Guanosine-specific DNA damage by a Co(II)·bithiazole complex

A bithiazole derivative structurally related to the bithiazole moiety of bleomycin (BLM) A₂ was prepared. This derivative contained a 2-(1,3-diaminopropyl) substituent, rather than the 2-(2-aminoethyl) substituent normally present in BLM, in order to facilitate metal coordination by the bithiazole moiety itself. In the presence of Co²⁺, the modified bithiazole mediated the production of alkali labile lesions on double-stranded DNA. Following treatment with alkali, guanosine-specific DNA strand scission was observed. DNA degradation by this Co(II)·bithiazole complex was not light dependent but did require molecular oxygen. DNA cleavage studies employing scavengers of activated oxygen species suggested that the observed DNA damage did not result from diffusible, activated forms of oxygen. Moreover, ESR spectroscopy utilizing a spin trapping reagent demonstrated conclusively that the Co(II)·bithiazole complex did not produce diffusible oxygen radicals. Absorption spectroscopy with a thiazole analog of the modified bithiazole suggested that, in the presence of oxygen, a O₂·Co·thiazole complex was formed. These mechanistic studies suggested that the (oxygenated) Co(II)·bithiazole complex mediated the oxidative alteration or liberation of the guanine base, producing an alkali labile site. Further, the guanosine specificity appeared to derive from preferential reactivity at guanosine sites, as opposed to a guanosine binding selectivity of the bithiazole. The oxidative degradation of G residues in DNA appears to proceed by an inner sphere mechanism.