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標題: | 脂質雙層膜在吸附表面的相態行為 Phase Behavior of Lipid Bilayers Adsorbed on a Surface |
作者: | Chia-Ling Chi 紀佳伶 |
指導教授: | 諶玉真(Yu-Jane Sheng) |
關鍵字: | 耗散粒子動力學法, Dissipative particle dynamics (DPD), |
出版年 : | 2013 |
學位: | 碩士 |
摘要: | 生物體的細胞膜是由脂質(lipid)所組成的雙層結構,脂質的構造是一端為親水的頭基和另一端為兩條疏水性的尾基。吸附在一固體表面的脂質雙層膜(supported lipid bilayers)因為固定於親水的基板,雙層膜在水溶液中很穩定且許多特性分析的儀器能適用,所以被廣泛地應用在細胞膜的研究。脂質雙層膜受到溫度的影響,有不同的相態存在,在低溫環境下,尾基的碳鏈很整齊的排列,膜厚度較高,稱之為凝膠態(gel phase),而在高溫時,尾巴則是沒有方向性的凌亂交錯,膜厚度較低,為液態(liquid phase)。介於凝膠態與液態之間,還有一種相態存在:波紋態(ripple phase),此溫度下有部分的脂質為凝膠態,其餘為液態,因此膜的厚度有高有低類似波浪的形狀。凝膠態轉變為波紋態的溫度為脂質的前相轉移溫度(pre-transition temperature),而波紋態到液態的溫度則為相轉移溫度(main transition temperature)。
每一種脂質有自己的相轉移溫度,本研究是利用耗散粒子動力學法探討不同種類的脂質之相轉移溫度的變化。我們發現許多物理性質,如膜的厚度、尾基排列的整齊度及吸附於親水表面的膜面積等等,都是溫度的函數,且在相轉移溫度時有明顯的劇烈變化,因此從這些性質與溫度的曲線中,藉由反曲點(inflection point)可以準確地得到脂質的相轉移溫度。其中熱容(heat capacity)這個性質隨溫度變化會出現兩個尖峰,可同時測得前相轉移溫度與相轉移溫度。研究結果發現,尾基的碳鏈長度愈短、脂質雙鍵愈多、疏水端愈不排斥水、頭基愈互相排斥以及頭基與親水板間的附著力愈強,相轉移溫度愈低。 自然界的生物細胞膜,大部分是由不同種類的脂質所組成的,由於不同結構的脂質在同一個環境溫度下有不同的相態存在,因此不同於單成分雙層膜,雙成分系統有相分離的現象。本研究發現雙成分混合膜分相的形態與兩成分之間的互溶度和混合比例有關。此部分的研究主要應用於生物感測器,由於這類的微機電儀器要進入生物體內做檢測,常會發生體內的排斥現象,因此這些裝置表面會塗抹一層脂質雙層做改良。利用兩種結構的脂質在一特定的溫度下有兩種相態的共存,可於感測器的表面上創造出較高(脂質相轉移溫度較高)與較低的雙層膜(脂質相轉移溫度較低)圖樣(patterning)。 The biological cell membrane is made of two layers of lipid molecules. A phospholipid comprises a hydrophilic head group and two hydrophobic tails. Since a supported lipid bilayer (SLB) is anchored to a hydrophilic solid surface, it is quite stable in an aqueous solution, allowing the use of some characterization tools. Therefore, SLB is a popular model for studying cell membranes. Depending on temperature, a lipid bilayer can exist in different phases. For most phospholipids, tails are highly ordered and the membrane thickness is larger at low temperature, which is called the gel phase, while at high temperature, the gel phase undergoes a transition to liquid phase where the tails are disordered and the membrane thickness is smaller. In addition, there is a phase called ripple phase between gel and liquid phases. Due to the coexistence of both gel (thick) and liquid (thin) states, the surface of the membrane is rippled. The transition temperature between gel and ripple phases is called the pre-transition temperature (Tp), whereas the transition temperature from ripple to liquid phase is referred to as the main transition temperature (Tm). All lipids have their own transition temperatures. In this work, we employed dissipative particle dynamics (DPD) method to investigate the transition temperatures of various lipids. We found that many physical quantities, such as membrane thickness, order parameter of tails, deposition area of lipids on a substrate, etc., are functions of temperature. Those properties exhibit great changes at Tm; thus, Tm can be determined by the inflection point of the properties vs. temperature curves. Heat capacity can be used to detect both Tp and Tm via two peaks in the plot. In conclusion, lower Tm will be found for lipids with: shorter tails, more double bonds, lower tail hydrophobicity, stronger repulsive force between lipid headgroups, and more attractive force between lipid head and the plate. Most natural membranes are composed of different types of lipids; hence, phase separation, which is not seen in single component systems, can be observed in two-component SLBs. The structure and morphology of binary mixtures are strongly dependent on the miscibility and the molar composition of the two constituents. Two-component SLBs are promising for applications in diagnostic devices. Since biosensors are implanted inside the human body, surface modifications such as coating a phospholipid bilayer on the non-biological materials are needed to prevent human body rejection. As a result, lipid patterning on a substrate can be created by the coexistence of the two phases at a given temperature. The higher domains are solid-like lipids (with higher Tm) while the lower areas are fluid-like ones (with lower Tm). |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/62355 |
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