Borden Mark A, Martinez Gary V, Ricker Josette, Tsvetkova Nelly, Longo Marjorie, Gillies Robert J, Dayton Paul A, Ferrara Katherine W
Department of Biomedical Engineering, University of California, Davis, California 95616, USA.
Langmuir. 2006 Apr 25;22(9):4291-7. doi: 10.1021/la052841v.
In the design of lipid-coated microbubble ultrasound contrast agents for molecular imaging and targeted drug delivery, the surface distribution of the shell species is important because it dictates such properties as ligand location, brush coverage, and amount of drug loading. We used a combination of spectroscopy and microscopy techniques to test the prevailing notion that the main phosphatidyl choline (PC) and lipopolymer species are completely miscible within the monolayer shell. NMR spectroscopy showed that the shell composition is roughly equivalent to the bulk lipid ratio. FTIR spectroscopy showed a sharp melting peak corresponding to the main phase-transition temperature of the main PC species, with no observed pretransitions while scanning from room temperature, indicating a single PC-rich ordered phase. Electron and fluorescence microscopy showed a heterogeneous microstructure with dark (ordered) domains and bright (disordered) regions. Domain formation was thermotropic and reversible. Fluorescent labeling of the lipopolymer following shell formation showed that it partitions preferentially into the disordered interdomain regions. The ordered domains, therefore, are composed primarily of PC, and the disordered interdomain regions are enriched in lipopolymer. Phase heterogeneity was observed at all lipopolymer concentrations (0.5 to 20 mol %), and the degree of phase separation increased with lipopolymer content. The composition and temperature dependence of the microstructure indicates that phase separation is driven thermodynamically rather than being a kinetically trapped relic of the shell-formation process. The overall high variation in microstructure, including the existence of anomalous three-phase coexistence, highlights the nonequilibrium (history-dependent) nature of the monolayer shell.
在用于分子成像和靶向药物递送的脂质包被微泡超声造影剂设计中，壳层物质的表面分布很重要，因为它决定了诸如配体位置、刷状覆盖度和药物负载量等性质。我们结合使用光谱学和显微镜技术来检验一种普遍观点，即主要的磷脂酰胆碱（PC）和脂聚合物在单层壳层内完全互溶。核磁共振光谱表明壳层组成大致等同于本体脂质比例。傅里叶变换红外光谱显示出一个尖锐的熔化峰，对应于主要PC物种的主要相变温度，在从室温扫描时未观察到预转变，表明存在单一的富含PC的有序相。电子显微镜和荧光显微镜显示出具有暗（有序）域和亮（无序）区域的异质微观结构。域的形成是热致性的且可逆。壳层形成后对脂聚合物进行荧光标记表明它优先分配到无序的域间区域。因此，有序域主要由PC组成，无序的域间区域富含脂聚合物。在所有脂聚合物浓度（0.5至20摩尔%）下均观察到相异质性，并且相分离程度随脂聚合物含量增加。微观结构的组成和温度依赖性表明相分离是由热力学驱动的，而不是壳层形成过程中动力学捕获的残余物。微观结构的整体高度变化，包括异常三相共存的存在，突出了单层壳层的非平衡（历史依赖性）性质。
