The effects of temperature, pressure and peptide incorporation on ternary model raft mixtures—A Laurdan fluorescence spectroscopy study

Recently, an increasing evidence accumulated for the existence of lipid microdomains, called lipid rafts, in cell membranes, which may play an important role in many important membrane-associated biological processes. Suitable model systems for studying biophysical properties of lipid rafts are lipid vesicles composed of three-component lipid mixtures, such as POPC/SM/cholesterol, which exhibit a rich phase diagram, including raft-like liquid-ordered/liquid-disordered phase coexistence regions. We explored the temperature, pressure and concentration-dependent phase behavior of such canonical model raft mixtures using the Laurdan fluorescence spectroscopic technique. Hydrostatic pressure has not only been used as a physical parameter for studying the stability and energetics of these systems, but also because high pressure is an important feature of certain natural membrane environments. We show that the liquid-disordered/liquid-ordered phase coexistence regions of POPC/SM/cholesterol model raft mixtures extends over a very wide temperature range of about 50 °C. Upon pressurization, an overall ordered membrane state is reached at pressures of ∼1000 bar at 20 °C, and of ∼2000 bar at 40 °C. Incorporation of 5 mol% gramicidin as a model ion channel slightly increases the overall order parameter profile in the lo + ld two-phase coexistence region, probably by selectively partitioning into ld domains, does not change the overall phase behavior, however. This behavior is in contrast to the effect of the peptide incorporation into simple, one-component phospholipid bilayer systems.