Tsunenori Nomaguchi

Molecular-scale imaging of unstained deoxyribonucleic acid fibers by phase transmission electron microscopy

The molecular structure of deoxyribonucleic acid (DNA) fibers was observed by a phase reconstruction method called three-dimensional Fourier filtering using a 200kV transmission electron microscope. The characteristic helical structure and the spacing of adjacent base pairs of DNA were partially resolved due to an improved signal-to-noise 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.

Estimation of minimum electron dose necessary to resolve molecular structure of deoxyribonucleic acid by phase transmission electron microscopy

The minimum electron dose that is necessary to resolve the molecular structure of deoxyribonucleic acid (DNA) was estimated based on experimental measurements of information limits and simulated DNA images, considering conditions of a low electron dose. From these results, a dose of ∼400e∕A2 was found to be necessary to achieve observation of DNA on a molecular scale under the present experimental setup. A DNA molecule was observed by a phase reconstruction method using through-focus images under the limited electron dose. In the reconstructed images, the helical structure and the intervals of the base pairs of DNA were partially resolved.

High-resolution wave field reconstruction using a through-focus series of images taken under limited electron dose

The three-dimensional Fourier filtering method and Schiskes Wiener filtering method are compared with the aim of high-resolution wave field reconstruction of an unstained deoxyribonucleic acid (DNA) molecular fiber using a through-focus series of images taken under a limited electron dose. There were some definite differences between the two reconstructed images, although the two kinds of processing are essentially equivalent except for the dimension and the filter used for processing. Through theoretical analyses together with computer simulations, the differences were proved to be primarily due to specimen drift during the experiment. Although the observed structure of the DNA molecular fiber was heavily damaged by electron beam irradiation, reconstructed images by the three-dimensional Fourier filtering method provided higher resolution information on the molecular structure even when relatively large specimen drift was included in the through-focus series. In contrast, in Schiskes Wiener filtering method, the detailed information of the structure was lost because of the drift, although the reconstructed image showed a higher signal-to-noise ratio. The three dimensional Fourier filtering method seems to be more applicable for observing radiation-sensitive materials under an extremely low electron dose, because specimen drift cannot be completely avoided.

Development of Compact Cs/Cc Corrector with Annular and Circular Electrodes

The spherical aberration (Cs) correction is indispensable to improve the spatial resolution in the electron microscopes. Some types of Cs correction devices have been proposed and developed, and the Cscorrectors consisted of multi-pole lenses have successfully realized sub-angstrom resolution in (S)TEMs [1-2]. However, these correctors require complex control of multiple optical components with high accuracy and stability. They also demand reconfiguration of the microscope columns to insert rather large additional components, resulting in huge cost. In order to solve these problems, Ikuta had proposed a very simple and compact Cs-corrector with axially-symmetric electrostatic-filed formed between annular and circular electrodes [3-4], as schematically shown in Fig. 1(a). We called it “ACE corrector” (the Cscorrector using Annular and Circular Electrodes). Furthermore, this simple device has an additional capability to reduce the effect of the chromatic aberration (Cc). In the present paper, we report the principle of Cs/Cc correction and preliminary results of the ACE corrector in simulations and experiments.