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Morphological Behavior of Lipid Bilayers Induced by Melittin near the Phase Transition Temperature

Shuichi Toraya ^*, Takashi Nagao , Kazushi Norisada , Satoru Tuzi , Hazime Saitô  , Shunsuke Izumi   and Akira Naito ^*

^* Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan; Department of Life Science, Graduate School of Science, Himeji Institute of Technology, Harima Science Garden City, Kamigori, Hyogo 678-1297, Japan; and Center for Quantum Life Sciences and Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan

Correspondence: Address reprint requests to Akira Naito, Fax: 81-45-339-4251; E-mail: naito{at}ynu.ac.jp.

Morphological changes of DMPC, DLPC, and DPPC bilayers containing melittin (lecithin/melittin molar ratio of 10:1) around the gel-to-liquid crystalline phase transition temperatures (Tc) were examined by a variety of biophysical methods. First, giant vesicles with the diameters of 20 µm were observed by optical microscopy for melittin-DMPC bilayers at 27.9°C. When the temperature was lowered to 24.9°C (Tc = 23°C for the neat DMPC bilayers), the surface of vesicles became blurred and dynamic pore formation was visible in the microscopic picture taken at different exposure times. Phase separation and association of melittin molecules in the bilayers were further detected by fluorescent microscopy and mass spectrometry, respectively. These vesicles disappeared completely at 22.9°C. It was thus found that the melittin-lecithin bilayers reversibly undergo their fusion and disruption near the respective Tcs. The fluctuation of lipids is, therefore, responsible for the membrane fusion above the Tc, and the association of melittin molecules causes membrane fragmentation below the Tc. Subsequent magnetic alignments were observed by solid-state ³¹P NMR spectra for the melittin-lecithin vesicles at a temperature above the respective Tcs. On the other hand, additional large amplitude motion induced by melittin at a temperature near the Tc breaks down the magnetic alignment.

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