Akinari Yokoya

Soft X-ray Beamline Specialized for Actinides and Radioactive Materials Equipped with a Variably Polarizing Undulator

This report presents the design of an undulator beamline at SPring-8 to be used for soft X-ray spectroscopy focused on radioactive materials. Photoemission spectroscopy experiments are carried out in a radioisotope (RI)-controlled area where actinide compounds as well as unsealed radioactive materials are usable. Intrusion of the radioactive materials into the electron storage ring or to the outside of the evacuated beamline components can be avoided by a specially devised RI protection/inspection mechanism. The combination of a variably polarizing undulator and a varied-line-spacing plane-grating monochromator provides linearly or circularly polarized soft X-rays with a high resolving power in the energy range 0.28-1.5 keV. The beamline will become operational in December 1997.

Synchrotron radiation beamline to study radioactive materials at the Photon factory

Abstract Design and construction of a new beamline have been described. The beamline is housed in a specially designed area controlled for radioactive materials at the Photon Factory (PF) in the National Laboratory for High Energy Physics (KEK). The beamline system consists of a front-end and two branchlines. One of the branchlines is used for X-ray photoelectron spectroscopy and radiation biology in the energy range of 1.8-6 keV and the other for X-ray diffractometry and XAFS studies as well as radiation biology in the range of 4–20 keV. The former was particularly equipped for the protection against accidental scattering of radioactive materials both inside and outside of the vacuum system.

Construction of the JAERI soft X-ray beamline for actinide material sciences.

An undulator beamline for spectroscopy studies focusing on the electronic structure of actinide materials is under construction. Linearly or circularly polarized soft X-rays are provided by employing a variably polarizing undulator. Varied-line-spacing plane gratings and a sagittal-focusing system are used to monochromatize the undulator beam, whose energy ranges from 0.3 to 1.5 keV. A resolving power of 10(4) is expected in the whole energy region. These components are methodically operated by the SPring-8 beamline control system. There are three experimental stations in the beamline. In one of the stations the photoemission spectroscopy experiments are carried out at a radioisotope-controlled area where actinide compounds as well as unsealed radioactive materials are usable. Other experimental stations are planned in the beamline for surface photochemical reactions and biological applications.

X-ray absorption near edge structure of DNA bases around oxygen and nitrogen K-edge

Abstract X-ray absorption near edge structure (XANES) spectra of DNA and the nucleobases (adenine, guanine, cytosine, thymine and uracil) are observed using monochromatic soft X-rays (400–560 eV). The photon energy range includes the electron binding energy of the core orbital of nitrogen and oxygen K shell. Obtained XANES spectra showing a characteristic structure were theoretically analyzed by the discrete variational density functional formalism (DV-Xα) method. These results clearly indicate that the photo-excitation of oxygen and nitrogen 1s electron to π* orbitals shows a sharp resonance profile rather than that to σ* orbitals, enough to realize selecting a photochemical reaction site at specific nucleobases in a DNA molecule. Novel experiments are proposed based on the obtained spectra to understand the molecular mechanisms of radiation damages on DNA bases.

LET dependence of the yield of single-, double-strand breaks and base lesions in fully hydrated plasmid DNA films by 4He2+ ion irradiation

Purpose: To characterize the complexity of DNA damage through determination of the yields of single (SSB) and double strand breaks (DSB), base lesions and clustered damage sites induced in fully hydrated plasmid DNA by direct radiation effects as a function of the ionizing density of the radiation using 4He2+ ion irradiation with linear energy-transfer (LET) values in the range 19 to 148 keV/μm. Materials and methods: Hydrated plasmid DNA (pUC18) containing 34.5 water molecules/nucleotide was irradiated with He2+ ions with LET values of 19, 63, 95, 121 and 148 keV/μm. From quantification of the conformational changes of the irradiated samples (closed circular, open or linear forms) analyzed by agarose gel electrophoresis, the yields of SSB and DSB were obtained. Base lesions were visualized as additional strand breaks by treatment with base excision repair enzymes (endonuclease III (Nth) and formamidpyrimidine DNA glycosylase (Fpg)). Results: The yield of prompt SSB does not depend significantly on LET of the 4He2+ ions, whereas the yield of prompt DSB increases with increasing LET. The yields of isolated base lesions, revealed by Nth and Fpg as additional SSB, decrease drastically with increasing LET. The sum of the yields of DSB and additional DSB revealed by Nth and Fpg increase with increasing LET of the 4He2+ ions except at the highest LET investigated. Conclusion: The yields of clustered damage, revealed as DSB and non-DSB clustered damage sites, but not isolated lesions, namely SSB, increase with increasing ionization density of the 4He2+ ions except at the highest LET investigated.

Mutagenic and transforming effects of soft-X-rays with resonance energy of phosphorus K-absorption edge.

Syrian golden hamster embryo (SHE) cells were exposed to synchrotron-produced monochromatic X-rays at 5.747 (2.159 keV), 5.763 (2.153 keV) and 5.779 A (2.147 keV). Although X-rays of all wavelengths induced mutations and chromatid aberrations in a dose-dependent manner, when cells were irradiated with 2.153 keV X-rays, which correspond to the resonance energy of the phosphorus K-absorption edge, the frequencies of mutation and chromatid aberration at equal dose levels were higher than for X-rays of the other wavelengths. At equal survival levels, however, there was no difference in the frequencies of mutations and chromatid aberrations in cells irradiated with soft X-rays. On the other hand, the frequency of morphological transformation in cells irradiated with 2.147 keV X-rays was higher than those irradiated with 2.153 keV and 2.159 keV X-rays. The relative biological effectiveness compared to cobalt-60 gamma-rays in morphological transformation was 2.8 for 2.147 keV, 1.1 for 2.159 keV and 1.0 for 2.153 keV at a 37% survival level.

Free radical scavenging reactions and antioxidant activities of silybin: Mechanistic aspects and pulse radiolytic studies

Silybin (extracted from Silybum marianum) is the major active constituent of silymarin which possesses a wide range of medicinal properties. These properties may be, in part, due to the potent scavenging capacity of oxidizing free radicals. In this context, scavenging radicals (hydroxyl, azide, dibromide anion radicals, nitrite, carbonate, etc.) of silybin have been studied to understand the mechanistic aspects of its action against free radicals. The transients produced in these reactions have been assigned and the rate constants have been measured by pulse radiolysis techniques. Reduction potential determined both by cyclic voltammetry gave a value 0.62±0.02 V vs NHE at pH 9. Quantum chemical calculations have been performed to further confirm the different activities of individual hydroxyl groups with the difference of heat of formation. Moreover, silybin also protected plasmid pUC18 DNA from soft X-ray radiation which induced strand breaks. These results are expected to be helpful for a better understanding of the anti-oxidative properties of silybin.

Induction of DNA Strand Breaks, Base Lesions and Clustered Damage Sites in Hydrated Plasmid DNA Films by Ultrasoft X Rays around the Phosphorus K Edge

Abstract To characterize the DNA damage induced by K-shell ionization of phosphorus atom in DNA backbone on the level of hydration, the yields of DNA strand breaks and base lesions arising from the interaction of ultrasoft X rays with energies around the phosphorus K edge were determined using dry and fully hydrated pUC18 plasmid DNA samples. Base lesions and bistranded clustered DNA damage sites were revealed by postirradiation treatment with the base excision repair proteins endonuclease III (Nth) and formamidopyrimidine-DNA glycosylase (Fpg). The yield of prompt single-strand breaks (SSBs) with dry DNA irradiated at the phosphorus K resonance energy (2153 eV) is about one-third that below the phosphorus K edge (2147 eV). The yields of prompt double-strand breaks (DSBs) were found to be less dependent on the X-ray energy, with the yields being about two times lower when irradiated at 2153 eV. Heat-labile sites were not produced in detectable amounts. The yields of base lesions were dependent on the energy of the X rays, especially when the DNA was fully hydrated. Bistranded clustered DNA damage sites, revealed enzymatically as additional DSBs, were produced in dry as well as in hydrated DNA with all three energies of X rays. The yields of these enzyme-sensitive sites were also lower when irradiated at the phosphorus K resonance energy. On the other hand, the yields of prompt SSBs and enzyme-sensitive sites for the two off-resonance energies were, larger than those determined previously for γ radiation. The results indicate that the photoelectric effect caused by X rays and dense ionization and excitation events along the tracks of low-energy secondary electrons are more effective at inducing SSBs and enzyme-sensitive sites. The complex types of damage, prompt and enzymatically induced DSBs, are preferentially induced by phosphorus K resonance at 2153 eV rather than simple SSBs and isolated base lesions, particularly in hydrated conditions. It is concluded that not only the phosphorus K resonance and resulting emission of low-energy LMM-Auger electrons (∼120 eV) but also the level of hydration plays an important role in the induction of complex damage in plasmid DNA.

X-ray absorption near edge structures of DNA or its components around the oxygen K-shell edge.

Abstract Akamatsu, K. and Yokoya, A. X-Ray Absorption near Edge Structures of DNA or its Components around the Oxygen K-shell Edge. The initial process of radiation damage in DNA was investigated by measuring the X-ray absorption near edge structures (XANES) within the energy region around the oxygen K-shell absorption edge for DNA, cytosine and 2-deoxy-d-ribose. Irradiation and XANES experiments were performed with the BL23SU soft X-ray beamline, using synchrotron radiation from the 8 GeV electron storage ring at SPring-8. Samples were mounted on gold-coated plates in a vacuum chamber. The XANES spectra were obtained by measuring the photoelectron current of the samples. 2-Deoxy-d-ribose was exposed to X rays at the absorption peak corresponding to the oxygen (O) 1s→σ* transition energy (538 eV); the XANES spectra were obtained after each irradiation. DNA and cytosine, possessing characteristic XANES spectra, both had two major energy bands corresponding to the O 1s→π* and 1s→σ* transitions. Two new peaks appeared and gradually increased in the XANES spectra of 2-deoxy-d-ribose during irradiation. These results suggest that C–O bonds in 2-deoxy-d-ribose are transformed to C=O bonds by O 1s→σ* transition, suggesting that the molecules undergo chemical changes into carbonyl-containing compounds.

Oxygen K-edge X-ray absorption near edge structures (XANES) of sublimated films of amino acids

Oxygen K-edge X-ray absorption near edge structures (XANES) spectra of amino acids (glycine, L-alpha-alanine, beta-alanine, L-serine, L-asparic acid and L-tyrosine) were measured. Several peaks of XANES spectra were successfully assigned on the basis of DV-Xalpha calculation.