Bleecker Joan V, Cox Phillip A, Foster Rami N, Litz Jonathan P, Blosser Matthew C, Castner David G, Keller Sarah L
Departments of Chemistry, ‡Chemical Engineering, and §Bioengineering, University of Washington , Seattle, Washington 98195, United States.
J Phys Chem B. 2016 Mar 17;120(10):2761-70. doi: 10.1021/acs.jpcb.5b10165. Epub 2016 Mar 3.
Lipid composition dictates membrane thickness, which in turn can influence membrane protein activity. Lipid composition also determines whether a membrane demixes into coexisting liquid-crystalline phases. Previous direct measurements of demixed lipid membranes have always found a liquid-ordered phase that is thicker than the liquid-disordered phase. Here we investigated noncanonical ternary lipid mixtures designed to produce bilayers with thicker disordered phases than ordered phases. The membranes were composed of short, saturated (ordered) lipids; long, unsaturated (disordered) lipids; and cholesterol. We found that few of these systems yield coexisting liquid phases above 10 °C. For membranes that do demix into two liquid phases, we measured the thickness mismatch between the phases by atomic force microscopy and found that not one of the systems yields thicker disordered than ordered phases under standard experimental conditions. We found no monotonic relationship between demixing temperatures of these ternary systems and either estimated thickness mismatches between the liquid phases or the physical parameters of single-component membranes composed of the individual lipids. These results highlight the robustness of a membrane's liquid-ordered phase to be thicker than the liquid-disordered phase, regardless of the membrane's lipid composition.
脂质组成决定膜的厚度，而膜厚度又会影响膜蛋白活性。脂质组成还决定了膜是否会分离成共存的液晶相。以往对分离脂质膜的直接测量总是发现一个比液相无序相更厚的液相有序相。在这里，我们研究了非典型三元脂质混合物，其设计目的是产生无序相比有序相更厚的双层膜。这些膜由短链饱和（有序）脂质、长链不饱和（无序）脂质和胆固醇组成。我们发现，在10℃以上，这些体系中很少有能产生共存液相的。对于确实分离成两个液相的膜，我们通过原子力显微镜测量了两相之间的厚度失配，发现在标准实验条件下，没有一个体系产生的无序相厚度比有序相厚。我们发现，这些三元体系的分离温度与液相之间估计的厚度失配或由单个脂质组成的单组分膜的物理参数之间没有单调关系。这些结果突出了膜的液相有序相比液相无序相更厚的稳健性，而与膜的脂质组成无关。
