Iron distances in hemoglobin: comparison of x-ray crystallographic and extended x-ray absorption fine structure studies

A comparison is presented of the structures obtained around the iron atom in deoxyhemoglobin (Hb). The data come from extended x-ray absorption fine structure (EXAFS) studies of the iron, which gave Fe-porphyrin nitrogen distances of 2.06 +/- 0.01 A, and from the most recent high-resolution x-ray crystallographic study, which gave exactly the same distance--2.06 +/- 0.02 A. The distance of Fe above the plane of the porphyrin nitrogens was 0.38 +/- 0.04 A from the crystallographic study; this value is not far from the upper limit of the distances 0.20 +/- (0.10)0.20 A calculated from the EXAFS experiment by triangulation. These distances above the nitrogen plane are shorter than those estimated in the earliest x-ray structures.