title: Detection of bilayer packing stress and its release in lamellar-cubic phase transition by time-resolved fluorescence anisotropy.
abstract: An introduction of nonlamellar-forming lipids into planar bilayers generates packing stress, which is important for the biological functions of plasma membranes and is a driving force for the lamellar-nonlamellar phase transition. We have investigated the phase behavior of a binary system consisting of egg yolk phosphatidylcholine and monoolein (MO) and the changes in the local orientation order of lipids in a lamellar-bicontinuous cubic phase transition. Small-angle X-ray scattering has revealed that the lamellar-bicontinuous cubic phase transition occurs at an MO molar fraction (X(MO)) between 0.6 and 0.7. These phases were dispersed to form liposomes and cubosomes to monitor the anisotropy of the incorporated fluorescence probe, in which Pluronic F127, used as a dispersion stabilizer of the cubic phase, has been proven not to alter the cubic structure and the location of the probes. Time-resolved fluorescence anisotropy measurements on these dispersions have revealed that the order parameter of the probe in the lamellar phase increases with increasing X(MO), and that it decreases during the transition to the cubic phase. This observation suggests that packing stress generated by the addition of the nonlamellar-forming lipid is released by the phase transition.
authors.affiliation: Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. mnakano@pharm.kyoto-u.ac.jp
authors.forename: Minoru
authors.initials: M
authors.lastname: Nakano
authors.forename: Tomoari
authors.initials: T
authors.lastname: Kamo
authors.forename: Atsuhiko
authors.initials: A
authors.lastname: Sugita
authors.forename: Tetsurou
authors.initials: T
authors.lastname: Handa
mesh_terms: D000465:Algorithms; D005454:Fluorescence Polarization; D005989:Glycerides / Q000737:chemistry; D008027:Light; D008051:Lipid Bilayers / Q000737:chemistry*; D008081:Liposomes / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D010713:Phosphatidylcholines / Q000737:chemistry; D012542:Scattering, Radiation; D013050:Spectrometry, Fluorescence
chemical_list: D005989:Glycerides; D008051:Lipid Bilayers; D008081:Liposomes; D010713:Phosphatidylcholines; C005953:monoolein