Akamatsu, K., Fujii, K. and Yokoya, A. Qualitative and Quantitative Analyses of the Decomposition Products that Arise from the Exposure of Thymine to Monochromatic Ultrasoft X Rays and 60Co Gamma Rays in the Solid State. Radiat. Res. 161, 442–450 (2004).
HPLC analyses of condensed thymine irradiated with monochromatic synchrotron ultrasoft X rays in the energy region around nitrogen and oxygen K-shell edges were performed. Cobalt-60 γ rays were used as a reference radiation. The radiation chemical dose response of each separated thymine decomposition product was also determined. Uracil (U), 5-(hydroxymethyl)uracil (HMU), 5,6-dihydrothymine (DHT), 5-formyluracil (foU) and four main unknown products were found in the HPLC chromatogram of the sample irradiated with ultrasoft X rays in vacuo. Similar spectra of the products were also found in the γ-ray experiment; however, some unknown products that appeared after elution of the thymine peak were significantly larger than those in the ultrasoft X- ray experiment. This result indicates the difference in radiation quality. The G value of DHT produced by γ radiation was 10 times larger than those produced by the ultrasoft X- ray photons with energies of 395 and 407 eV corresponding to below and on the nitrogen K-shell edge, respectively. This result suggests that the differences in the photon energy and/ or in the energy spectra of the secondary electron between ultrasoft X rays and γ rays are causing differences in the process of the radiation chemistry. Moreover, the yields of all the thymine decomposition products induced by 538 eV photons (oxygen K-shell edge) were significantly smaller than those induced by photons around the nitrogen K-shell edge. The K-shell excitation of oxygen in thymine may efficiently promote the production of small thymine fragments susceptible to desorption from the sample.