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

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

Abstract

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
Volume42
Issue number2
DOIs
StatePublished - Jan 27 2003

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