Structural Characterization of Ru−Bleomycin Complexes by Resonance Raman, Circular Dichroism, and NMR Spectroscopy

A series of spectroscopic techniques including absorption and CD spectra, resonance Raman spectra, and ¹H NMR as well as electrospray mass spectrometry have shown that Ru(II) ion binds to bleomycin, forming an equimolar complex, similarly to Fe(II), i.e., via the secondary amine nitrogen, the pyrimidine ring nitrogen, the deprotonated peptide bond nitrogen of the histydyl residue, and the histidine imidazole nitrogen, which are bound in the equatorial positions, and the α-amino nitrogen of β-aminoalanine, which coordinates in the apical position above pH 7. The reaction of Ru(II)−BLM with O₂, H₂O₂,or PhIO leads to formation of the oxy species in which only one oxygen atom is bound to metal ion. According to our data, the reaction of Ru(II)−BLM complex with oxygen species leads to different product than that suggested for Fe(II)−BLM. The formation of the BLM−Ru−O−Ru−BLM dimeric unit, similar to that found for sterically unhindered Ru porphyrins, seems to be the most likely.