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Magnetic, vibrational, and optical techniques are combined with density functional calculations to elucidate the electronic structure of the diamagnetic mononuclear side-on CuII-superoxo complex. The electronic nature of its lowest singlet/triplet states and the ground-state diamagnetism are explored. The triplet state is found to involve the interaction between the Cu xy and the superoxide pi v * orbitals, which are orthogonal to each other. The singlet ground state involves the interaction between the Cu xy and the in-plane superoxide pi v * orbitals, which have a large overlap and thus strong bonding. The ground-state singlet/triplet states are therefore fundamentally different in orbital origin and not appropriately described by an exchange model. The ground-state singlet is highly delocalized with no spin polarization.

Lipoxygenases (LOs) are non-heme iron enzymes which catalyze the reaction of dioxygen with cis,cis-1,4-pentadiene-containing fatty acids to form hydroperoxide products, which in mammals are the precursors to the inflammation-and immunity-mediating compounds lipoxins and leukotrienes. Recent X-ray crystal structures of ferrous soybean lipoxygenase-1 (SLO-1) offer two different descriptions of the active site: one four-coordinate and one five-or six-coordinate. Near-infrared (NIR) circular and magnetic circular dichroism (CDMCD) and variabletemperature, variable-field (VTVH) MCD have been used to study SLO-1 in solution which is found to exist as a 40/60% mixture of five-and six-coordinate forms, respectively. Addition of linoleate substrate or alcohols shifts the mixture to the purely six-coordinate form. NIR CDMCD studies of two mammalian LOs, rabbit reticulocyte and recombinant human 15-LOs, show that these exist as pure six-coordinate forms. X-ray absorption Fe K-edge and pre-edge data also show that the mammalian 15-LOs and SLO-1 in glycerol are six-coordinate. This is consistent with the extended X-ray absorption fine structure (EXAFS) results of SLO-1 in glycerol which show the iron active site to have 5 f 1 N/O at -2.16 A. VTVH MCD data on the six-coordinate sites show that the mammalian and soybean enzymes have very different ground-state splittings, indicative of differences in bonding interactions with the ligand set. These differences in ferrous site coordination in solution and ground-state splittings are attributed to the substitution of a stronger histidine ligand in the mammalian 15-LOs for an asparagine in SLO-1.

The magnetic circular dichroism (MCD) spectrum and saturation magnetization MCD data of the near-infrared d --d transitions of the hexaaquoferrous complex ferrous fluorosilicate are presented. These data are used to develop a theoretical framework for modeling saturation magnetization MCD data from randomly oriented, integer spin systems, such as ferrous metalloprotein active sites, that have positive zero-field splittings (ZFS) and hence a nondegenerate ground state. The sign of the ZFS coupled with the energies of the MCD transitions provides direct information about the energy ordering of the d-orbitals. Although ferrous fluorosilicate does not have a degenerate ground state, it exhibits increasing MCD intensity with decreasing temperature much like a Kramers system. The low-temperature MCD intensity is shown to be due to a temperature-dependent nonlinear 55-term mechanism which results from a z-polarized transition moment coupled with off-axis Zeeman effects. Since the data from a complex with positive ZFS can qualitatively resemble a system with negative ZFS, ways to distinguish the sign of the zero-field splitting from saturation magnetization MCD data are also presented. This extension of the current saturation magnetization MCD methodology to include cases with positive ZFS is important since MCD has proven to be a crucial technique for characterizing the geometric and electronic structures of mononuclear non-heme ferrous enzymes.