Ultraviolet Difference Spectral Studies in the Gamma Radiolysis of DNA and Model Compounds. I. Aqueous Solutions of DNA Bases

Summary

Ultraviolet difference spectra of gamma-irradiated, air-saturated aqueous solutions of DNA bases vs. unirradiated solutions of the same bases are shown to be a very sensitive supplemental tool with which to investigate the yields, postirradiation kinetics, and general nature of DNA base radiation products. Irradiated pyrimidines yield difference spectra which are approximately negative mirror-images of the base absorption spectra in the near-UV, indicating loss of ring conjugation. Difference spectra of irradiated purines yield a more complex pattern containing a positive long-wavelength peak, interpreted as radiation-induced extension of conjugation of the π electron system beyond that of the unirradiated purine. On the basis of the spectroscopic evidence from these studies, 8-hydroxyguanine appears to be the dominant UV-absorbing radiation product in air-saturated guanine solutions with a G-value of 0·3 molec (100 eV)⁻¹. Difference spectral studies provide isosbestic points which can be used in testing proposed radiation products and their yields. Such spectral studies are a rapid, non-invasive, supplemental tool which can be employed in conjunction with other analytical techniques in radiation-chemical studies, and which is one of the few tools able to detect short-lived chemical intermediates observed in oxygenated solutions of irradiated purines.