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

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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