Determination of Fe-ligand bond lengths and the Fe-N-O bond angles in soybean ferrous and ferric nitrosylleghemoglobin alpha using multiple-scattering XAFS analyses

The NO adducts of leghemoglobin (Lb) are implicated in biological processes , but only the adduct with ferrous Lb (Lb(II)NO) has been characterized pre viously. We report the first characterization of ferric nitrosylleghemoglob in (Lb(III)NO) and XAS experiments performed on frozen aqueous solutions of Lb(II)NO and Lb(III)NO at 10 K. The XANES and electronic spectra of the NO adducts are similar in shape and energies to the myoglobin (Mb) analogues. The environment of the Fe atom has been refined using multiple-scattering (MS) analyses of the XAFS data. Fdr Lb(II)NO, the MS analysis resulted in a n averaged Fe-N-p(pyrrole) distance of 2.02 Angstrom, an Fe-N-epsilon(imida zole) distance of 1.98 Angstrom, an Fe-N-No distance of 1.77 Angstrom, and an Fe-N-O angle of 147 degrees. The Fe-N-NO distance and Fe-N-O angle obtai ned from the analysis of Lb(II)NO are in good agreement with those determin ed crystallographically for [Fe(TPP)(NO)] (TPP, tetraphenylporphyrinato), w ith and without 1-methylimidazole (1-MeIm) as the sixth ligand, and the MS XAFS structures reported previously for the myoglobin (Mb(II)NO) analogue a nd [Fe(TPP)(NO)]. The MS analysis of Lb(III)NO yielded an average Fe-N-p di stance of 2.00 Angstrom, an Fe-N-epsilon distance of 1.89 Angstrom, an Fe-N -NO distance of 1.68 Angstrom, and an Fe-N-O angle of 173 degrees. These bo nd lengths and angles are consistent with those determined previously for t he myoglobin analogue (Mb(III)NO) and the crystal structures of the model c omplexes, [Fe-III(TPP)(NO)(OH2)](+) and [Fe(OEP)(NO)](+) (OEP, octaethylpor phyrinato). The final XAFS R values were 16.1 and 18.2% for Lb(II)NO and Lb (III)NO, respectively.