SPring-8, the large synchrotron radiation facility

（Note 1） Monochromatic X-rays from synchrotron radiation
Synchrotron radiation is a narrow and powerful electromagnetic wave (light) generated when electrons are accelerated to a speed comparable to that of light and their path is bent by an electromagnet. The generation of continuous X-rays of various wavelengths is possible. Continuous X-rays irradiated on a monocrystal (diffraction grating) are reflected (diffracted). The angle of diffraction depends on Bragg’s law and only waves with a specific wavelength can be reflected. In other words, only waves with a specific wavelength can be collected by diffraction from continuous X-rays of various wavelengths. The element used for this purpose is generally called a monochromator. In the X-ray wavelength range, silicon monocrystals are often used as the monochromator material. Monochromatic X-rays obtained using the monochromator are used in measurements employing the beamlines of general synchrotron radiation facilities.

（Note 2） X-ray dose
X-ray dose is the amount of radiation absorbed upon X-ray irradiation. At medical sites, X-ray dose is used as the unit of radiation absorbed by the organs of patients, because it is independent of the kind of irradiated material. An X-ray probe is sometimes called a nondestructive probe because the damage to the sample is extremely small compared with that of an electron beam, which is also a quantum probe. However, we cannot ignore the destruction of a sample caused by the photoelectric effect when the sample is irradiated with high–intensity high-energy X-rays. A countermeasure to this problem has been sought.

（Note 3） Diffracted X-ray tracking (DXT) method
In the DXT method, the molecules that are the target of dynamics evaluation are labeled with gold nanocrystals having a diameter of several nanometers; then, the motion of X-ray Laue spots diffracted from the gold nanocrystals is observed with a high-speed time resolution. The DXT method mainly detects the rotational motion with a sensitivity of less than milliradian order (0.01o), which is equivalent to a translation distance of picometer order. The time resolution is as fast as 100 ns, although it depends on the performance of the high-speed camera used. This technique was developed by Professor Yuji Sasaki in 1998 and published in 2000. His research group has already published many reports on the internal motion of a protein molecule (e.g., Physical Review Letters, Physical Review, BBRC, Cell, Biophysical J., and Scientific Reports).

The principle of the DXT method is shown in the figure below. It is the only method that detects the internal rotational motion of dynamic molecular assemblies (nonuniform structural materials). They named this new method in which DXT is realized using monochromatic X-rays the diffracted X-ray blinking (DXB) method. The mechanisms behind DXT and DXB are shown in the figure.

（Note 4） Diffraction X-ray blinking (DXB) method
In the 1990s, the blinking phenomenon at intervals of the order of milliseconds to seconds was observed in the visible light region. It was considered to be a typical phenomenon to be avoided because it made measurements unstable. The blinking phenomenon has been observed for various pigments used for the tracking of a single molecule, including green fluorescent protein (GFP), organic fluorescent molecules, and semiconductor quantum dots. Professor W. E. Moerner of Stanford University excited yellow fluorescent protein (YFP), a variant of GFP, with light of a wavelength of 488 nm and observed the luminescence property of a single molecule of YFP by a special optical microscopy method. He found that the fluorescence began to blink. He further found that after several blinks, it eventually stopped and the YFP became stable. He was awarded the Nobel Prize in Chemistry for this achievement in 2014. The study led by Professor Sasaki and his colleagues was the first instance of observing the blinking of X-ray diffraction intensity, as shown in the figure. It was also confirmed that the phenomenon is independent of the type of X-ray source and originates from the motion of the nanocrystals used as the X-ray diffraction source.

（Note 5） Large synchrotron radiation facility (SPring-8)
SPring-8, owned by RIKEN, is a large synchrotron radiation facility that delivers the most powerful synchrotron radiation in the world and is located in Harima Science Garden City in Hyogo Prefecture, Japan. JASRI operates and supports users of the facility. The name “SPring-8” is derived from “Super Photon ring-8 GeV”. The synchrotron radiation is in the form of a narrow and powerful beam of electromagnetic light generated when electrons are accelerated to a speed comparable to that of light and their path is bent by an electromagnet. The research conducted at SPring-8 covers a wide range of fields from nanotechnology to biotechnology to industrial applications.

（Note 6） Autocorrelation
Autocorrelation is a mathematical method of data analysis that is based on a time axis. Autocorrelation is the correlation of a signal with a delayed copy of itself and is represented as a function of the time shift. Autocorrelation is effective in revealing minute patterns included in signals. It is a fundamental primary analysis method that enables more detailed analysis using the results of the primary analysis. Autocorrelation is used in single-particle analysis by electron microscopy and dynamic optical scattering in the visible light region. In the X-ray region, X-ray photon correlation spectroscopy has been used, although the measurement conditions are limited and only special samples can be measured. In contrast, the DXB method developed and demonstrated in this study is expected to be a general-purpose method because it accommodates a wide range of measurement conditions.