Determination of Iron−Ligand Bond Lengths in Horse Heart Met- and Deoxymyoglobin Using Multiple-Scattering XAFS Analyses

XAFS data in the range 0 ≤ k ≤ 14.5 Å-1 have been obtained from frozen aqueous solutions (10 K) of horse heart myoglobin (Mb) in the Fe(III) (aqua-met) and Fe(II) (deoxy) forms. The structures of the Fe sites have been refined using both single-scattering (SS) and multiple-scattering (MS) analyses. The XAFS MS analyses yield more precise Fe−ligand bond lengths (estimated error 0.02−0.03 Å) than those determined crystallographically (estimated errors ≥0.1 Å). For met-Mb, the MS analysis results in an average Fe−N(pyrrole) distance of 2.05 Å, an Fe−N(imidazole) distance of 2.17 Å, and an Fe−O(aqua) distance of 2.08 Å. For deoxy-Mb, the MS analysis results in Fe−N(pyrrole) and Fe−N(imidazole) distances of 2.06 and 2.16 Å, respectively. The final XAFS R values are 18.8% and 17.8% for met- and deoxy-Mb, respectively. The robustness of the refinements was tested by varying the starting models, constraints, restraints, and k ranges.