1H NMR studies of gamma-irradiated polynucleotides and DNA in N2O-saturated aqueous solutions: release of undamaged and modified bases.

In the 400-MHz 1H NMR spectra of gamma-irradiated, N2O-saturated aqueous solutions of polyuridylic acid (polyU), polycytidylic acid (polyC), polyadenylic acid (polyA), and single-stranded DNA from calf thymus, well-resolved signals of the undamaged free bases were identified. Upon incubation of the solutions at 95 degrees C for 2 h after irradiation, the amount of release of undamaged bases increased and in the case of polyA and DNA additional signals in the region 7.7-8.3 ppm due to 4,6-diamino-5-(formylamino)-pyrimidine and 8-hydroxyadenine (8-OH-adenine) were identified. Probably because of their low solubility guanine and modified guanine bases were not detected, whereas the lack of signals of low molecular weight pyrimidine derivatives is explained by the stability of the corresponding glycosidic bonds toward hydrolysis under our conditions. From the spectral intensities G values for release of unaltered and modified nucleic bases were estimated for radiation doses corresponding to approximately 2.5% conversion for polyU and polyC, to approximately 6% conversion for polyA and to approximately 30-60% conversion for DNA. The NMR data were in agreement with G values reported for release of undamaged bases from irradiated polyU and for double-strand DNA obtained on the basis of chromatographic procedures in combination with UV absorption (D.J. Deeble, D. Schulz, and C. von Sonntag, Int. J. Radiat. Biol. 49, 915-926, 1986; J.F. Ward and I. Kuo, Radiat. Res. 66, 485-498, 1976). Broadening of the NMR peaks of the polymers at high radiation doses (> 10 kGy) was dependent on the magnetic field strength. Therefore, it is ascribed to chemical shift heterogeneity caused by a variety of irradiation products with strongly overlapping signals rather than to a decrease in internal mobility caused by crosslinking of the polymer chains.