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Characterization of a Quasicrystalline Phase in Codispersions of Phosphatidylethanolamine and Glucocerebroside

Ying Feng ^*, Dominique Rainteau , Claude Chachaty , Zhi-Wu Yu ^*, Claude Wolf  and Peter J. Quinn 

^* Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, China; Faculté de Medecine Saint Antoine, Universite Paris 6, Inserm U538, Paris 75012, France; and Department of Life Sciences, King's College, London SE1 9NN, United Kingdom

Correspondence: Address reprint requests to Professor Dr. Zhi-Wu Yu, Dept. of Chemistry, Tsinghua University, Beijing 100084, P. R. China. Tel.: +086-10-6279-2492; Fax: +086-10 6277-1149; E-mail: yuzhw{at}tsinghua.edu.cn.

Synchrotron x-ray diffraction, differential scanning calorimetry, and electron spin resonance spectroscopy have been employed to characterize a quasicrystalline phase formed in aqueous dispersions of binary mixtures of glucocerebroside and palmitoyloleoylphosphatidylethanolamine. Small- and wide-angle x-ray scattering intensity patterns were recorded during temperature scans between 20° and 90°C from mixtures of composition 2, 5, 10, 20, 30, and 40 mol glucocerebroside per 100 mol phospholipid. The quasicrystalline phase was characterized by a broad lamellar d-spacing of 6.06 nm at 40°C and a broad wide-angle x-ray scattering band centered at 0.438 nm, close to the gel phase centered at 0.425 nm and distinct from a broad peak centered at 0.457 nm observed for a liquid-crystal phase at 80°C. The quasicrystalline phase coexisted with gel and fluid phase of the pure phospholipid. An analysis of the small-angle x-ray scattering intensity profiles indicated a stoichiometry of one glucosphingolipid per two phospholipid molecules in the complex. Structural transitions monitored in cooling scans by synchrotron x-ray diffraction indicated that a cubic phase transforms initially into a lamellar gel. Thermal studies showed that the gel phase subsequently relaxes into the quasicrystalline phase in an exothermic transition. Electron spin resonance spectroscopy using spin labels located at positions 7, 12, and 16 carbons of phospholipid hydrocarbon chains indicated that order and motional constraints at the 7 and 12 positions were indistinguishable between gel and quasicrystalline phases but there was a significant decrease in order and increase in rate of motion at the 16 position on transformation to the quasicrystalline phase. The results are interpreted as an arrangement of polar groups of the complex in a crystalline array and a quasicrystalline packing of the hydrocarbon chains predicated by packing problems in the bilayer core requiring disordering of the highly asymmetric chains. The possible involvement of quasicrystalline phases in formation of membrane rafts is considered.