Model Studies of Iron-Tyrosinate Proteins

The phenolate-to-iron(III) charge-transfer transition in a series of iron(III) phenolate complexes has been investigated. NMR contact shifts for the phenolate protons are well correlated with the visible absorption maxima of the complexes and the Fe^III/Fe^IIredox potentials. The results indicate that the energy of the phenolate-to-iron(III) charge-transfer band is sensitive to the crystal field strength of the other ligands coordinated to the ferric center. The stronger the other ligands are, the higher the energy of the phenolate charge-transfer band. The blue shift of the charge-transfer band is also reflected in smaller contact shifts for the phenolate protons and a more negative Fe^III/Fe^IIredox potential. On the basis of these studies, the probable identities of ligating species in transient dioxygenase intermediates are deduced. These studies also demonstrate that a square-pyramidal complex with an apical and a basal phenolate can give rise to phenolate charge-transfer bands of quite different energies. A dioxygenase active site approaching such a structure is proposed. Lastly, the axial ligand in Fe(salen)OC₆H₄-4-CH₃is shown to be an excellent model for tyrosine in resonance Raman studies of iron-tyrosinate proteins. Isotopic substitution studies on the model complex show that the ca. 570-cm^-1feature found in these proteins cannot be assigned solely to an Fe-0 stretching vibration.