混合脂質雙層膜在吸附表面相行為

Abstract

生物膜是由不同脂質(lipid)所組成的雙層結構，脂質的構造為一端親水的頭基與另一端兩條疏水性的尾基。吸附在基材上的脂質雙層膜因為固定於親水的基材上(SLB)，雙層膜不易受到溶液的影響且性質穩定可以長達數周或數個月，因此廣泛的運用於細胞膜的研究以及分析。脂質雙層膜在不同溫度下有不同的相態，低溫時，脂質的尾基排列整齊，膜厚度高，此狀態稱為凝膠態(gel phase)。在高溫時，脂質交錯排列，膜厚度低，為流體態(liquid phase)。而介於此兩種相態間，有一波紋態(ripple phase)，此溫度下，有些脂質尾基排列整齊，有些為凌亂的，因此膜厚度有高有低。在凝膠態轉換為波紋態的溫度為預相轉移溫度(pre-transition temperature)，波紋態轉變為流體態的溫度為相轉移溫度(main transition temperature)。 在混合脂質雙層膜中，隨著混合比例不同，雙層膜的相轉換溫度也會隨之改變，本研究利用耗散動力學法探討混合系統下脂質的相轉移溫度變化以及相分離的現象。發現許多物理性質，如膜厚度、尾基的排列整齊度以及膜的面積變化都是溫度的函數，在相轉移溫度時都有明顯的變化，因此可以藉這些曲線的反曲點，進而找到脂質雙層膜的相轉移溫度。也可以從機械性質的分析，如彎曲係數以及面積擴張係數得到相轉移溫度，因為在相轉移溫度時整體膜會變得比較柔軟，因此在機械性質分析的最低點可以求得相轉移溫度。研究結果發現，依脂質混合比例不同相轉換溫度會隨之改變，但轉換溫度是介於純脂質的轉移溫度之間。混合的脂質碳鏈長度差越多、脂質雙鍵越多，越容易相分離。混合系統下，改變不飽和脂質的雙鍵親疏水度，會造成更明顯的相分離現象。使用具有兩條不同碳鏈長度尾基的脂質形成雙層膜時，性質會比較接近短的碳鏈脂質。

Biological membranes are composed of different types of lipids. Lipids comprise a hydrophilic head and two hydrophobic tails. A lipid bilayer anchored to a hydrophilic solid surface is called a supported lipid bilayer (SLB). A SLB can exist stably within a solution for more than several weeks or months. As a consequence, a SLB is an ideal model for studying biological membrane. Lipid bilayers can exhibit different phases depending on temperature. Generally, a lipid bilayer can be in either a liquid or a solid (gel) phase at a given temperature. The bilayer can take on a solid-ordered phase state at low temperatures but exhibit a liquid-disordered state at high temperatures. That is, the lipid tails are highly ordered in the gel phase and become disordered in the liquid phase. For some lipids, there exists a rippled phase between the gel and liquid phases. The temperature at which a transition from the gel phase to the rippled phase takes place is called the pre-transition temperature (Tp) and it is linked to the formation of periodic membrane ripples. Subsequently the melting of the bilayer from the rippled phase to the liquid phase occurs at the main transition temperature (Tm). In this work, we use dissipative particle dynamics to investigate the phase behaviors of supported mixed lipid bilayers. It is found that certain physical properties, such as membrane thickness, membrane area, lipid order parameter are a function of temperature. Tmcan be accurately determined from the inflection point of the property versus temperature curves. Our results reveal that Tms of lipids with unsaturated bonds are greatly reduced and Tm of a mixed lipid bilayer situates between Tms of constituent pure lipids depending on the mixing ratio. Phase separation becomes more significant as difference in lipid tail length increases or number of unsaturated bonds grows. Moreover, the mechanical properties, such as bending and stretching modulus, dip to a minimum value as temperature reaches Tm indicating the membrane becomes most soft and stretchable at the main transition temperature.