SPECTROSCOPIC DEFINITION OF THE GEOMETRIC AND ELECTRONIC-STRUCTURE OF THE NONHEME IRON ACTIVE-SITE IN IRON(II) BLEOMYCIN - CORRELATION WITH OXYGEN REACTIVITY

The geometric and electronic structure of high-spin ferrous complexes of bleomycin (Fe(II)BLM) and the structural analog PMAH ([Fe(II)PMA](+), 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. From the excited state ligand field transition energies in the low-temperature MCD spectra, the XAS pre-edge shapes and intensities, and EXAFS analysis, solid [Fe(II)PMA](+) has been determined to have a five-coordinate, square-pyramidal geometry (E(dx2-y2) - E(dz2) = 6100 cm(-1)) with a short Fe-N bond (1.93 Angstrom), while in solution [Fe(II)PMA](+) binds a solvent molecule at the sixth position to form a distorted octahedral complex (E(dx2-y2) - E(dz2) = 2110 cm(-1)) with an expanded coordination sphere which still maintains one short Fe-N bond (2.00 Angstrom). Similar spectral features consistent with a six-coordinate geometry (E(dx2-y2) - E(dz2) = 2650 cm(-1)) are also present for Fe(II)BLM in solution, suggesting parallel ligation to the Fe2+ center in [Fe(II)PMA](+) including one relatively short Fe-N bond (2.06-2.08 Angstrom). The magnetic field and temperature dependence of the MCD intensity reveals an unusually small zero-field-splitting of the S = 2, M(s) = +/- 2 non-Kramers doublet ground state of Fe(II)BLM and [Fe(II)PMA](+) in solution (delta = 2.4 cm(-1)) indicating a large splitting of the d pi orbitals (E(dxz,yz) - E(dxy) = 800 and 950 cm(-1), respectively) resulting from strong metal-ligand pi-bonding interaction. The presence of moderately-intense, low-energy metal-to-ligand charge transfer (MLCT) transitions in the low-temperature Abs and MCD spectra of [Fe(II)PMA](+) and Fe(II)BLM represents an important deviation from other non-heme iron centers and reflects high covalency. The MLCT transition energies and intensities determine the degree of metal-ligand pi-backbonding which decreases along the series solid [Fe(II)PMA](+) > solution [Fe(II)PMA](+) > Fe(II)BLM. Assignment of these bands as iron(II) --> pyrimidine MLCT transitions is derived from the strong resonance enhancement of the pyrimidine normal modes at 680, 744, 1519, and 1542 cm(-1) in the [Fe(II)PMA](+) Raman spectrum, thereby implicating pyrimidine as the specific ligand associated with the large d pi splitting and short Fe-N bond. This pyrimidine pi-backbonding mediates the electron density localized on the Fe2+ center which contributes to the unique chemistry of Fe(II)BLM relative to other non-heme iron sites. This includes its ability to bind pi-acceptor exogenous ligands resulting in the conversion to a low-spin state and its formation of a long-lived oxygen intermediate.