||Lateral heterogeneities exist in biological membranes of living cell. The heterogeneity is proposed to be a coexistence of lipid domains with differing degrees of order, lipid/protein compositions, and physical properties in the membrane. Recent evidence suggests that not only cholesterol, the major sterol found in mammals, but other sterols such as β-sitosterol are important for the formation of a specific domain called lipid raft. β-sitosterol is one of plant sterols and its chemical structure is similar to that of cholesterol. While most study of lipid-sterol interaction focuses on cholesterol, little is known about plane sterols. In addition, it is pointed out that β-sitosterol decreases serum of cholesterol and reduced cardiovascular disease. We investigate the phase behavior of model membranes composed of 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and β-sitosterol using deuterium nuclear magnetic resonance (2H NMR). The sn-1 chain of DPPC is deuterium labeled. The 2H NMR spectra were taken as a function of temperature and β-sitosterol concentration. Our data shows that addition of β-sitosterol promotes the formation of the lo phase. Moreover, β-sitosterol has opposite ordering effect on the DPPC membranes below and above Tm: It decreases the order of DPPC membranes below Tm, whereas increases the chain orders of DPPC above Tm. Finally, the partial phase diagram is determined from 2H NMR spectra. Coexistence of so+ld phase is observed at low β-sitosterol concentration. On the other hand, there are two two-phase coexistence regions, so+lo and lo+ld, found below and above Tm, respectively, at intermediated β-sitosterol concentration. A three-phase line separates these two regions is observed at 37°C.|
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