Contrast inversion in the epifluorescence of cholesterol-phospholipid monolayers.

A number of dihydrocholesterol-phospholipid mixtures have been examined using the epifluorescence microscopy of monolayers at the air-water interface. These mixtures form two coexisting liquids. Fluorescence contrast was provided using a variety of different lipid probes. With increasing monolayer pressure, all of the charged probes show contrast inversion at higher dihydrocholesterol concentrations. That is, with increasing pressure the charged probes transfer from one liquid to the other, reversing the fluorescence contrast. A wide variety of phospholipids were studied, and the inversion was seen in all cases. In the inverted state and at the higher dihydrocholesterol concentrations, the immiscibility persists to the highest pressures employed, 30-40 mN/m. The data show that binary dihydrocholesterol-phospholipid mixtures can form three distinct liquids, one of which is interpreted as a phase rich in condensed complex.