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Mesons and Muons Open Up a New World
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| Building a star in the laboratory | |||
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"Don't you think it would be great fun to create a star?" said Dr. Iwasaki, with a happy smile. "It seems that we have succeeded in creating the same state as in a neutron star or a quark star." A neutron star, considered to be a dense star made up only of neutrons, is a celestial body remaining after a supernova explosion, which may occur at the end of a star's life. Today, some celestial bodies, higher in density than neutron stars, are predicted to exist. These are what we call quark stars, which are made up of quarks. Dr. Iwasaki and other scientists used kaons to create a super-dense state similar to that in the neutron star or quark star. Many kinds of mesons are known to date; probably the best known is the pion that Dr. Hideki Yukawa predicted. His achievement was later to earn him the Nobel Prize in Physics. Pions play the role of 'glue' that sticks protons and neutrons together in the nuclei. Kaons are heavier than pions. There has been a mystery surrounding kaons. "We did not know for years whether kaons and protons attract or repel each other. In 1997, we demonstrated for the first time that kaons and protons strongly attract each other," Dr. Iwasaki said, thinking back over past years. "Then, what will happen if we embed a kaon into a nucleus?" Working out the solution turned into his next research target. "Theoretically, it is possible that kaons attract surrounding protons, thereby forming a very dense state. (The figure on the upper left of the cover page illustrates the dense state, whereas the figure on the upper right represents a normal nucleus.) Any nuclei known to date have the same density. Therefore, it is really interesting if super-dense nuclei are created. Furthermore, this experiment may enable us to trace the origins of mass." Our universe was born 13.7 billion years ago. All the particles created at that time were thought to be massless, whereas the material around us has some amount of mass. The "Higgs mechanism" was proposed. Vacuum is not simple empty space, but it is filled with Higgs which give particles their mass. However, it is not the whole story. Dr. Iwasaki explains, "It is only the basic particles like quarks that Higgs particles can give mass to. Protons consist of three quarks. The total mass of three quarks experimentally known to us, however, accounts for only about 2% of the mass of a proton." Clearly an additional mechanism to create mass is required. One of the possible theoretical scenarios is that the vacuum is yet filled with quark-antiquark pairs which give rest of the mass to the particles like protons or mesons. The particles acquire mass because they drag them around. "If a nucleus is squeezed, it should become lighter because the 'mass source' is pushed out. Creating a super-dense kaonic nucleus will serve to verify this scenario." |
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| Is it the discovery of a dense nucleus? | |||
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An experiment was performed at the High Energy Accelerator Research Organization (KEK) in Tsukuba City by an international research team composed of Japanese, Korean, and US members under the leadership of Dr. Iwasaki. They stopped negative K-mesons in the helium (composed of two protons and two neutrons) target and measured the energy of proton emitted from the target. From this measurement, one can tell the mass of the object produced in this reaction. This mass is called missing mass and the result is shown in Figure 1. "Please look at the red arrow. This represents a state where a proton, two neutrons, and a K-meson are strongly bounded," said Dr. Iwasaki. He termed this state a "strange tribaryon." "Baryon" refers to particles consisting of three quarks like protons or neutrons, and the number of baryons is three; a single proton plus two neutrons. A meson is composed of a quark and an antiquark, and a K-meson has a strange quark. Here, "strange" refers to the strange quark included in the state, something that normal nuclei do not have. "Unfortunately we could not tell directly from the experiment whether it is a dense state or not," are Dr. Iwasaki's introductory remarks, and he continues to explain. "At least, we are sure that it is a strange state that has never been observed before. Some theorists view the state as a completely new state where nine quarks exist rather than a single proton plus two neutrons. Dr. Iwasaki explains, "As the density of the nucleus increases, the mass decreases to zero because 'mass source' spills out from it. We may be observing that process. We think this is one of the most important discoveries in the world. The greater the discovery, the more its credibility will be questioned. We have conducted another experiment to verify this discovery in May, the results of which are now being analyzed." Dr. Iwasaki plans many experiments in the future, involving measuring the density of the new state, increasing the number of K-mesons embed into a nucleus, and using a heavier nucleus as a target. "To clarify the state we observed, we plan to continue experiments at J-PARC in Tokai-mura, Ibaraki Prefecture, which will be operational in 2008."
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| Generating nuclear fusion by using muons as a catalyst | |||
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Another major activity in the Iwasaki Advanced Meson Science Laboratory is the research using muons. The muon is one of elementary particles called leptons along with the electrons, and is generated when the pion decays. "It was in 1995 that Dr. Kanetada Nagamine, former Chief Scientist responsible for opening up the new field of Muon Science, built the RIKEN-RAL Muon Facility at the Rutherford Appleton Laboratory. This facility can generate the world's strongest pulsed muon beam, which is used in a wide range of cutting-edge research fields into solid-state properties, exotic atom, and elementary particles; muon-catalyzed fusion is one of them. When light nuclei fuse together, an enormous amount of energy is released. Therefore, if we can handle nuclear fusion reaction in hydrogen isotopes, which are abundant, human beings will surely obtain an almost endless supply of energy. However, nuclear fusion has not yet been put into practical use. This is mainly because positively-charged hydrogen isotopes repel each other. "Plasma and laser techniques have been used to achieve a very high temperature, in which nuclei can overcome repulsive force and fuse together, but practical use is still a long way off," says Dr. Iwasaki. "However, muons act as mediators to achieve nuclear fusion even at lower temperatures." When a negative muon, having negative electric charge, is injected into hydrogen target, an electron of the hydrogen atom is replaced by the muon and forms so called muonic hydrogen atom. As a muon is 207 times heavier than an electron, it revolves in an orbit closer to the hydrogen nucleus. The size of the muonic atom is reduced to 1/207 of a normal hydrogen atom. Since the muonic hydrogen atoms do not have electric charge, they can come close to each other. This makes the distance of two nuclei short enough to initiate nuclear fusion. Furthermore, the remaining negative muons can be used as a mediator for the next nuclear fusion. Dr. Iwasaki states, "There still remain some challenges such as how to develop efficient methods to use muons as a catalyst. So far, we are at the basic research stage, but the results of our research are proving to be promising. The day may come when human beings can obtain an endless supply of energy derived from muon catalyzed fusion technology." |
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| Probing magnetic phase transitions by muons | |||
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The muon is also drawing attention as a powerful tool for probing physical properties such as magnetic properties of substances. In a substance, magnetic field is produced by the nuclear and electronic spins. On the other hand, muon is sensitive to the magnetic field and behaves like a small compass. The motion of muon spins gives the direction (and strength) of magnetic field in the substance, like a compass, where muons are located. Thus, the magnetism of a substance can be studied by implanting muon into the substance. Dr. Iwasaki pointed to a figure (Figure 2), saying, "This is one of my favorite figures. It clearly verifies the fact that the muon can serve as a very powerful tool for the study of material science." The figure represents the magnetism in an organic magnet, Me4P[Pd(dmit)2]2 measured by the muon spin relaxation method (µSR method). When the temperature of the organic magnet is lowered from 70 K to 39 K, no significant changes in the time evolution of the muon spin direction are shown on the graph, whereas when lowered to 38.4 K, significant change is observed. Dr. Iwasaki comments on the phenomenon, "The muons were sensitively detected that the electronic spins were frozen up, in order, only within the temperature difference of 0.6 K. With muons, you can clearly observe the magnetic phase transition of this organic magnet." Applications for organic magnets look promising in the field of recording media or as superconductors. Since conventional methods such as the neutron scattering and specific heat measurements are unsuitable for organic materials, muon-based measuring methods are highly promising. "Application to Surface Science is also interesting," says Dr. Iwasaki. We have developed an ultra-low energy muon beam facility and succeeded in separately measuring the magnetism in a silica substrate (SiO2) and in a 40 nano-meter aluminum film deposited on the substrate (Figure 3). Dr. Iwasaki refers to the significance of this study like this: "Although there are many challenges such as increasing the intensity of the muon beam further, we may become able to measure the magnetism in each layer of molecules, and we may be acquiring the ideal measuring means of magnetic properties of ultra-thin multilayer system. For example, understanding the multilayer film magnetism is vitally important to develop thin mass-storage recording media. Muon Science is a science familiar to the public." Dr. Iwasaki places importance on communicating with other researchers working at RIKEN in different fields. "I am very happy because I have the chance to keep good company with Dr. Reizo Kato, the Chief Scientist of Condensed Molecular Materials Laboratory, and Dr. Maki Kawai, the Chief Scientist of Surface Chemistry Laboratory. We share information by making numerous comments on each other's research work. I like RIKEN because researchers from various fields can communicate well, which is very stimulating and informative. I would like to use these amenable circumstances to move forward on research into subjects what we find interesting. 
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Advanced-Meson Science Laboratory is promoting their research using two different tools, namely mesons and muons. Operating the RIKEN-RAL Muon Facility at the Rutherford Appleton Laboratory has allowed the Laboratory to carry out studies of muon catalyzed fusion and material science. The Iwasaki Laboratory also used an accelerator in the High Energy Accelerator Research Organization (KEK) to embed K-mesons into nuclei, leading to the world's first observation of a new object which can be considered as a super-dense nucleus caused by the strongly attractive interaction between K-meson and nucleus. This observation is attracting worldwide attention as one of the biggest discoveries that may enable us to trace the origins of mass. "We want to conduct extensive cutting-edge research into elementary particle property using mesons. We also want to expand accelerator driven research into study on material science using muons. We want to advance our scientific research beyond the current border and open a new world. That is why we named the laboratory 'the Advanced Meson Science Laboratory,' said Dr. Iwasaki, Chief Scientist. Here, let us introduce the cutting-edge research.
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SPring-8 to Shed Light on Protein Structural Analysis in the Near Future
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| Impacts of protein structural analysis | |||
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"Protein is an excellent 'molecular machine' of the highest efficiency," says Yamamoto. Protein is a polymeric compound comprising a sequence of about 20 kinds of amino acids arranged according to genetic information, and folded to form a three-dimensional structure. "Genetic information is no more than a 'parts chart' that specifies amino acids as protein constituents, and does not suffice to explain protein functions. To understand protein functions, we must elucidate protein three-dimensional structures as a 'parts assembly diagram' to explain how the 'parts' are assembled into the excellent functioning of the molecular machine." Elucidating protein structures and functions would increase the comprehensibility of biological phenomena at molecular levels and, more importantly, offer significant benefits to our life and society. "I think the ultimate goal of the life sciences must be to improve our quality of life. The major target for structural analysis is proteins associated with disease." For example, elucidating the structure and function of a protein associated with a disease would enable the design of a drug that enhances or suppresses the function. "If functional differences in enzymes (proteins that catalyze chemical reactions in the body) due to structural variation are revealed, it is theoretically possible to modify their structures to produce enzymes offering sophisticated functions that are not found in nature." The ability to design such excellent artificial proteins would have immeasurable impacts on all industrial sectors, including chemistry, foods, and materials, and make significant contributions to resolving the challenging issues concerning food, energy, and the environment. |
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| The two major barriers to X-ray crystallographic analysis | |||
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The representative approach to determine protein structures is X-ray crystallographic analysis. This is a method in which protein three-dimensional structures are explored with atomic resolutions using X-rays, which have shorter wavelengths than visible light. Even if an X-ray beam is applied to a single protein molecule, however, signals from the protein cannot be observed because the interaction between the protein and the X-ray is too weak. Hence, a crystal comprising many regularly-arranged protein molecules is artificially produced and irradiated with X-rays. Then, the X-rays collide with the atoms in the crystal, and the waves of the diffracted and scattered X-rays are superposed on each other, resulting in an interference pattern that produces a dotted diffraction image. From this diffraction image, the protein structure can be determined (Figure 1). The X-ray crystallographic analysis technique was successfully applied to the determination of a protein structure for the first time at the end of the 1950s. By 1990, however, only 500 proteins in total had been structurally analyzed. This was because of two major barriers to X-ray crystallographic analysis. One barrier hampers protein crystallization. "We have not yet discovered the principles or rules by which protein crystals are formed. Even now, protein crystals are obtained only accidentally through many trials and errors based on our experience." Another barrier concerns the difficulty in deriving the protein structure from diffraction images. "For light (visible light), a lens as an excellent means is available that converges scattered light to a single point to form an image. However, no such means are available for X-rays. Hence, we process data of diffracted and scattered X-rays on a computer using a mathematical technique known as a Fourier transform to visualize the structure." However, the dot pattern and intensity shown in the diffraction image do not provide sufficient information to visualize the structure using the Fourier transform. The degree of crest-crest and trough-trough shifts (phase) of waves of diffracted and scattered X-rays remains unknown. Traditionally, the multiple isomorphous replacement technique has been used to obtain such phase information. This is a method in which two or more samples are prepared by labeling a protein crystal with respective heavy metals, which produce the difference of diffraction intensities and their diffraction images are compared. However, we have often encountered the problem of the excessive time required to prepare a sample - even of the order of years - due to unsuccessful labeling with the heavy metal and to crystal deformation caused by the labeling.
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| SPring-8 produced an innovation | |||
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Although the two major barriers to X-ray crystallography remain to be overcome in full, the speed of structural analysis has increased rapidly since the 1990s. This is thanks to two technical innovations: advances in gene engineering have made it possible to produce highly pure proteins necessary for protein crystallization in large amounts, and brilliant X-rays, known as synchrotron radiation, have become fully available. Synchrotron radiation is a type of light emitted from electrons when their orbits are curved by a magnetic force, first discovered in an accelerator ring. "Now, we can produce X-rays 1000 to 10000 times as bright as those produced from the X-ray generator I used in my laboratory when I was a university student in the 1980s, at existing ordinary synchrotron radiation facilities. In the past, one to two months of laboratory work was required to obtain a diffraction image; now, we can accomplish the same purpose in only 10 minutes using synchrotron radiation." Another big feature of synchrotron radiation resides in the fact that the capability of freely changing the X-ray wavelength has enabled us to use a new method of obtaining phase information, known as multi-wavelength anomalous diffraction method (MAD). This method is quite simple in that only one kind of sample is prepared by crystallizing a protein previously labeled with selenium by gene engineering. The time required to prepare the sample can thus be reduced significantly. Selenium exhibits significantly variable diffraction intensity over the wavelength range of the X-rays used. When measurements are taken with X-rays of different wavelengths applied to a selenium-labeled crystal, useful phase data is obtained based on the variable diffraction intensity of the selenium. "However, because the wavelength-related variation in the diffraction intensity of selenium is relatively large but up to several percent, highly accurate data with little errors must be taken. Until the mid-1990s, the use of second-generation synchrotron radiation facilities was often unsuccessful in applying MAD technique." In October 1997, SPring-8, built jointly by RIKEN and the Japan Atomic Energy Research Institute, went into actual operation. SPring-8 is a third-generation synchrotron radiation facility. It is configured with an accelerator ring through which electrons are running, and a large number of insertion devices, apparatus for increasing radiation brilliancy and parallelity. Among all third-generation synchrotron radiation facilities, SPring-8 is capable of producing radiation of the world's highest brilliance, and of producing small, highly parallel X-ray beams. If an X-ray beam wider than the size of a crystal is applied to the crystal, the beam collides not only with the crystal, but also with the air and solution, resulting in scattered X-ray, all of which becomes noise. As SPring-8 enables highly accurate measurements with low noise, we are now able to use MAD technique at full scale. |
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| Elucidating protein structures using microcrystals | |||
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"To realize what cannot be done without SPring-8 as early as possible. That's our mission. The most promising thing with the highly brilliant light source SPring-8 is to analyze very small crystals." There are many important proteins that cannot be structurally analyzed because crystals as small as 10 μm cannot be grown to greater sizes, though their preparation is often relatively easy. Additionally, analyzing small crystals would save the protein sample for crystallization and shorten the time to form crystals. Hence, crystallization costs and time can be reduced significantly. Currently, the crystal size must be at least about 300 mm for laboratory X-ray generators and about 200 to 300 mm for second-generation synchrotron radiation facilities; however, SPring-8 permits structural analysis even for crystal sizes of slightly less than 100 mm. Yamamoto and others aim at further improvements to enable structural analysis for crystal sizes of from several tens of micrometers to several micrometers. To this end, it is necessary to improve the beamline, an apparatus for introducing radiation beam generated from the storage ring to the sample. "Some reports are available from overseas on successful structural determinations for 20 to 30 mm crystals using a newly constructed beamline dedicated to the analysis of microcrystals. We want to allow everyone routine access to all the beamlines of SPring-8 to analyze microcrystals." For structural analysis, it is necessary to narrow the X-ray beam to microcrystal size, and to accurately apply it to the target crystal. "In addition, microcrystal analysis encounters the major barrier of radiation damage," says Yamamoto. Smaller crystals contain fewer protein molecules. To obtain diffraction data necessary for analysis, more intense X-rays must be applied to the crystal. However, X-rays of excess intensity result in the collapse of the crystal. In principle, the crystal unavoidably undergoes a collapse as the X-rays collide with the atoms in the crystal. In addition, an unwanted chemical reaction proceeds in which water in the spaces between the crystal constituent proteins is activated by X-ray radiation and damages the proteins. The influence of this reaction can be reduced by shortening the exposure time. With its excellent beam stability, SPring-8 is capable of taking highly accurate data in a short time. "At SPring-8, we will soon be able to obtain a set of diffraction data in approximately 300 seconds. Aiming at even shorter measuring time, we are working to improve the detector jointly with a commercial enterprise. We have the ultimate goal of reducing the measurement time to the order of 10 seconds." Yamamoto and others are aiming at enabling everybody to measure several tens of micrometers of crystals in two to three years, and several micrometers in 10 years using SPring-8. |
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| Toward achieving fully automated measurements | |||
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As the base sites for comprehensive analysis in the Protein 3000 national project to explore protein structures and functions, the Harima Institute and the Yokohama Institute in cooperation are analyzing important proteins at high speeds in large volumes. To support these activities, the Division of Synchrotron Radiation Instrumentation has been engaged in developing an automation technology for the necessary measurements. "To date, it has been common practice for the researcher to visit SPring-8 here and take measurements with individual crystal samples set in the instrument one by one. Sample exchanges have also been done manually." Yamamoto and others have developed a dedicated sample tray that accommodates 52 samples, and the SPACE sample exchanger, which automatically picks up samples from the tray and loads and unloads the sample for the measuring apparatus (Figure 2 and lower panel on cover page). They are also working to develop a software program for centralized management of data collection and beamline operation. "A researcher sends a tray containing samples to SPring-8 by home delivery service. At SPring-8, a beamline operator sets the tray on the apparatus and begins fully automated measurements. The data is transferred on line to the researcher. I hope such a system will be realized in the near future." Yamamoto says that their ultimate goal is to fully automate structural analysis. "Expanding the database on protein structures and functions would enable us to determine the phase and to derive the structure of a protein based on accumulated patterns. This is my ultimate goal." A new era of structural analysis is about to be opened through activities at the Division of Synchrotron Radiation Instrumentation under the leadership of Yamamoto.
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Having exquisitely folded three-dimensional structures, proteins exhibit unique functions that drive all reactions involved in biological phenomena. However, conventional approaches to protein structural analysis have been painstaking, time-consuming tasks. "Nothing was better than the process of determining the structure of a single protein over several years," says Masaki Yamamoto, Leader of the Division of Synchrotron Radiation Instrumentation at RIKEN Harima Institute, looking back upon the second half of the 1980s, when he was a student. Until the second half of the 1980s, only up to several tens of proteins were structurally determined per year worldwide. More recently in 2004, as many as about 5,500 proteins were structurally determined within the year. One of the major forces leading to this innovation was the recent advance in structural analysis using synchrotron radiation. At the Division of Synchrotron Radiation Instrumentation, Yamamoto and others are working to open the frontiers of structural analysis making use of SPring-8, which is capable of producing the synchrotron radiation of the world's highest brilliance.