Determination of the Fe-ligand bond lengths and Fe-N-O bond angles in horse heart ferric and ferrous nitrosylmyoglobin using multiple-scattering XAFS analyses

The structural characterizations of the Fe environments in the NO adducts of horse heart ferric (Fe(III)) and ferrous (Fe(II)) nitrosylmyoglobin (MbNO) have been achieved by multiple-scattering (MS) analyses of XAFS data obtained from frozen aqueous solutions. For Mb(II)NO, the MS analysis resulted in an averaged Fe-N(p) (pyrrole) distance of 2.00 Angstrom, an Fe-N(epsilon) (imidazole) distance of 2.05 Angstrom, an Fe-N(NO) distance of 1.75 Angstrom, and an Fe-N-O angle of 150 degrees. For comparison, the MS analysis of the XAFS data of [Fe(II)(TPP)(NO)] (TPP = tetraphenylporphyrinato) resulted in bond lengths of 2.01 Angstrom for Fe-N(p) and 1.74 Angstrom for Fe-N(NO) and an Fe-N-O bond angle of 155 degrees. The Fe-N(NO) and N-O distances obtained from the analysis of Mb(II)NO are in good agreement with those determined crystallographically for [Fe(II)(TPP)(NO)] with and without 1-methylimidazole (1-MeIm) as the sixth ligand. The MS analysis of Mb(III)NO yielded an average Fe-N(p) distance of 2.00 Angstrom, an Fe-N(epsilon) distance of 2.04 Angstrom, an Fe-N(NO) distance of 1.69 Angstrom, and an Fe-N-O angle of 180 degrees. These bond lengths and angles are consistent with the crystal structures of model complexes [Fe(III)(TPP)(NO)(OH(2))](+) and [Fe(III)(OEP)(NO)](+) (OEP = octaethylporphyrinato). The final XAFS R values were 14.5, 14.6, and 12.7% for Mb(III)NO, Mb(II)NO, and [Fe(II)(TPP)(NO)], respectively.