NO forms reversible complexes with non-heme ferrous enzymes and model complexes which exhibit unusual S = 3/2 ground states. These nitrosyl derivatives can serve as stable analogs of possible oxygen intermediates in the non-heme iron enzymes. Two complexes, Fe(Me(3)TACN)(NO)(N-3)(2) and FeEDTA-NO, have been studied in detail using X-ray absorption, resonance Raman, absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopies and SQUID magnetic susceptibility. These studies have been complemented by spin restricted and spin unrestricted SCF-X alpha-SW electronic structure calculations. The description generated involves high spin Fe3+ antiferromagnetically coupled to NO- S = 1. This description is strongly supported by experiment (X-ray absorption pre-edge multiplets and edge energy and shape, extended X-ray absorption fine structure bond lengths, resonance Raman force constants, ligand field and charge transfer spectral assignments), which also show that starting from the Fe3+-NO- limit the NO- is involved in a strong donor bonding interaction to the Fe3+: This donor bonding interaction involves the 4 sigma(+) molecular orbital of the NO- which is sigma antibonding with respect to the NO bond and greatly strengthens this bond. The Fe-NO bond is bent in both complexes with an angle of 156 degrees which the resonance Raman studies and SCF-X alpha-SW calculations show derives from the stabilizing effect of bending on the in-plane bonding interaction of the NO- pi* orbital with the d orbital on the iron. Magnetic circular dichroism data allow this study to be correlated with the nitrosyl derivative of the non-heme iron site in soybean lipoxygenase and demonstrate an analogous electronic structure description. As these calculations have been strongly supported by experiment for the nitrosyl complexes, they have been extended to possible oxygen intermediates. In parallel with the Fe3+-NO- complexes, the description of the intermediate obtained involves superoxide antiferromagnetically coupled to a high spin ferric center with a strong sigma donation of charge from the superoxide to the iron. These studies allow spectral data on the nitrosyl complexes to be used to estimate bonding differences in possible oxygen intermediates of different non-heme iron proteins and provide insight into the activation of superoxide by coordination to the ferric center for reaction or further reduction.
