Metal coordination environment and dynamics in 113cadmium bleomycin: relationship to zinc bleomycin.

The 13C chemical shifts of Cd- and ZnBlm A2 are almost identical throughout the entire molecule, suggesting that these structures adopt similar conformations. Nuclear magnetic resonance experiments with 113Cd-bleomycin have defined part of the metal-ligand environment of the molecule. Nitrogen atoms from the primary amine, pyrimidine, and imidazole are bound to 113Cd according to 13C spectra showing 113Cd-13C spin-spin couplings. Bound and free forms of the secondary amine nitrogen may be in equilibrium, as suggested by temperature-dependent 13C studies with Cd-bleomycin. In addition, a number of other carbon resonances are in chemical exchange over the temperature range 5-54 degrees C. The temperature dependence of the line widths of carbon atoms of Zn-bleomycin strongly resembles that of Cd-bleomycin. Examination of the 113Cd resonance as a function of temperature also supports the presence of at least two differently coordinated forms of cadmium in the molecule. According to the position of the 113Cd chemical shift, at most four nitrogen atoms are bound to Cd at low temperature. Titrations of 113Cd-bleomycin with chloride or acetate demonstrate that these anions can bind to major and minor forms of the structure and that a minor species exists which does not associate with chloride.