雙成份脂質雙層膜在固體表面的相態及動態行為

Abstract

生物細胞膜是由不同種類的脂質、膽固醇、蛋白質和其他物質所組成，脂質是由一親水的頭基和兩條疏水的尾基所組成的，因此由脂質所組成的脂質雙層麼可於水溶液中存在。支撐性脂質雙層模是將一脂質雙層膜嵌合於一親水基板上，嵌合於親水基板上的脂質雙層膜在水溶液中相當穩定，可供實驗數週甚至數月之久，其平面且固定的特性方便於許多實驗儀器來測量雙層膜的性質，因此支撐性脂質雙層膜在有關細胞膜的相關研究中是相當受歡迎的實驗模組。於此研究中，我們使用了耗散粒子動力學法(dissipative particle dynamics)來模擬支撐性雙成份脂質雙層膜系統並觀察其相轉移溫度和分相行為。純物質脂質雙層膜在不同溫度下會有不同的相態，而雙成份脂質雙層膜在不同溫度及組成下也會有分相的狀況。系統中兩成分脂質的結構與組成對於整體雙層膜的形態有很大的影響。對於多數的脂質來講，在低溫的凝膠態下其尾基會較整齊的排列且整體膜厚度也會較高；而在高溫的流體態下其尾基會較紊亂的排列且整體膜厚度也較低，然而相轉移溫度即是發生相轉移時的溫度，不同於純物質脂質雙層膜的分相行為，雙成份脂質雙層膜除了既有的相態之外還會出現兩相共存區，當雙成份兩成份尾基差距越大，兩相共存區的範圍也越大。 我們利用分子模擬來觀察雙成份脂質雙層膜的分相行為，首先我們以差示掃描熱量分析儀原理測定雙成分系統的相轉移溫度，接著我們便思考是否有其他方法也可以來測量雙成分系統的相轉移溫度，由於分別在凝膠態及流體態整齊度參數和擴散系數會有明顯的差異，於此研究中我們將使用這兩者來測量相轉移溫度。 最後我們會觀察在不同尾基組合下的雙成份脂質雙層膜在形態上會有甚麼不同，我們將從膜厚度、膜厚度標準差、膜厚度分佈圖、整齊度參數來觀察。

Cell membranes are composed of different types of lipids, protein, cholesterol and other different substances. A phospholipid comprises a hydrophilic head group and two hydrophobic tails, so it can form a bilayer in aqueous environment. A supported lipid bilayer (SLB) is a lipid membrane fixed on a hydrophilic solid surface, so the lipid membrane is quite stable in an aqueous solution, allowing the use of some characterization tools. Therefore, SLB is a popular model for studying cell membranes. In this work, we employed dissipative particle dynamics (DPD) method to investigate the transition temperatures and phase behavior of supported binary lipid mixtures. Depending on temperature, a lipid bilayer can exist in different phases, and the phase separation of lipid can be observed in two-component supported lipid bilayers. The morphology of binary mixtures are strongly dependent on the structure and the molar composition of the two constituents. For most phospholipids, the tails of lipid are highly ordered and the membrane thickness is larger at low temperature, which is called the gel phase. When the bilayer at high temperature, the gel phase undergoes a transition to liquid phase where the tails are disordered and the membrane thickness is smaller. The main transition temperature is the temperature where the phase transition occurred. We also found that phase transition in mixed lipid bilayers is different from single component lipid bilayers, there is no longer a single transition from gel phase to liquid phase. A gel and liquid coexistence region separated from gel and liquid phase. As increasing the difference between two lipid tails length, the coexistence region is more distinctly. In our previous work, we have confirmed the validity of the pure lipid bilayer in simulation. Confirming the validity of our binary system with thermodynamics is our next step. In order to measure the main transition temperature, we used to use DSC to measure the heat capacity in experiment, and use statistical thermodynamics relation in simulation. In this work, we tried to use different methods to measure the main transition temperature, likes order parameter and diffusivity. And then, we studied the morphology of binary lipid bilayer with two different tails length by thickness, standard deviation of thickness, thickness distribution and order parameter.