Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 5608531, Japan.
Department of Chemical Engineering and Materials Science, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 6712280, Japan.
Langmuir. 2020 Mar 31;36(12):3242-3250. doi: 10.1021/acs.langmuir.9b03710. Epub 2020 Mar 20.
Bicelles are submicrometer-sized disc-shaped molecular self-assemblies that can be obtained in aqueous solution by dispersing mixtures of certain amphiphiles. Although phospholipid bicelle and phospholipid vesicle assemblies adopt similar lipid bilayer structures, the differences in bilayer characteristics, especially physicochemical properties such as bilayer fluidity, are not clearly understood. Herein, we report the lipid ordering properties of bicelle bilayer membranes based on induced circular dichroism (ICD) and fluorescence polarization analyses using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe. Bicelles were prepared by using 1,2-dimyristoyl--glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl--glycero-3-phosphocholine (DHPC), while pure DMPC vesicles and pure DHPC micelles were used as references. At temperatures below the phase transition temperature of DMPC, the bicelles showed lower membrane fluidities, whereas DHPC micelles showed higher membrane fluidity, suggesting no significant differences in bilayer fluidity between the bicelle and vesicle assemblies. The ICD signals of DPH were induced only when the membrane was in ordered (solid-ordered or ripple-gel) phases. In the bicelle systems, the ICD of DPH was more significant than that of the DMPC vesicle. The induced chirality of DPH was dependent on the chirality of the bilayer lipid. Compared to that of the DMPC/DHPC bicelle, the ICD of the 1,2-dipalmitoyl--glycero-3-phosphocholine/DHPC bicelle was higher, while that of the bovine sphingomyelin/DHPC bicelle was lower. Because the lipids are tightly packed in the ordered phase, the ICD intensity reflects the molecular ordering state of the lipids in the bicelle bilayer.
双分子层囊泡是亚微米大小的盘状分子自组装体，可以通过在水溶液中分散某些两亲分子的混合物来获得。虽然磷脂双分子层囊泡和磷脂囊泡组装体采用类似的脂质双层结构，但双层特性的差异，特别是双层流动性等物理化学性质，尚不清楚。在此，我们使用 1,6-二苯基-1,3,5-己三烯（DPH）作为探针，通过诱导圆二色性（ICD）和荧光偏振分析，报告了双分子层囊泡双层膜的脂质有序性。双分子层囊泡由 1,2-二肉豆蔻酰基-sn-甘油-3-磷酸胆碱（DMPC）和 1,2-二己酰基-sn-甘油-3-磷酸胆碱（DHPC）制备，而纯 DMPC 囊泡和纯 DHPC 胶束则用作参考。在低于 DMPC 相变温度的温度下，双分子层囊泡显示出较低的膜流动性，而 DHPC 胶束显示出较高的膜流动性，这表明双分子层囊泡和囊泡组装体之间的双层流动性没有显著差异。只有当膜处于有序（固体有序或波纹凝胶）相时，DPH 的 ICD 信号才会被诱导。在双分子层囊泡体系中，DPH 的 ICD 比 DMPC 囊泡更为显著。DPH 的诱导手性取决于双层脂质的手性。与 1,2-二棕榈酰基-sn-甘油-3-磷酸胆碱/DHPC 双分子层囊泡相比，1,2-二肉豆蔻酰基-sn-甘油-3-磷酸胆碱/DHPC 双分子层囊泡的 ICD 更高，而牛脑苷脂/DHPC 双分子层囊泡的 ICD 更低。由于脂质在有序相中紧密堆积，ICD 强度反映了双分子层囊泡双层中脂质的分子有序状态。
