Enhanced Killing Effect on 5-Bromodeoxyuridine Labelled Bacteriophage T1 by Monoenergetic Synchrotron X-Ray at the Energy of Bromine K-shell Absorption Edge

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5-Bromo-2'-deoxyuridine labelled bacteriophage T1 was irradiated with monoenergetic X-rays obtained from synchrotron radiation with the energies at 13.51 keV and 12.40 keV, just above and below the K-shell absorption edge (13.41 keV) of bromine, respectively. Phage samples were irradiated under three conditions, wet N_2 gas, dry N_2 gas and in vacuum, with water contents (g H_2O/g sample) of 60, 2.6 and 0%. At 12.40 keV the D0 in kGy were 1.1 ("wet"), 1.9 ("dry") and 2.5 ("vacuum") for the Br-labelled phages and 1.6 ("wet"), 2.4 ("dry") and 6.2 ("vacuum") for unlabelled phages. The results clearly demonstrated that the X-ray sensitivities decreased with decrease in water contents. Br-enhancement ratios, ER=D_0 (unlabelled)/ D0 (Br-labelled), varied from 1.4 ("wet", 12.40 keV) to 2.5 ("vacuum", 13. 51 keV). Auger enhancements which were defined by energy-dependent enhancement factor, Fen=[ER(13.51 keV)-ER(12.40 keV)]/ ER(12.40 keV), were 0.09±0.09, 0.29±0.07 and 0.02±0.03 under "wet", "dry" and "vacuum" conditions, respectively. The change in Auger enhancement under "dry" condition in comparison with "wet" condition could be explained due to less of water. However the Auger enhancement decreased sharply under "vacuum" condition as the water content was zero. The reason for the sharp decrease in Auger enhancement under "vacuum" condition is difficult to understand. A possible explanation is discussed in the text.
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