Comparison of Cholesterol and 25-Hydroxycholesterol in Phase Separated Phospholipid Monolayers

Abstract

Experimental studies of the phase separation of coexisting liquid phases in mixed phospholipid/sterol monolayer systems have contributed significantly to our understanding of the unique role that cholesterol plays within lipid membranes. Cholesterol is not unique in its ability to promote phase separation in these model systems. Several cholesterol analogs display similar liquid-liquid phase coexistence in monolayer and bilayer systems. One particularly interesting example of this is 25-hydroxycholesterol (25OH), which has been previously noted to have a kink in its monolayer pressure-area isotherm corresponding to the miscibility phase transition as well as for its pathological effect on the plasma cell membrane. We present the results of experiments using traditional Langmuir film-balance techniques (pressure-area isotherms) and surface potential measurements to identify changes in molecular orientation during monolayer compression. Fluorescence microscopy experiments complement these studies with comparisons of domain size distributions, area fraction, and line tension measurements. From our preliminary work it is clear that there are many similarities between the phase behavior of these two systems as well as many significant differences.