Development of bio electron microscopy under ultra low electron dose conditions

由轻原子组成的生物样品在透射电子显微镜（TEM）中显示出非常小的对比度，并且由于结构容易被电子束照射破坏，因此仅可容许小的电子剂量用于拍摄图像。此外，由于传统TEM中缺乏理想的相位板，聚焦图像显示出很小的对比度。因此，生物样品的高分辨率观察是极其困难的，尽管直接观察未染色的分子结构似乎是识别单个分子的局部结构和理解其功能的有前途的方法之一。Ikuta和川崎提出的三维傅里叶滤波方法（3DFFM）是最有前途的方法。在该方法中，对多个记录的离焦图像执行3D傅立叶变换，并且将所获得的3D傅立叶光谱乘以具有像差校正因子的滤波形状函数。最后，对修正后的三维傅立叶谱进行三维傅立叶逆变换，得到相位图像。在本研究项目中，利用透射电子显微镜，通过三维傅立叶滤波的相位重建方法，观察了脱氧核糖核酸（DNA）纤维的分子结构。尽管电子束辐照破坏了DNA的分子结构，但由于相位重构提高了信噪比和分辨率，DNA的特征螺旋结构和相邻碱基对的间距得到了部分分辨。在球面无像差相位像中，有时观察到形成正弦曲线的单原子大小的斑点的排列，这似乎是源于硫原子沿主链沿着的对比度

Biological samples that are composed of light atoms show very little contrast in transmission electron microscopy (TEM) and only a small electron dose is tolerable for taking the image because the structures are easily damaged by electron beam irradiation. Furthermore, an in-focus image shows little contrast due to a lack of an ideal phase plate in conventional TEM. Therefore, high-resolution observation of biological samples is extremely difficult, although the direct observation of unstained molecular structures seems to be one of the promising approaches for identifying the localized structures of individual molecules and for understanding their functions. The three-dimensional Fourier filtering method (3DFFM) developed by Ikuta and Kawasaki seems to be the most promising approach for this purpose. In the method, a 3D Fourier transform is performed for a number of recorded through-focus images and the obtained 3D Fourier spectrum is multiplied by a filtering shape function with aberration correction factors. Finally, the phase image is obtained by performing an inverse 3D Fourier transform on the corrected 3D Fourier spectrum.In the preset research project, the molecular structure of deoxyribonucleic acid (DNA) fibers were observed by a phase reconstruction method called three-dimensional Fourier filtering using a transmission electron microscope. The characteristic helical structure and the spacing of adjacent base pairs of DNA were partially resolved due to an improved S/N ratio and resolution enhancement by the phase reconstruction although the molecular structure was damaged by the electron beam irradiation. In the spherical aberration-free phase images, the arrangements of single atom-sized spots forming sinusoidal curves were sometimes observed, which seem to be the contrast originating in the sulfur atoms along the main chains