Catechol dioxygenases spectroscopy

The asymmetric bonding motif of the catecholate to the ferric ion in these complexes duplicates the binding motif in the enzyme-substrate structures that have been characterized. While the original belief that this asymmetry in Fe—O bond lengths was critical to the reactivity of the systems has since proven incorrect,the complexes remain as functional models for the catechol dioxygenases due to both their structural and reactivity features. All complexes have been characterized as high-spin ferric catecholate complexes by UV-visible, EPR and NMR spectroscopy, and an examination of C—O bond length in these complexes shows all of the complexes to bind in the catecholate form with no semiquinonate character. ... 

Two intermediates have been detected in the reaction of catechol 1,2-dioxygenase with pyrogallol and dioxygen [88]. The active site in this enzyme has been studied by NMR spectroscopy a high-spin ferric center gives rise to paramagnetically shifted resonances [89]. 

The character of the coordinated catechols has been studied by Mossbauer spectroscopy [38]. Since the value of A is a measure of the hyperfine interaction of the iron electron with Fe, it reflects the covalency of the metal-ligand bands. The value of A/gnPn for the catechol complex is -18.9 T, that is the smallest observed for a dioxygenase complex -20.8 T for the phenol complex and -20.0 T for the thiophenol complex. The lowest values are explained by a greater delocalization of unpaired spin density away from the ferric center onto the catechol, generating a radical character of the catechol. The unpaired spin density delocalization is consistent with the paramagnetic shifts of protons of the catecholate ligand observed by NMR. 

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