Phase behavior of binary bilayer membrane of dipalmitoylphosphatidylcholine and stigmasterol

We investigated the phase behavior of a binary bilayer membrane of dipalmitoylphosphatidylcholine (DPPC) and stigmasterol (Stig) as a function of the Stig concentration (X-St) by means of differential scanning calorimetry (DSC) and fluorescence spectroscopy using 6-propionyl-2-(N,N-dimethylamino)naphthalene (Prodan) as a membrane probe. The DSC results revealed that the pretransition and the main transition are abolished at X-St=ca. 0.08 and ca. 0.50, respectively. The main-transition enthalpy (H-m) of the DPPC-Stig binary bilayer decreased with X-St in a nonlinear manner, which is contrast to the almost linear decrease in H-m for the DPPC-cholesterol (Chol) binary bilayer. This suggests that Stig has a stronger effect of disturbing the chain packing in a gel state and a weaker effect of suppressing the chain melting as compared to Chol. The X-St dependence of the peak width at half height (T-1/2) suggests that a drastic change in the phase behavior occurs at X-St=ca. 0.14, which is similar to the DPPC-Chol binary bilayer. The Prodan fluorescence spectra changed depending on the phase state, which was characterized by the wavelength at the emission maximum and allowed us to determine the transition temperatures of the binary bilayers. The compositional phase diagram of the DPPC-Stig binary bilayer was found to be similar to that of the DPPC-Chol binary bilayer. The phase behavior of the DPPC-Stig binary bilayer could be well explained by applying the so-called superlattice view to interpret the phase diagram.