Percolation and diffusion in three-component lipid bilayers: effect of cholesterol on an equimolar mixture of two phosphatidylcholines

• Published in: Biophysical journal Vol. 64; no. 2; pp. 399 - 412
• Main Authors: Almeida, P.F., Vaz, W.L., Thompson, T.E.
• Format: Journal Article
• Language: English
• Published: Bethesda, MD Elsevier Inc 01.02.1993; Biophysical Society
• Subjects: Biological and medical sciences; Biophysical Phenomena; Biophysics; cholesterol; Cholesterol - chemistry; composition; Diffusion; dimyristoylphosphatidylcholine; Dimyristoylphosphatidylcholine - chemistry; distearylphosphatidylcholine; distribution; Fundamental and applied biological sciences. Psychology; lipid bilayers; Lipid Bilayers - chemistry; Models, Chemical; Molecular biophysics; Molecular Probes; Phosphatidylcholines - chemistry; Photochemistry; Physico-chemical properties of biomolecules; Thermodynamics; Differential scanning calorimetry; Stoichiometry; Temperature; Lateral diffusion; Mixed bilayer; Artificial membrane; Cholesterol; Fluorescence spectrometry; Fluorescent labelling; Thermodynamic parameter; Phosphatidylcholine; Percolation; Models; Physicochemical properties
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/S0006-3495(93)81381-X

The lateral diffusion of a phospholipid probe is studied in bilayers of binary mixtures of dimyristoylphosphatidylcholine (DMPC)/cholesterol and distearoylphosphatidylcholine (DSPC)/cholesterol and in the ternary system DMPC/DSPC/cholesterol using fluorescence recovery after photobleaching. An approximate phase diagram for the ternary system, as a function of temperature and cholesterol concentration, was obtained using differential scanning calorimetry and the phase diagrams of the binary systems. This phase diagram is similar to those of the phospholipid/cholesterol binary mixtures. In bilayers where solid and liquid phases coexist, the diffusion results are interpreted in terms of phase percolation. The size of the liquid-phase domains is estimated using percolation theory. In the ternary system, addition of cholesterol up to approximately 20 mol% shifts the percolation threshold to lower area fractions of liquid, but the size of the liquid-phase domains does not change. Above approximately 20 mol% cholesterol, the liquid phase is always connected. The size of solid-phase domains clusters is estimated using a model recently developed (Almeida, P.F.F., W.L.C. Vaz, and T.E. Thompson. 1992. Biochemistry. 31:7198–7210). For cholesterol concentrations up to 20 mol%, the size of solid-phase domain units does not change. Beyond 20 mol%, cholesterol causes the size of the solid units to decrease.