Phase studies of model biomembranes: macroscopic coexistence of Lalpha+Lbeta, with light-induced coexistence of Lalpha+Lo Phases.
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, {Lalpha+Lbeta}. 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.
Zhao
Jiang
J
Field of Biophysics, Cornell University, Ithaca, NY 14853, USA.
Wu
Jing
J
Shao
Huilin
H
Kong
Fanrong
F
Jain
Nieraj
N
Hunt
Geoffrey
G
Feigenson
Gerald
G
D002784:Cholesterol / Q000737:chemistry; D008027:Light; D008051:Lipid Bilayers / Q000737:chemistry*; D021962:Membrane Microdomains / Q000737:chemistry*; D044367:Phase Transition; D010713:Phosphatidylcholines / Q000737:chemistry*; D011435:Propyl Gallate / Q000494:pharmacology; D013109:Sphingomyelins / Q000737:chemistry; D013696:Temperature
D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D013109:Sphingomyelins; D011435:Propyl Gallate; D002784:Cholesterol; C028694:1-palmitoyl-2-oleoylphosphatidylcholine