Determination of Iron−Ligand Bond Lengths in Ferric and Ferrous Horse Heart Cytochrome c Using Multiple-Scattering Analyses of XAFS Data

X-ray absorption fine structure (XAFS) data were obtained from frozen aqueous solutions (10 K) of horse heart ferri- and ferrocyt c . Models of the structure about the Fe center were refined to optimize the fit between the observed XAFS in the range 0 ≤ k ≤ 16.3 Å -1 and the XAFS calculated using both single-scattering (SS) and multiple-scattering (MS) calculations. The bond lengths obtained are more accurate and precise than those determined previously for cyt c from various species using X-ray crystallography. The Fe−N bond lengths are 1.98−1.99 Å for both oxidation states of cyt c . The Fe−S bond of ferricyt c (2.33 Å) is significantly longer than that of ferrocyt c (2.29 Å). The small changes in the bond lengths are consistent with the small reorganizational energy required for the fast electron-transfer reaction of cyt c .