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 aqueo us solutions (10 K) of horse heart ferri- and ferrocyt c. Models of the str ucture about the Fe center were refined to optimize the fit between the obs erved XAFS in the range 0 less than or equal to k less than or equal to 16. 3 Angstrom(-1) and the XAFS calculated using both single-scattering (SS) an d multiple-scattering (MS) calculations. The bond lengths obtained are more accurate and precise than those determined previously for cyt c from vario us species using X-ray crystallography. The Fe-N bond lengths are 1.98-1.99 Angstrom for both oxidation states of cyt c. The Fe-S bond of ferricyt c ( 2.33 Angstrom) is significantly longer than that of ferrocyt c (2.29 Angstr om). The small changes in the bond lengths are consistent with the small re organizational energy required for the fast electron-transfer reaction of c yt c.