Alternative Fe-0₂ Bond Lengths in 0₂ Adducts of Iron Porphyrins: Implications for Hemoglobin Cooperativity

The extended fine structure (EXAFS) of the Fe K edge X-ray absorption spectrum has been analyzed for Ot adducts of “picket-fence” iron(II) porphyrin complexes with hindered bases: Fe(0₂)(TpivPP)(2-MeIm)-C₂H₅0H and Fe(0₂)-(TpivPP)(1,2-Me₂Im) (TpivPP = 5,10,15,20-(a,a,a,a)-(o-pivaloylamidophenyl)porphyrinate, 2-MeIm = 2-methylimidazole, 1,2-Me₂Im = 1,2-dimethylimidazole). The first shell peaks of the Fourier transforms were filtered and backtransformed, and analyzed for the distances from the Fe to the coordinated atoms (one O, one imidazole N, and four pyrrole N). For the 2-MeIm complex at -150 °C, the Fe-O distance was 1.90 A, in agreement with X-ray crystallography. For the 1,2-Me₂Im complex, however, the Fe-O distance was 1.77 A, the same as the crystallographically determined distance for the O₂ adduct with the unhindered base, 1-Melm (1-methylimidazole). These results establish that two Fe-O distances are available to O₂ adducts with hindered bases: One with a shorter Fe-O bond and a consequently longer Fe-N_Im bond, and the second vice versa. These two structures offer a plausible explantation for the cooperativity in O₂ binding previously observed for the solid (ethanol-free) adducts of these complexes; the low- and high-affinity binding regions may be due to the formation of the long and short Fe-O bonded structures successively. A similar structure change may take place within the T state of hemoglobin for which there exists substantial evidence of restraint on the proximal imidazole; initial ligand binding may occur via a long Fe-O bond, which could be followed by rearrangement to a short Fe-O bonded structure.