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Nuclear Resonance Vibrational Spectroscopic Definition of Peroxy Intermediates in Nonheme Iron Sites
Sutherlin, Kyle D.,Liu, Lei V.,Lee, Yong-Min,Kwak, Yeonju,Yoda, Yoshitaka,Saito, Makina,Kurokuzu, Masayuki,Kobayashi, Yasuhiro,Seto, Makoto,Que, Lawrence,Nam, Wonwoo,Solomon, Edward I. American Chemical Society 2016 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.138 No.43
<P>Fe-III-(hydro)peroxy intermediates have been isolated in two classes of mononuclear nonheme Fe enzymes that are important in bioremediation: the Rieske dioxygenases and the extradiol dioxygenases. The binding mode and protonation state of the peroxide moieties in-these intermediates are not well-defined, due to a lack of vibrational structural data. Nuclear resonance vibrational spectroscopy (NRVS) is an important technique for obtaining vibrational information on these and other intermediates, as it is sensitive to all normal modes with Fe displacement. Here, we present the NRVS spectra of side-on Fe-III-peroxy and end-on Fe-III-hydroperoxy model complexes and assign these spectra using calibrated DFT calculations. We then use DFT calculations to define and understand the changes in the NRVS spectra that arise from protonation and from opening the Fe-O-O angle. This study identifies four spectroscopic handles that will enable definition of the binding mode and protonation state of Fe-III-peroxy intermediates in mononuclear nonheme Fe enzymes. These structural differences are important in determining the frontier molecular orbitals available for reactivity.</P>
Hong, Seungwoo,Yan, James J.,Karmalkar, Deepika G.,Sutherlin, Kyle D.,Kim, Jin,Lee, Yong-Min,Goo, Yire,Mascharak, Pradip K.,Hedman, Britt,Hodgson, Keith O.,Karlin, Kenneth D.,Solomon, Edward I.,Nam, W Royal Society of Chemistry 2018 Chemical science Vol.9 No.34
<▼1><P>X-ray structures of nonheme {FeNO}<SUP>6</SUP> and iron(<SMALL>III</SMALL>)-nitrito complexes bearing a tetraamido macrocyclic ligand are reported along with three different generation pathways.</P></▼1><▼2><P>While the synthesis and characterization of {FeNO}<SUP>7,8,9</SUP> complexes have been well documented in heme and nonheme iron models, {FeNO}<SUP>6</SUP> complexes have been less clearly understood. Herein, we report the synthesis and structural and spectroscopic characterization of mononuclear nonheme {FeNO}<SUP>6</SUP> and iron(<SMALL>III</SMALL>)–nitrito complexes bearing a tetraamido macrocyclic ligand (TAML), such as [(TAML)Fe<SUP>III</SUP>(NO)]<SUP>–</SUP> and [(TAML)Fe<SUP>III</SUP>(NO<SUB>2</SUB>)]<SUP>2–</SUP>, respectively. First, direct addition of NO<SUB>(g)</SUB> to [Fe<SUP>III</SUP>(TAML)]<SUP>–</SUP> results in the formation of [(TAML)Fe<SUP>III</SUP>(NO)]<SUP>–</SUP>, which is sensitive to moisture and air. The spectroscopic data of [(TAML)Fe<SUP>III</SUP>(NO)]<SUP>–</SUP>, such as <SUP>1</SUP>H nuclear magnetic resonance and X-ray absorption spectroscopies, combined with computational study suggest the neutral nature of nitric oxide with a diamagnetic Fe center (<I>S</I> = 0). We also provide alternative pathways for the generation of [(TAML)Fe<SUP>III</SUP>(NO)]<SUP>–</SUP>, such as the iron–nitrite reduction triggered by protonation in the presence of ferrocene, which acts as an electron donor, and the photochemical iron–nitrite reduction. To the best of our knowledge, the present study reports the first photochemical nitrite reduction in nonheme iron models.</P></▼2>