Effects of the K-shell X-ray absorption of phosphorus on the scission of the pentadeoxythymidylic acid.

The scission of pentadeoxythymidylic acid, d(pT)5, by monochromatic soft X-rays on (2153 eV) and below (2147 eV) the K-shell absorption peak of phosphorus was studied as a model of strand breakage in DNA. Samples dried on glass plates were irradiated by monochromatic soft X-rays in vacuo, and the products were analysed by HPLC. The main products, in ascending order of the retention time, were thymine, 5'-dTMP, d(pT)2, d(pT)3, d(pT)4, d(pTp) and three unknown products (UK 1, 2 and 3), which were presumed to be d(pTpTp), d(pTpTpTp) and d(pTpTpTpTp), respectively. No difference between 2147 and 2153 eV irradiation in the nature of the induced products was detected, indicating that the K-shell absorption of phosphorus and the following Auger process produced the same types of products as those produced by photoabsorption at the other shells of phosphorus and at other atoms. The cross-sections for the induction of products at 2153 eV were 3.3-4.0 times larger than those at 2147 eV, the ratios of these values being scattered around the ratio (3.65) of absorbance of the sample between 2153 and 2147 eV. The dependence on the X-ray energy, however, almost disappeared after conversion from exposure to absorbed dose; the ratios of the G-values (number of products per 100 eV) of the products were 0.92-1.11. Photoabsorption at the K-shell of phosphorus induced products comparable or slightly less effectively than photoabsorption at the others. These results indicate that the K-shell absorption of phosphorus and the following Auger process do not have any characteristic effect on strand breakage in dry DNA, either qualitatively or quantitatively.