Yoshiya Furusawa

Effects of K-shell X-ray absorption of intracellular phosphorus on yeast cells.

The effects of K-shell absorption of phosphorus atoms on yeast cells were investigated using synchrotron X-rays that were tuned to the resonance absorption peak (2153 eV). Three types of cellular effect (cell inactivation, induction of gene conversion at the trp-5 locus, and cell membrane impairment (changes in the permeability] were measured. It was demonstrated that the enhancement factor was 1.4 at the resonance peak regarding both lethality and the induction of gene conversion in reference of off-peak irradiation (2146 and 2160 eV). No difference was found between the two off-peak irradiation energies. No cell membrane impairment was detected, irrespective of the X-ray photon energies employed within the fluence range tested. These results strongly suggest that K-shell X-ray absorption in the resonance mode by cellular phosphorus atoms causes significantly more cellular effects than the off-resonance mode of absorption, probably via some specific changes induced in the phosphates of the DNA strand. Calculations using the number of phosphorus atoms in a defined size of the trp locus (2127 base pairs) on the DNA and the absorption cross-section of the resonance mode of phosphorus showed that gene conversion is inducible at a rate of 0.13 per X-ray photon absorption per locus. These results are discussed regarding the modes of K-shell photoabsorption.

Effects of Monoenergetic X-rays with Resonance Energy of Bromine K-absorption Edge on Bromouracil-labelled E. Coli Cells

In order to examine enhanced killing that might be induced by Auger cascades in the incorporated atoms in cells, bromouracil(BrU)-labelled E. coli cells were irradiated with monoenergetic X-rays at 13.49 and 12.40keV, just above and below the K-absorption edge of bromine. In both cases BrU-labelled cells were more sensitive for killing than were normal cells. However, when the degree of BrU-sensitization was compared between the two energies of X-rays, the enhanced killing at 13.49 keV was only small, 2 +/- 8 per cent based on the D0 value in saline. By the addition of DMSO, which is believed to suppress radical-mediated effects, killing of BrU-labelled cells was enhanced at 13.49 keV by 8 +/- 4 per cent as compared with 12.40 keV, based on D0. These results have been examined in terms of absorbed energy in BrU-labelled cells and in terms of the number of induced Auger events.

Choice of coatings for the optical elements in the irradiation system of vacuum-ultraviolet radiation above 50 nm.

The spectral throughput of a vacuum‐ultraviolet irradiation system at the SOR‐RING facility was examined with various combinations of aluminum‐and gold‐coated optical elements in a 2.2‐m modified Wadsworth monochromator. We found that the optimum was a combination of an aluminum‐coated collimating mirror, concave grating, and plane deflecting mirror, and a combination of a gold‐coated collimating mirror, concave grating and an aluminum‐coated plane deflecting mirror in the wavelength regions 190‐110 nm and 110‐50 nm, respectively.

LETHAL AND MUTAGENIC ACTION OF VACUUM-AND FAR-ULTRAVIOLET RADIATIONS ON DRY X174 PHAGE

Killing and mutation of dry X174 phages (amber mutant) were investigated with vacuum‐UV (130, 150 and 190 nm) and far‐UV (254 nm) radiations. The sensitivity to killing was greatest at 130 nm; the sensitivity (in terms of energy fluence) at 130 nm was about 17 times higher than that at 150 nm. The reversion frequency of amber mutants to pseudo‐wild type at 190 nm was lower than at 254 nm. Comparison of the induction rate of revertants per survivor showed that mutagenicity after 130 nm radiation, which may raise the ionization process, and after X‐rays was similar.

Vacuum Ultraviolet Photoacoustic Spectroscopy of Organic Solids Using Synchrotron Radiation as a Light Source

The vacuum ultraviolet (UV) region below 190nm has been one of the most difficult portions of the photon spectrum on which to perform spectroscopic studies. It is well known, however, that the major part of the oscillator strength of organic substances due to the valence electrons is localized in this spectral region, where photon energies are high enough to produce a variety of higher excited states. It is an intriguing subject to study relaxations of these higher excited states by photoacoustic (PA) method.