| Abstract
| - The 13C chemical shifts of Cd- and ZnBlmA2 are almost identical throughout the entiremolecule, suggesting that these structures adopt similar conformations.Nuclear magnetic resonanceexperiments with 113Cd−bleomycin have defined partof the metal−ligand environment of the molecule.Nitrogen atoms from the primary amine, pyrimidine, and imidazoleare bound to 113Cd according to13Cspectra showing 113Cd−13C spin−spincouplings. Bound and free forms of the secondary aminenitrogenmay be in equilibrium, as suggested by temperature-dependent13C studies with Cd−bleomycin. Inaddition,a number of other carbon resonances are in chemical exchange over thetemperature range 5−54 °C.The temperature dependence of the line widths of carbon atoms ofZn−bleomycin strongly resemblesthat of Cd−bleomycin. Examination of the 113Cdresonance as a function of temperature also supportsthe presence of at least two differently coordinated forms of cadmiumin the molecule. According to theposition of the 113Cd chemical shift, at most fournitrogen atoms are bound to Cd at low temperature.Titrations of 113Cd−bleomycin with chloride oracetate demonstrate that these anions can bind to majorand minor forms of the structure and that a minor species exists whichdoes not associate with chloride.
|
