Structural studies of of "active complex" of bleomycin: assignment of ligands to the ferrous ion in a ferrous-bleomycin-carbon monoxide complex.

Proton NMR studies at 360 MHz establish the binary Fe(II)-bleomycin complex to be paramagnetic with a spectrum covering 70 ppm. Addition of carbon monoxide generates a stable, diamagnetic Fe(II)-bleomycin-CO complex that is a putative structural analog of the "active" Fe(II)-bleomycin-O2 complex. The following six groups have been determined to be coordinated to the Fe(II) ion from analysis of the highly resolved 1H NMR spectra of this complex: CO, the primary and secondary amine nitrogens of the beta-aminoalanine moiety, the carbamoyl moiety on the 3-position of mannose, the pyrimidine N-1, and the imidazole N-1. The Fe(II)-bleomycin-CO complex binds to DNA, as shown by fluorescence quenching experiments, but Fe(II)-bleomycin-CO does not mediate thymine release. These results necessitate a major revision in the current model for metal coordination to bleomycin.