Spectroscopic studies of the effect of ligand donor strength on the Fe-NO bond in intradiol dioxygenases

Erik C. Wasinger, Mindy I. Davis, Monita Y.M. Pau, Allen M. Orville, Jeffrey M. Zaleski, Britt Hedman, John D. Lipscomb, Keith O. Hodgson, Edward I. Solomon

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20 Scopus citations


The geometric and electronic structure of NO bound to reduced protocatechuate 3,4-dioxygenase and its substrate (3,4-dihydroxybenzoate, PCA) complex have been examined by X-ray absorption (XAS), UV-vis absorption (Abs), magnetic circular dichroism (MCD), and variable temperature variable field (VTVH) MCD spectroscopies. The results are compared to those previously published on model complexes described as {FeNO}7 systems in which an S = 5/2 ferric center is antiferromagnetically coupled to an S = 1 NO-. XAS pre-edge analysis indicates that the Fe-NO units in FeIIIPCD{NO-} and FeIIIPCD {PCA,NO-} lack the greatly increased pre-edge intensity representative of most {FeNO}7 model sites. Furthermore, from extended X-ray absorption fine structure (EXAFS) analysis, the FeIIIPCD{NO-} and FeIIIPCD{PCA,NO-} active sites are shown to have an Fe-NO distance of at least 1.91 Å ≈0.2 Å greater than those found in the model complexes. The weakened Fe-NO bond is consistent with the overall lengthening of the bond lengths and the fact that VTVH MCD data show that NO- → FeIII CT transitions are no longer polarized along the z-axis of the zero-field splitting tensor. The weaker Fe-NO bond derives from the strong donor interaction of the endogenous phenolate and substrate catecholate ligands, which is observed from the increased intensity in the CT region relative to that of {FeNO}7 model complexes, and from the shift in XAS edge position to lower energy. As NO is an analogue of O2, the effect of endogenous ligand donor strength on the Fe-NO bond has important implications with respect to O2 activation by non-heme iron enzymes.

Original languageEnglish (US)
Pages (from-to)365-376
Number of pages12
JournalInorganic chemistry
Issue number2
StatePublished - Jan 27 2003

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