Transition-Metal Binding Site of Bleomycin A2. A Carbon-13 Nuclear Magnetic Resonance Study of the Zinc(II) and Copper(II) Derivatives†

The l3C NMR spectra at 25.2 MHz of the Zn(II) and Cu(II) complexes of the antitumor antibiotic bleomycin A2 are discussed. Complexation of the drug to Zn(II) causes 38 of the 52 resonance lines of bleomycin A2 to shift to new positions. All but ten of these shifted lines have been assigned in the Zn(II) bleomycin complex. Although the specific donor sites of the drug cannot be identified from the l3C NMR data, the analysis clearly shows that the pyrimidine-imidazole portion of the molecule is affected by chelation. This finding is in agreement with the previously reported metal-binding site of the antibiotic. The analysis also shows that carbon atoms which have large through-bond distances from the binding site can experience substantial chemical-shift changes upon metal binding. Complexation of the drug to Cu(II) eliminates 23 resonances from the spectrum of the molecule. All of these resonances emanate from carbon atoms which are located in the pyrimidine-imidazole portion of the drug.