Some Radiation Effects on Dna and Its Constituents.

The first use of electron spin resonance for the study of radiation effects on the nucleic acids and their constituents was made by Shields and Gordy', 2on powdered samples. Quantitative measurements of the free-radical yield in powdered samples have recently been made by Muller.3 In an effort to gain more specific information with this method about the effects of radiation on these genetically important compounds, we have undertaken the slower, more difficult program of the study of oriented radicals in polymers and in single crystals. We have succeeded in growing single crystals of both thymidine and cytidine which are large enough to give strong ESR signals induced by gamma-irradiation. The measurements on thymidine crystals have been completed and analyzed; the results on cytidine are nearing completion. Because of limitation of space, details of these studies cannot be included in the present paper. They will be described in separate papers. Use will be made, however, of these results in the interpretation of the ESR signals obtained in irradiated DNA. In the earlier studies2 both DNA and RNA were found to have a relatively broad resonance, -30 gauss, with g = 2.00, and no resolved hyperfine structure. In contrast, certain of the constituent nucleosides were found to have resolvable, or partially resolvable, hyperfine structure. That for thymidine had the largest number of resolved components. Shields and Gordy2 concluded that this hyperfine structure must arise from a free radical having five hydrogen nuclei coupling with the electron spin moment, three of which have equivalent coupling of about 23 gauss and two of which have equivalent coupling of about 42 gauss. From the fact that thymine gave the same characteristic pattern as thymidine, it was evident that the electron spin density was concentrated in the basic ring group in thymidine and hence might be expected to occur in the same ringed group in irradiated DNA. However, no signals were found from DNA which could be definitely identified as characteristic of its thymidine rings. In later studies, a thymidinelike resonance was detected in irradiated DNA by Salovey, Shulman, and Walsh,4 and by Ehrenberg, Ehrenberg, and Lofroth,5 who applied special physical and/or chemical treatment not applied by Shields and Gordy. It should be noted that the unresolved resonance like that observed by Shields and Gordy has been observed by other researchers using various kinds of DNA samples. It is the predominant pattern observed for samples of DNA irradiated under vacuum at room temperature. The factors which lead to the production of the thymidinelike resonance have not been entirely clear. The implication of the results by Ehrenberg et al.5 is that special chemical preparation or treatment of the sample is required for observation of the thymidinelike resonance. Nevertheless, Salovey, Shulman, and Walsh4 with their special method of physical treatment (irradiation at lowered temperature followed by annealing at 3250K) obtained the thymidinelike resonance in several commercial samples, although the signals they observed for dosage of 1 M rads were like those of Shields and Gordy.