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 secondar y and primary amine functionalities with either the 3-O-carbamoyl subs tituent 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 o f the perturbed complexes. This is correlated with altered DNA cleavin g capabilities. Additionally, altering the binding of the axial primar y amine significantly affects the iron coordination sphere as evidence d by reduced pi-back-bonding interactions specifically with the pyrimi dine ligand. This pyrimidine pi-back-bonding appears to play a key rol e in mediating the electron density localized on the ferrous center, w hich contributes to the unique oxygen chemistry and reactivity exhibit ed by (FeBLM)-B-II relative to other non-heme iron sites. Oxygen bindi ng to derivatives in which the beta-aminoalanine fragment has been rem oved leads to a high-spin ferric complex and no observed DNA strand sc ission, in contrast to the long-lived low-spin activated BLM intermedi ate that precedes DNA degradation.