Direct Observation of DNA at a Molecular Scale Level by Three-Dimensional Fourier Filtering Method

高分辨率透射电子显微镜（HRTEM），目前，使原子水平的观察。然而，所得到的图像并不总是精确地显示原子结构。因为，原子结构的象常常由于晶体中电子波的多次散射和成象透镜的象差而失真。为了解决后者的要求，我们提出了一种新的方法，称为三维傅里叶滤波方法（3D-FFM），它使我们能够观察球面像差的相位和振幅图像分开。在处理过程中，滤波形状函数只提取出现在所谓的Ewald球上的线性分量，该函数通过排除图像的非线性分量和量子噪声来提高信噪比，有效地用于Au极律电子剂量条件下的生物样品观察。三维傅立叶滤波方法（3DFFM）已经成功地应用于在分子尺度水平上观察脱氧核糖核酸（DNA）。为了避免由支撑膜的对比度引起的图像质量的劣化，使用多孔纳米尺寸的金网格，并观察通过冷冻干燥方法桥接在多孔网格上的DNA。该处理使得可以观察到在聚焦条件下具有高对比度的DNA的无球面像差的相位图像。在DNA的相位图像中，部分观察到间隔为3.4-4.0 A的周期性精细结构，这似乎对应于相邻碱基对之间的距离，尽管它似乎受到严重破坏。如果我们能用更小的电子剂量重建DNA的图像，就能分辨出DNA更多的细节结构，这一点正在研究之中

High-resolution transmission electron microscopy (HRTEM), at present, enables atomic level observation. However, the resulting image do not always show atomic structures precisely. Because, the image of the atomic structure is often distorted by multiple scattering of electron waves in the crystal and by aberrations ill the image-forming lenses. To solve the latter requirement, we proposed a new method called three-dimensional Fourier filtering method (3D-FFM), which enables us to observe spherical aberration-free phase and amplitude images separately. In the processing, a filtering shape function extracts only the linear component that appears -on a so-called Ewald sphere and the function works well for improving the S/N ratio by excluding the non-linear components and, quantum noises of the image, which effectively works for the biological sample observation under au extremely law electron dose conditionIn this study, the three-dimensional Fourier filtering method (3D FFM) has been successfully applied to observe a deoxyribonucleic acid (DNA) in the molecular scale level. To avoid the deterioration of the image quality cased by the contrast of the supporting film, a holey nano-sized gold grid was used and the DNA that was bridged over on the holey grid by a freeze drying method was observed. The processing made it possible to observe a spherical aberration-free phase image of the DNA with high contrast at in-focus condition. In the phase image of the DNA, periodic fine structures with intervals of 3.4-4.0 A, which seem to correspond to the distance between adjacent base pairs, were partially observed, though it seems to be heavily damaged. If we can reconstruct the DNA image by smaller dose of the electron, more,detail structures of the DNA will be, resolved, which is now under investigation