Interacting effects of temperature, pressure and cholesterol content upon the molecular order of dioleoylphosphatidylcholine vesicles.

Dioleoylphosphatidylcholine (DOPC) multilamellar vesicles containing varying amounts of cholesterol (0-50 mol%) were studied by measuring the polarisation of diphenylhexatriene fluorescence at 6, 23.5, and 35.5 degrees C, and at hydrostatic pressures up to 1.5 kbar . Interactions between temperature and pressure were quantified as the temperature-pressure equivalence which was approximately 19-23 K X kbar -1 for all binary mixtures of cholesterol and DOPC. Polarisation was linearly related to cholesterol/DOPC ratio, except at low temperature. In all cases pressure caused an increase in polarisation (i.e., an increase in molecular order) but did not alter the slope of the graph relating polarisation to cholesterol/DOPC ratio. The relative ordering effect of cholesterol and pressure was quantified by calculating the cholesterol-pressure equivalence. An increase in cholesterol/DOPC ratio of approximately 0.35-0.50 increased polarisation by an amount equivalent to an increase in pressure of 1 kbar . Cholesterol-pressure equivalence tended to decrease as temperature decreased and pressure increased; that is, as membrane order increased.