X-ray-induced unpaired spins in nucleic acid bases and in 5-bromouracil
Quantitative determinations were made of the energy required for the formation by x or gamma rays of a free radical (unpaired spin) in nucleic acid bases and in one analog: 5-bromouracil. The materials were irradiated in the form of dry powders with 100- to 150-kv x rays or with gamma rays from a Co/sup 60/ source. The energies required for the production of one unpaired spin in nucleic acid bases range from 120 to 1400 ev. These values are compared to those obtained for amino acids containing an aromatic ring (170 to 2500 ev), whereas the values for aliphatic amino acids fall between 20 and 80 ev. The pair cytosine-guamine (C:350 ev and G: 150 ev) require considerably less energy for the formation of an unpaired spin than the pnir thymine-adenine (T: 1400 ev and A: 1200 ev). This difference may be of importance for explaining the observation that in bacteria the mean lethal dose for x rays decreases with increasing ratio CG/TA. The energy required for the formatlon of one unpaired spin in 5bromouracil is 160 ev as compared to 1400 ev for thymine. The difference observed between the natural base and its analog has a bearing on the sensitization towardx-ray damage after incorporation of the analog into phage, micro-organisms, or mammalian cells. The results are not due to an increase of total energy absorption by photoelectric absorption in Br which is negligible, if not absent, with Co/sup 60/ gamma rays. With relatively soft x rays the additional effect of photoelectric absorption is very noticeable in pure bromouracil (42% Br by weight) though in heavy T phage (containing about 1% Br by wt) the additional effect of photoelectric absorption would be much less pronounced even with x rays. Cousequently, the sensitization towards radiation damage after incorporation of 5bromouracil into biological entities is, even with soft x rays, not likely due to the smaller energy needed for radical formation and not to additional absorption of energy by photoelectric processes. The energy required for the formation by x or gamma rays of one unpaired spin in the bases (120 to 1400 ev) is higher than the energy needed for the same effect in whole nucleic acid. The latter was 160 ev in calf thymus DNA, 110 ev in yeast RNA, and 8 ev to 30 ev in different preparations of DNA from T phage. Therefore, free radicals formed in the bases would not be expected to be the most frequent source of radiation damage to DNA. (BBB)
- Research Organization:
- Kernforschungszentrum, Karlsruhe, Ger.
- Sponsoring Organization:
- USDOE
- NSA Number:
- NSA-17-039125
- OSTI ID:
- 4657177
- Journal Information:
- Journal of Molecular Biology, Vol. 7, Issue 1; Other Information: Orig. Receipt Date: 31-DEC-63; ISSN 0022-2836
- Publisher:
- Elsevier
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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