Translational molecular diffusion in phospholipid monolayers : substrate coupling and phase transitions

Abstract

The results of an investigation of the phases of a monolayer of the phospholipid dipalmitoylphosphatidylcholine, deposited by Langmuir-Blodgett techniques onto hydrocarbon-derivatized silicon substrates, are described Measurements of the long-range translational diffusion of fluorescent lipid analogs in such monolayers reveal the presence of a phase transition not previously seen in phospholipids. It is marked by a large increase of lipid mobility at a temperature characteristic of the employed combination of lipid and substrate-attached hydrocarbon residue. A second transition into a high temperature phase, characterized by a rapid, homogeneous diffusivity, is observed at a temperature slightly above the temperature of the so-called chain melting transition in hydrated multi-bilayers of the lipid. In the novel intermediate state between the two transitions, long-range translational diffusion in the uniformly fluorescent monolayers is best described in a manner analogous to transport in composite materials. Lipid analogs containing a fluorophore in the polar headgroup portion of the molecule sample coexisting populations of host lipid whose diffusivities differ by as much as an order of magnitude in this temperature range. A chain labelled lipid analog is found to exhibit completely different behaviour, remaining largely immobile on the scale of the experimental observation times at temperatures below 43 °C. In samples cycled through the high temperature phase, this label exhibits increased mobilities at lower temperatures for several hours before returning to its original state. The measured diffusivities are compared with corresponding values obtained for monolayers spread at the air-water interface for which the average density is fixed, and also with available data for hydrated multibilayers. In natural bilayer membranes the two monolayer leaflets are generally not identical. They thus exhibit the same type of asymmetry as the one inherent in the system described here.