 M NaCl) were particularly studied. The photobleaching results show that the lateral diffusion coefficient of the probe molecules decreases drastically within the liquid-condensed phase, i.e., from the end of the liquid-expanded-liquid-condensed phase transition to the beginning of the solid-condensed phase. The molecular areas at which the phase transition occurs under the various experimental conditions, together with a parallel analysis of the hydration states and related molecular areas of the DPPC molecules in multilayers, strongly suggest that the steric hindrance associated with the hydration water of the polar head of DPPC molecules in the monolayer is responsible for the drastic decrease in diffusion coefficient in the liquid-condensed phase. Furthermore, the fluorescence characteristics of the probe molecules also show that, together with the aforementioned reorganization of the polar head, a structural reorganization of the aliphatic chains of the lipid molecules also takes place in the liquid-condensed phase. The liquid-condensed phase therefore appears as a transition region from liquid to solid phases in which the lipid molecules present a significant decrease in their lateral diffusion related to a structural reorganization of both their polar and aliphatic components.