Molecular-level pictures of the phase transitions of saturated and unsaturated phospholipid binary mixtures
2014
Wu, Fu-Gen | Sun, Hai-Yuan | Zhou, Yu | Deng, Geng | Yu, Zhi-Wu
Binary lipid mixtures consisting of saturated and unsaturated lipids are important models for natural cell membranes. However, a detailed molecular picture for the phase transition process of such lipid binary mixtures remains unclear. Herein, by using deuterated dipalmitoylphosphatidylcholine (DPPC-d₆₂) and hydrogenated dioleoylphosphatidic acid (DOPA), we expect to separately analyze the changes of the two lipid components during thermotropic phase transitions by temperature-dependent Fourier transform infrared (FTIR) spectroscopy, and uncover the hidden secrets of a seemingly single endothermic peak observed in differential scanning calorimetry (DSC) experiments. We found that at low DOPA concentrations (10–20 mol%), a gel to fluid conformational transition of DPPC-d₆₂ and a fluidization transition of DOPA by DPPC-d₆₂ were observed. The two lipids were found to have nonsynchronous conformational rearrangements in the tail regions upon thermotropic phase transitions, with the change of DOPA earlier than DPPC-d₆₂. Besides, in the mixed fluid phase at 40 °C, the unsaturated DOPA can be fluidized (loosened) by the saturated DPPC-d₆₂, and such an isothermal fluidization effect is more pronounced at elevated DPPC-d₆₂ concentrations. At higher DOPA contents (30–50 mol%), only DPPC-d₆₂ molecules have conformational transitions upon heating. The present work demonstrates for the first time that the unsaturated lipid component can have significant conformational reorganizations in the phase transition process of a saturated–unsaturated binary lipid mixture.
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