Ye Weihua, Lind Jesper, Eriksson Jonny, Mäler Lena
Department of Biochemistry and Biophysics, Center for Biomembrane Research, The Arrhenius Laboratory, Stockholm University , SE-10691 Stockholm, Sweden.
Langmuir. 2014 May 20;30(19):5488-96. doi: 10.1021/la500231z. Epub 2014 May 7.
Small, fast-tumbling bicelles are frequently used in solution NMR studies of protein-lipid interactions. For this purpose it is critical to have information about the organization of the lipids within the bicelle structure. We have studied the morphology of small, fast-tumbling bicelles containing DMPC and DHPC as a function of temperature, lipid concentration, and the relative ratio (q value) of lipid (DMPC) to detergent (DHPC) amounts. Dynamic light scattering and cryo-transmission electron microscopy techniques were used to measure the size of the bicelles and to monitor the shape and dispersity of the particles in the samples. The stability and size of DMPC-containing bicelle mixtures were found to be highly dependent on temperature and the total lipid concentration for mixtures with q = 1 and q = 1.5. Stable DMPC/DHPC bicelles are only formed at low q values (0.5). Bicelle mixtures with q > 0.5 appear to be multidisperse containing more than one component, one with r(H) around 2.5 nm and one with r(H) of 6-8 nm. This is interpreted as a coexistence of small (possibly mixed micelles) bicelles and much larger bicelles. Incubating the sample at 37 °C increases the phase separation. Moreover, low total amphiphile concentrations and low q values lead to the formation of a temperature-independent morphology, interpreted as the formation of small particles in which the DHPC and DMPC are more mixed. On the basis of these results, we propose the existence of a critical bicelle concentration, a parameter that determines the existence of bilayered bicelles, which varies with q value. This polymorphism was not observed at any concentrations for q = 0.5 bicelles, for which a small but detectable temperature dependence was observed at high concentrations. The results demonstrate that q = 0.5 mixtures predominantly form "classical" bicelles, but that caution is needed when using fast-tumbling mixtures with q values higher than 0.5.
小型、快速翻滚的双分子层膜泡常用于蛋白质 - 脂质相互作用的溶液核磁共振研究。为此，了解双分子层膜泡结构内脂质的组织信息至关重要。我们研究了含有二肉豆蔻酰磷脂酰胆碱（DMPC）和二己酰磷脂酰胆碱（DHPC）的小型、快速翻滚双分子层膜泡的形态，该形态是温度、脂质浓度以及脂质（DMPC）与去污剂（DHPC）量的相对比例（q值）的函数。动态光散射和低温透射电子显微镜技术用于测量双分子层膜泡的大小，并监测样品中颗粒的形状和分散性。对于q = 1和q = 1.5的混合物，发现含DMPC的双分子层膜泡混合物的稳定性和大小高度依赖于温度和总脂质浓度。稳定的DMPC/DHPC双分子层膜泡仅在低q值（0.5）时形成。q > 0.5的双分子层膜泡混合物似乎是多分散的，包含不止一种成分，一种r(H)约为2.5 nm，另一种r(H)为6 - 8 nm。这被解释为小的（可能是混合胶束）双分子层膜泡和大得多的双分子层膜泡共存。将样品在37°C下孵育会增加相分离。此外，低总两亲物浓度和低q值会导致形成与温度无关的形态，这被解释为形成了其中DHPC和DMPC混合程度更高的小颗粒。基于这些结果，我们提出存在一个临界双分子层膜泡浓度，这是一个决定双层双分子层膜泡存在的参数，它随q值变化。对于q = 0.5的双分子层膜泡，在任何浓度下都未观察到这种多态性，在高浓度下仅观察到小的但可检测到的温度依赖性。结果表明，q = 0.5的混合物主要形成“经典”双分子层膜泡，但在使用q值高于0.5的快速翻滚混合物时需要谨慎。
