||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 £]-sitosterol are important for the formation of a specific domain called lipid raft. £]-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 £]-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 £]-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 £]-sitosterol concentration. Our data shows that addition of £]-sitosterol promotes the formation of the lo phase. Moreover, £]-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 £]-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 £]-sitosterol concentration. A three-phase line separates these two regions is observed at 37¢XC.|
 Keisuke Matsuoka,Tomomi Nakazawa, Ai Nakamura, Chikako Honda,
Kazutoyo Endo, and Masamichi Tsukada. Study of thermodynamic parameters for solubilization of plant sterol and stanol in bile salt micelles. Chemistry and Physics of Lipids 154 (2008) 87¡V93.
 Robert A. Moreaua, Bruce D. Whitaker, and Kevin B. Hicks. Phytosterols, phytostanols, and their conjugates in foods: structural diversity, quantitative analysis, and health-promoting uses. Progress in Lipid Research 41 (2002) 457¡V500.
 Weihrauch JL, Gardner JM. Sterol content of foods of plant origin. J Am Diet Assoc. 73 (1978) 39-47.
 Katarzyna Ha˛c-Wydro, Paweł Wydro, Patrycja Dynarowicz-Ła˛tka, Maria Paluch. Cholesterol and phytosterols effect on sphingomyelin/phosphatidylcholine model Membranes-Thermodynamic analysis of the interactions in ternary monolayers. Journal of Colloid and Interface Science 329 (2009) 265¡V272.
 Chanatip Rujanavech and DavFid. Silbert. Effect of sterol structure on the partition of sterol between phospholipid vesicles of different composition. The Journal of Biological Chemistry 261 (1986) 7215-7219.
 F. Jeffrey Field and Satya N. Mathur. £]-Sitosterol: esterification by intestinal acylcoenzyme A:cholesterol acyltransferase (ACAT) and its effect on cholesterol esterification. Journal of Lipid Research 24 (1983) 409-417.
 Katrin K. Halling, J. Peter Slotte. Membrane properties of plant sterols in phospholipid bilayers as determined by differential scanning calorimetry, resonance
energy transfer and detergent-induced solubilization. Biochimica et Biophysica Acta 1664 (2004) 161¡V171.
 Claudia Silva, Francisco J. Aranda, Antonio Ortiz, Vicente Martinez, Micaela Carvajal, Jose A. Teruel. Molecular aspects of the interaction between plants sterols and DPPC bilayers an experimental and theoretical approach. Journal of Colloid and Interface Science 358 (2011) 192¡V201.
 S.J. singer and G. L. Nicolson. The fluid mosaic model of the structure of cell membranes. Science. 175 (1972) 720-731.
 N. Campbell, J. Reece, and L. Mitchell. Biology (3rd edn). Addison Wesley Longman, New York, 1999.
 Simons, K and E. Ikonen. Functional rafts in cell membranes. Nature. 387 (1997) 569-572.
 D. L. Nelson and M. M. Cox. Lehninger principles of biochemistry (4th edn). Worth Publisher, New York, 2005.
 Ya-Wei Hsueh, Kyle Gilbert, C. Trandum, M. Zuckermann, and Jenifer Thewalt. The effect of ergosterol on dipalmitoylphosphatidylcholine bilayer: a deuterium NMR
and calorimetric study. Biophy. J. 88 (2005) 1799-1808.
 David A. Mannock, Ruthven N. A. Lewis, and Ronald N. McElhaney. Comparative calorimetric and spectroscopic studies of the effects of lanosterol and cholesterol on the thermotropic phase behavior and organization of dipalmitoylphosphosphatidylcholine bilayer membranes. Biophy. J. 91 (2006) 3327-3340.
 Mary Elizabeth Beattie, Sarah L. Veatch, Benjamin L. Stottrup, and Sarah L. Keller. Sterol structure determines miscibility versus melting transitions in lipid vesicles. Biophy. J. 89 (2005) 1760-1768.
 Ya-wei Hsueh, Mei-Ting Chen, Philipus J. Patty, Christian Code, John Cheng, Barbara J. Frisken, Martin Zuckermann, and Jenifer Thewalt. Ergosterol in POPC membranes: physical properties and comparison with structurally similar sterols. Biophy. J. 92 (2007) 1606-1615.
 J. Eisenblätter and R. Winter. Pressure effects on the structure and phase behavior of DMPC-Gramicidin lipid bilayers: a synchrotron SAXS and 2H-NMR spectroscopy study. Biophy. J. 90 (2006) 956-966.
 Sarah L. Veatch and Sarah L. Keller. Organization in lipid membranes containing cholesterol. Phys. Rev. Lett. 89 (26) (2002) 268101.
 Sarah L. Veatch and Sarah L. keller. Miscibility phase diagrams of giant vesicles containing sphingomyelin. Phys. Rev. Lett. 94 (14) (2005) 148101.
 Frédérick de Meyer and Berend Smit. Effect of cholesterol on the structure of a phospholipid bilayer. Pnas. 106 (2009) 3654-3658.
 Hans-Josachim Lehmler and Paul M. Bummer. Mixing behavior of 10-(perfluorohexyl)-decanol and DPPC. Colloids Surf B Biointerfaces. 44 (2005) 74-81.
 Miho Yanagisawa, Masayuki Imai, and Takashi Taniguchi. Shape deformation of ternary vesicles coupled with phase separation. Phys. Rev. Lett. 100 (14) (2008) 148102.
 J. L. Rubenstein, B. A. Smith, and H. M. McConnell. Lateral diffusion in binary mixtures of cholesterol and phosphatidylcholines. Proc. Natl. Acad. Sci. 76 (1976) 15-18.
 Juan M. Vanegas, Roland Faller, and Marjorie L. Longo, Influence of ethanol on lipid/sterol membranes: phase diagram construction from AFM imaging. Langmuir Lett. 26 (2010), 10415¡V10418.
 C. P. Slichter. Principles of magnetic resonance (3rd edn). Springer-Verlag, Berlin, 1990.
 James H. Davis. The description of membrane lipid conformation, order and dynamics by 2H-NMR. Biochimica et Biophysica Acta 737 (1983) 117¡V 171.
 P.J.Hore. Nuclear magnetic resonance. Oxford University Press, New York, 1995.