Phase behavior of cholesterol-containing binary membrane of an ether-linked phospholipid, dihexadecylphosphatidylcholine

The thermotropic phase behavior of the cholesterol (Chol)-containing binary membrane of dihexadecylphosphatidylcholine (DHPC) was investigated as a function of the mole fraction of Chol Xch by DSC and fluorescence spectroscopy using 6-propionyl-2-(N,N-dimethylamino)naphthalene (Prodan). The main-transition enthalpy ΔHm was almost constant at Xch < ca. 0.05, but it decreased almost linearly with increasing Xch above 0.05 until it reached 0 at Xch = ca. 0.50, suggesting that the interaction between DHPC and Chol is relatively weaker than that between dipalmitoylphosphatidylcholine (DPPC) and Chol. The Prodan fluorescence spectra changed depending on the phase state, which was characterized by the wavelength at the emission maximum. Particularly, the two-phase state of the interdigitated (LβI) and non-interdigitated (Lβ) gel was sensitively detected as the superposition of the two elementary spectra corresponding to each phase state, allowing us to explain the Xch dependent non-linear behavior of the Chol-induced Lβ phase by introducing a proportionality factor ξ(Xch) and by assuming a simple probabilistic process. By comparing the phase diagram constructed for the DHPC–Chol binary membrane with that for the DPPC–Chol binary bilayer, a difference was observed in the minimum Xch value for the entire formation of the liquid ordered (Lo) phase. This difference could be explained in terms of the hydrophobic mismatch at the bilayer midplane by taking into account the difference in the capability of hydrogen bonding with the 3β-OH group of Chol due to the presence or absence of the carbonyl oxygens at the glycerol backbone.