Spectroscopic Definition of the Geometric and Electronic Structure of the Non-Heme Iron Active Site in Iron(II) Bleomycin: Correlation with Oxygen Reactivity

Abstract

The geometric and electronic structure of high-spin ferrous complexes of bleomycin (Fe{sup II}BLM) and the structural analog PMAH ([Fe{sup II}PMA]{sup +}, where PMAH is a macrocyclic ligand with pyrimidine, imidazole, deprotonated amide, and secondary and primary amine functionalities) have been investigated by optical (Abs) and X-ray (XAS) absorption, magnetic circular dichroism (MCD), and resonance Raman (rR) spectroscopies. The lability of high-spin iron combined with steric constraints of the BLM ligand framework and its weaker axial interaction with solvent support a dissociative mechanism for O{sub 2} reactivity. Our spectroscopic studies of solid [Fe{sup II}PMA]{sup +} have defined the nature of such a five-coordinate intermediate as square pyramidal which provides an open coordination position for reaction with O{sub 2}. A major electronic structure difference between Fe{sup II}BLM (and [Fe{sup II}PMA]{sup +}) and other non-heme ferrous sites is the presence of low-energy CT transitions which reflect strong iron(II) {yields} pyrimidine backbonding. Despite generally being considered a non-heme iron system due to the absence of an extensive delocalized {pi} network, the existence of low-energy MLCT transitions with reasonable intensity, hence the presence of some backbonding, identifies BLM as an important link bridging the chemistry of non-heme and heme active sites. 113 refs., 17 figs.,more » 4 tabs.« less