Hemoglobin 7-coordination

Dr. Spijkerman I would be wilhng to have hemoglobin 7-coordinated because we worked on the iron EDTA salts, which show a very asymmetric quadrupole splitting, and they are 7-coordinated. The spectra of hemoglobin and the iron EDTA salts are quite similar. I think that spin-spin relaxation occurs, which is more pronounced with 7-coordination than one would expect for 6 or 4. If this is the case, there might be some indication of it, but the Mossbauer spectrum certainly doesn t indicate it from the chemical shift or quadrupole splitting. 

Each heme unit in myoglobin and hemoglobin contains one ion bound to four nitrogen donor atoms in a square planar arrangement. This leaves the metal with two axial coordination sites to bind other ligands. One of these sites is bound to a histidine side chain that holds the heme in the pocket of the protein. The other axial position is where reversible binding of molecular oxygen takes place. 

The binding and release of oxygen by hemoglobin can be represented as a ligand-exchange equilibrium at the sixth coordination site on the Fe ion. Each of the four polypeptide chains of hemoglobin contains one heme unit, so... 

The MM calculations show (77) that for a metal ion the size of five-coordinate high-spin Fe(II) in a porphyrin, there is a very small difference in energy between whether the Fe(II) rises up out of the plane or remains in the plane of the porphyrin. Thus, a small modification of the bell-pull mechanism (80) of hemoglobin may be that the high-spin Fe(II) with no oxygen coordinated to it does not rise out of the... 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

P. R. Girodot (The University of Texas at Arlington) A recent report stated that hemoglobin contained three globins and not two attached to the central iron atom, as shown by the Sorel eflFect. If so, it would not be octahedral but 7-coordinated. In this case, is there any evidence to support this idea or disprove it from the Mossbauer data presently available ... 

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