Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Science Faculty, Utrecht University, P.O. Box 80000, 3508 TA, Utrecht, The Netherlands.
Chem Phys Lipids. 2011 Jan;164(1):9-15. doi: 10.1016/j.chemphyslip.2010.09.007. Epub 2010 Oct 7.
Temperature-controlled Atomic Force Microscopy (TC-AFM) in Contact Mode is used here to directly image the mechanisms by which melting and crystallization of supported, hydrated DPPC bilayers proceed in the presence and absence of the model peptide WALP23. Melting from the gel L(β)' to the liquid-crystalline L(α) phase starts at pre-existing line-type packing defects (grain boundaries) in absence of the peptide. The exact transition temperature is shown to be influenced by the magnitude of the force exerted by the AFM probe on the bilayer, but is higher than the main transition temperature of non-supported DPPC vesicles in all cases due to bilayer-substrate interactions. Cooling of the fluid L(α) bilayer shows the formation of the line-type defects at the borders between different gel-phase regions that originate from different nuclei. The number of these defects depends directly on the rate of cooling through the transition, as predicted by classical nucleation theory. The presence of the transmembrane, synthetic model peptide WALP23 is known to give rise to heterogeneity in the bilayer as microdomains with a striped appearance are formed in the DPPC bilayer. This striated phase consists of alternating lines of lipids and peptide. It is shown here that melting starts with the peptide-associated lipids in the domains, whose melting temperature is lowered by 0.8-2.0°C compared to the remaining, peptide-free parts of the bilayer. The stabilization of the fluid phase is ascribed to adaptations of the lipids to the shorter peptide. The lipids not associated with the peptide melt at the same temperature as those in the pure DPPC supported bilayer.
在这里，我们使用温度控制原子力显微镜（TC-AFM）在接触模式下直接观察在存在和不存在模型肽 WALP23 的情况下，水合 DPPC 双层支持的熔融和结晶过程的机制。在没有肽的情况下，从凝胶 L(β) '到液晶 L(α)相的熔融从预先存在的线型包装缺陷（晶界）开始。确切的转变温度显示受到 AFM 探针对双层施加的力的大小的影响，但由于双层-基底相互作用，在所有情况下都高于非支持 DPPC 囊泡的主要转变温度。在冷却流动的 L(α)双层时，在不同凝胶相区域之间的边界处形成线型缺陷，这些缺陷起源于不同的核。这些缺陷的数量直接取决于通过转变的冷却速率，如经典成核理论所预测的那样。众所周知，跨膜合成模型肽 WALP23 的存在会导致双层出现异质性，形成 DPPC 双层中的条纹外观的微域。这个条纹相由脂质和肽交替的线组成。结果表明，熔融从域中与肽相关的脂质开始，其熔融温度比双层中剩余的、无肽的部分降低了 0.8-2.0°C。将脂质适应较短的肽，从而稳定了流体相。与纯 DPPC 支持双层中的脂质相比，未与肽结合的脂质在相同温度下熔融。
