Bio-AFM Frontier Research Center, Kanazawa University, Kanazawa, Japan.
ACS Nano. 2012 Oct 23;6(10):9013-20. doi: 10.1021/nn303229j. Epub 2012 Oct 2.
At biological interfaces, flexible surface structures and mobile water interact with each other to present non-uniform three-dimensional (3D) distributions. In spite of their impact on biological functions, molecular-scale understanding of such phenomena has remained elusive. Here we show direct visualization of such interfacial structures with subnanometer-scale resolution by 3D scanning force microscopy (3D-SFM). We measured a 3D force distribution at an interface between a model biological membrane and buffer solution by scanning a sharp tip within the 3D interfacial space. We found that vertical cross sections of the 3D image taken along a specific lateral direction show characteristic molecular-scale contrasts tilted at 30° to the membrane surface. Detailed analysis of the 3D image reveals that the tilted contrast corresponds to the time-averaged conformation of fluctuating lipid headgroups. On the basis of the obtained results, we discuss the relationships among the hydration structure, headgroup fluctuation, molecular fluidity, and mechanical strength of the membrane. The results demonstrate that 3D-SFM is capable of visualizing averaged 3D distribution of fluctuating surface structures as well as that of mobile water (i.e., hydration structure) at interfaces between biological systems and water.
在生物界面上，柔性表面结构和可移动的水相互作用，呈现出非均匀的三维（3D）分布。尽管这些分布对生物功能有影响，但对这些现象的分子尺度理解仍然难以捉摸。在这里，我们通过 3D 扫描力显微镜（3D-SFM）显示了具有亚纳米级分辨率的这种界面结构的直接可视化。我们通过在 3D 界面空间内扫描尖锐的尖端，测量了模型生物膜和缓冲溶液之间界面处的 3D 力分布。我们发现，沿着特定的横向方向拍摄的 3D 图像的垂直横截面以 30°的角度显示出与膜表面倾斜的特征分子尺度对比度。对 3D 图像的详细分析表明，倾斜的对比度对应于不断变化的脂质头部基团的时间平均构象。基于所得到的结果，我们讨论了水合结构、头部基团波动、分子流动性以及膜的机械强度之间的关系。结果表明，3D-SFM 能够可视化生物系统和水之间界面处的波动表面结构以及可移动水（即水合结构）的平均 3D 分布。
