Gradual change or phase transition: Characterizing fluid lipid-cholesterol membranes on the basis of thermal volume changes

Cholesterol has been reported to govern biomembrane permeability, elasticity, and the formation of lipid rafts. There has been a controversy whether binary lipid-cholesterol membranes should better be described in terms of a phase separation (liquid-ordered and liquid-disordered phases) or of gradual changes in largely homogeneous membranes. We present a new approach for detecting and characterizing phase equilibria in colloidal dispersions using pressure perturbation calorimetry (PPC). We apply this to the study of the thermal expansivity of mixtures of 1-palmitoyl-2-oleoyl sn-glycero-3-phosphatidylcholine (POPC) and cholesterol as a function of composition and temperature. We show that cholesterol can condense lipids not only laterally (with respect to interfacial area) but also in volume. A quantitative comparison with expansivity curves simulated assuming either phase separation or random mixing within one phase reveals that the real system shows an intermediate behavior due to submicroscopic demixing effects. However, both models yield consistent system parameters and are thus found to be useful for describing the systems to a similar approximation. Accordingly, one cholesterol may condense 3 +/- 1 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphatidylcholine molecules by similar to -(1.4 +/- 0.5) vol% at 2 degrees C; both absolute values decrease with increasing temperature.