Novel lipid nanotubes in dispersions of DMPC
Novel lipid nanotubes in dispersions of DMPC
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DOI：
10.1002/cphc.200400235
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发表时间：
2004-08-20
期刊：
影响因子：
2.9
通讯作者：
Ulrich, AS
中科院分区：
文献类型：
--
作者：
Lauf, U;Fahr, A;Ulrich, AS
脂质和表面活性剂的自组装产生了许多具有纳米尺度的三维结构，这些结构可以用作药物载体或分子反应容器，或作为生成进一步规则纳米材料的模板众所周知，由合成磷脂制成的脂质体根据温度、化学成分和添加剂的不同，具有不同的大小和形状，具有不同的表面形态1,2 -二肉豆醇-sn-甘油-3-磷脂酰胆碱（DMPC）在水分散体中的相行为在过去已经被彻底地表征。像其他饱和两性离子磷脂一样，它没有形成高度弯曲结构或非球形的倾向。在这里，我们报告了DMPC在高压均质和接近完全水合散装脂质转变前温度（Tp% 138C）的温度下意外地自组装成一种新型管状形态。在500bar高压均质后，新鲜的DMPC分散体在宏观上呈现均匀、白色和微半透明。光子相关光谱测量得到的颗粒尺寸为157 nm，多分散指数为0.21，表明其分布广泛或多模态。图1的冷冻断裂电镜（EM）显示了许多小的单层囊泡（suv,< 50 nm）和一些较大的脂质体，以及一些扩展的波纹双层结构。在一周内，138C下的分散体在宏观上变得不均匀，与68C和388C下的对照样品相反。在一个月的时间里，在138C的温度下形成了半固态的凝胶，在摇晃时变成了粘稠的块状物质。图2中的冷冻断裂图像显示了具有水性内核的新型管状结构的外观，我们将其称为™脂质纳米管∫。它们在非均相样品中的比例难以用电镜定量，但它们是丰富的，它们的数量随着储存时间的增加而增加，并与宏观凝胶的形成有关。纳米管呈现出规则的螺旋状表面，节距约为23 nm，与DMPC对称纹波相的典型25 ~ 30 nm相当。[2,3]图3的EM阴性染色表明，纳米管完全是单层的，直径约为40 nm，长度可超过10 μm。虽然在阴性染色制备中，长管容易断裂，但半球形端部的出现表明纳米管是封闭的囊泡结构。图4表明，未支链结构交织成一个网络，从而解释了宏观凝胶化现象。一旦形成，纳米管在138℃或冷却至68℃的条件下，在形态上至少可以保持20个月的稳定。相反，存储温度的增加会引起形态变化，如图5所示。178C孵育24 h，管状末端呈棒状膨大，节距变宽。进一步加热不可逆地将纳米管转化为大的球形脂质体，伴随着凝胶的宏观流化。
Self-assembly of lipids and surfactants yields a multitude of three-dimensional structures with nanoscopic dimensions, which may be used as drug carriers or molecular reaction vessels, or as templates for generating further regular nanomaterials.[1] Liposomes made of synthetic phospholipids are known to assume various sizes and shapes with different surface morphologies, depending on temperature, chemical composition and additives.[2] The phase behavior of 1, 2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) in aqueous dispersions has been thoroughly characterized in the past. Like other saturated zwitterionic phospholipids it shows no tendency to form highly curved structures or nonspherical shapes. Here, we report that DMPC unexpectedly self-assembles in a novel type of tubular morphology upon high pressure homogenization and storage at a temperature close to the pretransition temperature (Tp% 138C) of the fully hydrated bulk lipid. After high pressure homogenization at 500bar the fresh DMPC dispersions appeared macroscopically homogeneous, white, and slightly translucent. Photon correlation spectroscopy measurements yielded a particle size of 157 nm with a polydispersity index of 0.21, which indicates a broad or multimodal distribution. Freeze-fracture electron microscopy (EM) in Figure1 shows many small unilamellar vesicles (SUVs,< 50 nm), and a few larger liposomes, and some extended rippled bilayer structures. Within a week the dispersions stored at 138C became macroscopically inhomogeneous, in contrast to the control samples stored at 68C and 388C. Within a month, a semisolid gel was formed at 138C, which turned into a viscous and lumpy mass upon shaking. The freeze-fracture images in Figure 2 reveal the appearance of novel tubular structures with aqueous inner cores, which we shall call™ lipid nanotubes∫. Their proportion in the heterogeneous sample is hard to quantify by EM, but they are abundant, and their number increased with storage time and correlates with the macroscopic-gel formation. The nanotubes exhibit a regular, screwlike surface with a pitch distance of about 23 nm, comparable to the typical 25ą 30 nm of the symmetric ripple phase of DMPC.[2, 3] Negative staining EM in Figure 3 shows that the nanotubes are exclusively unilamellar, have a diameter of about 40 nm, and can be more than 10 μm in length. Although the long tubes tend to be broken in the negative staining preparation, the occurrence of hemispherical capped ends indicates that the nanotubes are closed vesicular structures. Figure4 illustrates that the unbranched structures are interwoven as a network, thus explaining the macroscopic gelation phenomenon. Once formed, nanotubes are morphologically stable for at least 20 months when kept at 138C or cooled to 68C. In contrast, an increase in storage temperature induces morphological changes, as depicted in Figure 5. Incubation at 178C for 24 h resulted in clublike expansions of the tubular ends and a widening of the pitch distance. Further heating irreversibly converted the nanotubes into large spherical liposomes, accompanied by a macroscopic fluidization of the gel.