Mohammed Ibrahim - New Light on NO Bonding in Fe(III) Heme Proteins from Resonance Raman Spectroscopy and DFT Modeling

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      Publication Details (including relevant citation   information):

      Journal of the American Chemical Society  2010, 132, 4614-4625



      Visible and ultraviolet resonance Raman (RR) spectra are reported   for FeIII

      (NO) adducts of myoglobin variants with altered polarity in the   distal heme pockets. The stretching frequencies of the   FeIII

      −NO and N−O bonds, νFeN

      and νNO

      , are negatively correlated, consistent with backbonding.   However, the correlation shifts to lower νNO

      for variants lacking a distal histidine. DFT modeling reproduces   the shifted correlations and shows the shift to be associated   with the loss of a lone-pair donor interaction from the distal   histidine that selectively strengthens the N−O bond. However,   when the model contains strongly electron-withdrawing   substituents at the heme β-positions, νFeN

      and νNO

      become positively correlated. This effect results from   FeIII

      −N−O bending, which is induced by lone-pair donation to the   NNO

      atom. Other mechanisms for bending are discussed, which likewise   lead to a positive νFeN


      correlation, including thiolate ligation in heme proteins and   electron-donating meso

      -substituents in heme models. The νFeN


      data for the Fe(III) complexes are reporters of heme pocket   polarity and the accessibility of lone pair, Lewis base donors.   Implications for biologically important processes, including NO   binding, reductive nitrosylation, and NO reduction, are   discussed.

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