Spectroscopic investigation of the metal ligation and reactivity of the ferrous active sites of bleomycin and bleomycin derivatives

The geometric and electronic structures of high-spin ferrous complexes of bleomycin ((FeBLM)-B-II) and a series of systematically perturbed BLM derivatives have been investigated by optical absorption, circular dichroism (CD), and magnetic circular dichroism (MCD) spectroscopies. The active site of the unmodified drug complex is six-coordinate with the coordination sphere completed by at least five endogenous ligands including the pyrimidine, imidazole, deprotonated amide, and secondary and primary amine functionalities with either the 3-O-carbamoyl substituent of the mannose sugar or solvent bound at the sixth site. This weak sixth ligand is the exchangeable site of exogenous small molecule binding. Perturbing the carbamoyl substituent alters the coordination environment of the metal and decreases the azide binding affinities of the perturbed complexes. This is correlated with altered DNA cleaving capabilities. Additionally, altering the binding of the axial primary amine significantly affects the iron coordination sphere as evidenced by reduced pi-back-bonding interactions specifically with the pyrimidine ligand. This pyrimidine pi-back-bonding appears to play a key role in mediating the electron density localized on the ferrous center, which contributes to the unique oxygen chemistry and reactivity exhibited by (FeBLM)-B-II relative to other non-heme iron sites. Oxygen binding to derivatives in which the beta-aminoalanine fragment has been removed leads to a high-spin ferric complex and no observed DNA strand scission, in contrast to the long-lived low-spin activated BLM intermediate that precedes DNA degradation.