Copper is an important biological metal that tightly binds to DNA. Its reaction with endogenously generated hydrogen peroxide may thus lead to the formation of DNA damage. To gain insights into the underlying mechanisms, a comparative study of the damage produced within isolated DNA upon exposure to gamma-radiation in aqueous solution, a source of hydroxyl radicals, and incubation with Cu(I) or Cu(II) complexes in the presence of hydrogen peroxide was carried out. Several relevant base modifications were quantified by HPLC-tandem mass spectrometry. It was first shown that addition of copper ions only slightly modified the profile of radiation-induced lesions within DNA. However, the distribution of base modifications was drastically different upon incubation of DNA with Cu(I) or Cu(II) complexes in the presence of H(2)O(2). Indeed, guanine degradation products were produced in much higher yield than lesions of the other bases. These observations are rationalized in terms of the occurrence of one electron oxidation with Cu(I) complexes, as confirmed by the study of the degradation of free thymidine. In contrast, the formation of the sole 8-oxo-7,8-dihydroguanine upon incubation of DNA with Cu(II) ions and H(2)O(2) strongly suggests the production of singlet oxygen as the predominant reactive oxygen species.