DETERMINATION OF IRON-LIGAND BOND LENGTHS IN HORSE HEART METMYOGLOBINAND DEOXYMYOGLOBIN USING MULTIPLE-SCATTERING XAFS ANALYSES

XAFS data in the range 0 less than or equal to k less than or equal to 14.5 Angstrom(-1) have been obtained from frozen aqueous solutions (1 0 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 usin g both single-scattering (SS) and multiple-scattering (MS) analyses. T he XAFS MS analyses yield more precise Fe-ligand bond lengths (estimat ed error 0.02-0.03 Angstrom) than those determined crystallographicall y (estimated errors greater than or equal to 0.1 Angstrom). For met-Mb , the MS analysis results in an average Fe-N(pyrrole) distance of 2.05 Angstrom, an Fe-N(imidazole) distance of 2.17 Angstrom, and an Fe-O(a qua) distance of 2.08 Angstrom. For deoxy-Mb, the MS analysis results in Fe-N(pyrrole) and Fe-N(imidazole) distances of 2.06 and 2.16 Angstr om, respectively. The final XAFS R values are 18.8% and 17.8% for met- and deoxy-Mb, respectively. The robustness of the refinements was tes ted by varying the starting models, constraints, restraints, and k ran ges.