Development of Super Resolution Bio-Phase Transmission Electron Microscopy

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Biological samples that are composed of light atoms show very little contrast in transmission electron microscopy(TEM). Only a small electron dose is allowed to obtain the image because the samples are easily damaged by the electron beam irradiation. Therefore, high resolution observation of biological samples is extremely difficult. In addition, an in-focus image shows little contrast due to the lack of an ideal phase plate. However, direct observation of unstained molecular structures seems to be a promising approach in identifying the segments of individual molecules and to understand their functions. We proposed a three-dimensional Fourier filtering method (3D-FFM) as a phase reconstruction method. In this method, a 3D Fourier transform is performed for a number of recorded through-focus images and a filtering shape function with aberration correction factors is applied to the obtained 3D Fourier spectrum. The phase image is finally obtained by performing an inverse 3D Fourier tran … More sform on the corrected 3D Fourier spectrum. Since the filtering shape function extracts only the linear image components that appear on the Ewald sphere, the image processing works well for improving the S/N ratio by excluding the non-linear image components and quantum noise in the images. This seems to be an indispensable advantage for observing unstained biological samples on a molecular scale.In the present project, aberration-free phase images of deoxyribonucleic acid(DNA) were observed on a molecular scale level by a three-dimensional Fourier filtering method using a 200 kV high-resolution transmission electron microscope(TEM). To avoid the noise produced by the supporting film of the TEM sample, we prepared a special gold thin film with nano-sized holes. The DNA molecular fibers hanging across the holes were observed. The total dose required to obtain the image was about 200-300 electrons/Å^2. Although the molecular structure of the DNA seems to be heavily damaged, a helical-like structure is partially confirmed and fine structures with intervals of 3.4-4.5Å are partially resolved in the helical-like structure, which is similar to the DNA molecular model determined by J.D.Watson and F.H.C.Crick. In conclusion, super resolution bio-phase transmission electron microscopy could be successfully established by the present project. Less