Phase studies of model biomembranes: Macroscopic coexistence of Lα + Lβ, with light-induced coexistence of Lα + Lo Phases

• Published in: Biochimica et biophysica acta Vol. 1768; no. 11; pp. 2777 - 2786
• Main Authors: Zhao, Jiang, Wu, Jing, Shao, Huilin, Kong, Fanrong, Jain, Nieraj, Hunt, Geoffrey, Feigenson, Gerald
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.11.2007
• Subjects: Antioxidant; Lipid peroxidation; Liquid-disordered phase; Liquid-ordered phase; Phase boundary shift; Photo-induced; DSPC; DOPG; NPG; RSE; DSPG; 16:0,Bodipy-PC; Lipid peroxidation; SOPC; GUV; TLC; Antioxidant; Phase boundary shift; C20:0-DiI; POPC; Liquid-disordered phase; SM; FRET; TR-DHPE; DLPC; Photo-induced; Liquid-ordered phase; DOPC; DPPC
• ISSN: 0005-2736; 0006-3002; 1879-2642
• DOI: 10.1016/j.bbamem.2007.07.009

Phase diagrams of 3-component lipid bilayer mixtures containing cholesterol reveal major differences among the different types of lipids. Here we report that mixtures of cholesterol together with POPC and a high-melting temperature PC or sphingomyelin show different phase behavior from similar mixtures that contain DOPC or di-phytanoyl-PC instead of POPC. In particular, only one region of macroscopic phase coexistence occurs with POPC, a region of coexisting liquid disordered and solid phases, {Lα + Lβ}. Fluorescence microscopy imaging is useful for these studies, but is subject to artifactual light-induced domain formation, as reported by Ayuyan and Cohen [A.G. Ayuyan, F.S. Cohen, Lipid peroxides promote large rafts: Effects of excitation of probes in fluorescence microscopy and electrochemical reactions during vesicle formation, Biophys. J. 91 (2006) 2172-2183.]. This artifact can be attenuated by decreased illumination and low dye concentration. The use of the free radical scavenger n-propyl gallate can reduce the artifact, but this molecule enters the bilayer and itself perturbs the phase behavior. We suggest that the light-induced domain separation artifact might actually arise from pre-existing lipid clusters that are induced to coalesce, and therefore indicates highly nonrandom mixing of the lipid components.