Calcium(II) and the Trivalent Lanthanide Ion Complexes of the Bleomycin Antibiotics. Potentiometric, Fluorescence, and ¹H NMR Studies

The interactions of Ca(II), Tb(III), and several other lanthanides with the bleomycins have been monitored by a combination of potentiometric, fluorescence, and NMR methods. From the potentiometric titrations we conclude that on complex formation, a single proton is displaced from the -amino group of the diaminopropionamide group of the antibiotics. The results of Tb(III) emission experiments indicate that there is energy transfer between the fluorophores of the antibiotics and the Tb(III) emission manifold, resulting in large enhancements in the emission spectrum of the Tb(III). We have made use of this enhancement to determine the binding constants for the Tb(III) complexes of the antibiotics directly. Binding constants for other lanthanide ions were determined from an analysis of competition experiments in which these ions displace Tb(III) from its complexes with the bleomycins, thus decreasing the intensity of the Tb(III) emission lines. Proton NMR experiments conducted at 400 MHz indicate that the Pr(III) complex is in fast exchange on the NMR chemical shift time scale, while the Yb(III) complex is found to be in slow exchange. The lifetimes of the various lanthanide complexes have been estimated from a comparison with the Yb(III) case. The observed Gd(III)-induced line broadenings are shown to be exchange limited, i.e., the lifetime of the complex is longer than the transverse relaxation times in the complex and hence the relaxation rates are approximately independent of the distances of protons from the metal ion binding site.