Gamma irradiation of DNA solutions containing copper causes changes in DNA conformation in oligonucleotides and in natural and synthetic DNAs. Diagnostic for these conformational changes is a ubiquitous 187-nm peak in the circular dichroism (CD) difference spectrum that has been predicted for a transformation from a right-handed to a left-handed helical DNA conformation. Changes in CD are correlated with changes in the UV spectrum. Reduction of DNA-bound Cu(II) to Cu(I) with ascorbic acid produces similar changes in CD spectra. These changes can be produced by the peroxy radical anion (O2*-) and the OH radical in the presence of copper. O2*- is approximately twice as efficient as *OH in initiating these changes in natural DNA. The changes in DNA conformation induced by ionizing radiation are remarkable in that they are dependent on the copper-ion concentration in a highly nonlinear manner at low copper concentrations and are not observed in the absence of copper ions. Possible implications of our results for radiobiological and oxidative damage in the cell nucleus are discussed.

Metal-ion assisted, gamma-radiation-induced B-Z conformation changes have been observed with UV and circular dichroism spectroscopy for poly (dGdC), calf thymus and herring testis DNA. These conformational changes are similar to those induced by increasing multivalent metal ion concentrations in DNA containing alternating purine and pyrimidine base sequences. In both the metal-ion-induced and the metal-ion-assisted, radiation-induced conformation changes, the conversions were from the right-handed B-DNA to the left-handed Z-DNA conformation. It is proposed that radiation-induced DNA strand breaks markedly reduce the high activation energy barrier in the metal ion-driven B-Z conformation conversion and allow much smaller metal ion concentrations to induce this conversion than in the absence of strand breaks. The biological importance of such radiation-induced conformational changes is discussed in terms of the potential significance of the Z-DNA conformation in the control of the DNA transcription process.