以耗散粒子動力學法研究囊胞之物理性質與融合機制

Abstract

以耗散粒子動力學法研究尺寸影響囊胞的行為，包括脂質膜上的特性和機械性質。藉由控制脂質的濃度可以自發形成不同尺寸的囊胞，自發形成的囊胞處於介穩狀態。當囊胞的尺寸下降時，膜厚度愈來愈薄且表面的親水頭基密度下降。然而，內層表面的親水頭基密度高於外層。從內外層排列參數結果表明，外層的脂質像一個被截斷的圓錐形，但在內部的脂質類似於一個倒截錐形。根據局部範圍的整齊度參數可以發現隨著囊胞尺寸變小而整齊度下降，而脂雙層變薄的主要原因是來自於脂質無序的排列。而脂質膜的張力可通Young-Laplace方程式計算出，囊胞張力隨著囊泡的尺寸下降而張力上升。因此，小顆囊胞較不穩定易發生融合。也可以使用充水的方法計算出拉伸模數和彎曲模數，而拉伸模數和彎曲模數隨著囊胞尺寸下降而上升。但是彎曲模數、拉伸模數和膜厚仍然具有固定常數的關係。最後，利用簡單介穩狀態的囊胞模型推測說明囊胞尺寸對膜厚、整齊度和張力的影響行為。 利用耗散粒子動力學法探討小顆囊胞的充水行為，包括膨脹、破裂、復原和融合機制。將自發所形成的囊胞充水時，囊胞的尺寸會增加且膜厚度會下降。當充水過程中，內層表面的親水性頭基密度下降的比外層迅速。還有內層與外層的排列參數都變成截錐形即使內層原先為倒截錐形而外層為截錐形。從局部範圍的整齊度參數可以發現隨著充水的量增加，脂質排列性變得更凌亂無序。從疏水尾端分佈圖可以發現內層與外層發生了交叉穿插的結構。脂質膜的張力可通過Young-Laplace方程式計算出，囊胞張力隨著充水量增加而上升。當充水越多囊胞越不穩定，更容易發生融合現象。最後不論囊胞尺寸為何，只要由同一種脂質所組成的囊胞在達破裂前其所有的物理及機械性質幾乎達到一致。 研究發現只有適當的界面活性劑才具有能力可以溶解囊胞形成小顆的混合微胞，當界面活性劑不夠殊水時，只會有少許界面活性劑會進入囊胞內其他大多傾向停留在囊胞外。因此囊胞結構不受界面活性劑濃度的影響。相反地，疏水的界面活性劑會使囊胞結構隨著活性劑濃度增加而混合聚集成空心圓柱狀，甜甜圈狀，平面雙層結構…等。此外對於中度疏水性界面活性劑，囊胞溶解形成混合微胞前會先看到開孔的囊胞，因此囊胞溶解機制比之前提的三階段假說更複雜。當界面活性劑達到臨界微胞濃度，界面活性劑開始發生聚集且囊胞同時發生扭曲行為，當界面活性劑濃度達到適量時，囊胞開始出現穿孔，且隨著界面活性劑的濃度增加囊胞的開孔越大，當界面活性劑濃度更高時，脂質開始離開囊胞並且與界面活性劑混合形成微胞，最後達到整個囊胞溶解。

The size-dependent behavior of small unilamellar vesicles is explored by dissipative particle dynamics, including the membrane characteristics and mechanical properties. The spontaneously formed vesicles are in the metastable state and the vesicle size is controlled by the concentration of model lipids. As the vesicle size decreases, the bilayer thickness is getting thinner and the area density of heads declines. Nonetheless, the area density in the inner leaflet is higher than that in the outer. The packing parameters are calculated for both leaflets. The result indicates that the shape of lipid in the outer leaflet is like a truncated cone but that in the inner leaflet resembles an inverted truncated cone. Based on a local order parameter the orientation order is found to decay with reducing vesicle size and this fact reveals that the thinner bilayer is mainly attributed to the orientation disorder. The membrane tension can be obtained through the Young-Laplace equation. The tension is found to grow with reducing vesicle size. Therefore, small vesicles are less stable against fusion. Using the inflation method, the area stretching and bending moduli can be determined and those moduli are found to grow with reducing size. Nonetheless, the general relation among the bending modulus, area stretching modulus, and bilayer thickness are still followed with a small numerical constant. Finally, a simple metastable model is proposed to explain the size-dependent behavior of bilayer thickness, orientation, and tension. The swelling of small unilamellar vesicles is explored by dissipative particle dynamics, including the growth, rupture, healing, and fusion process. The spontaneously formed vesicles are inflated by adding water into the inner water region. As the water is added into the vesicle, the vesicle size increases and the bilayer thickness decreases. It is also found that the area density of the tail group in the outer leaflet is greater than that of the inner leaflet during the swelling process; however, the area density of the outer head is less than that of the inner head. The results of the packing parameters for both leaflets reveal that the shape of lipid in the inner leaflet resembles an inverted truncated cone and in the outer leaflet is like a truncated cone. The outcome of local order parameter points out the orientation order is found to decay as the vesicle is inflated demonstrating that the bilayer becomes thinner partly attributed to the orientation disorder. The distribution of the lipid tail beads for the inner and outer leaflets indicates an interdigitated formation takes place within the bilayer of an inflated vesicle. The membrane tension can be obtained through the Young-Laplace equation is applied to estimate the membrane tension and the tension is found to grow with the inflation of the vesicle. Therefore, inflated vesicles are less stable against fusion. Finally, it is found that spontaneously formed vesicles of different sizes rupture at distinct degree of inflation. However, certain membrane properties, such as packing parameter, order parameter, and membrane thickness need to reach the same critical values before rupture. It is found that only surfactants with suitable hydrophobicity are able to solubilize vesicles by forming small mixed micelles. Surfactants with inadequate hydrophobicity tend to stay in the bulk solution and only a few of them enter into the vesicle. Consequently, the vesicle structure remains intact for all surfactant concentrations studied. On the contrary, surfactants with excessive hydrophobicity are inclined to incorporate with the vesicle and thus the vesicle size continues to grow as the surfactant concentration increases. Instead of forming discrete mixed micelles, lipid and surfactant are associated into large aggregates taking the shapes of cylinders, donuts, bilayers, etc. For addition of surfactant with moderate hydrophobicity, perforated vesicles are observed before the formation of mixed micelles and thus the solubilization mechanism is more intricate than the well-known three-stage hypothesis. As the apparent critical micellar concentration is attained, pure surfactant micelles form and the vesicle deforms because the distribution of surfactant within the bilayer is no longer uniform. When the surfactant concentration reaches medium concentration, the vesicle perforates. The extent of perforation grows with increasing surfactant concentration. The solubilization process begins at excess of concentration and lipids leave the vesicle and join surfactant micelles to form mixed micelles. Eventually total collapse of the vesicle is observed.