Alternative Fe-O2 bond lengths in O2 adducts of iron porphyrins: Implications for hemoglobin cooperativity

Geoffrey L. Woolery, Marc Walters, Kenneth S. Suslick, Linda S. Powers, Thomas G. Spiro

Research output: Contribution to journalArticle

Abstract

The extended fine structure (EXAFS) of the Fe K edge X-ray absorption spectrum has been analyzed for O2 adducts of "picket-fence" iron(II) porphyrin complexes with hindered bases: Fe(O2)(TpivPP)(2-MeIm)·C2H5OH and Fe(O2)(TpivPP)(1,2-Me2Im) (TpivPP = 5,10,15,20-(α,α,α,α)-(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 Å, in agreement with X-ray crystallography. For the 1,2-Me2Im complex, however, the Fe-O distance was 1.77 Å, the same as the crystallographically determined distance for the O2 adduct with the unhindered base, 1-MeIm (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-NIm 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.

Original languageEnglish (US)
Pages (from-to)2370-2373
Number of pages4
JournalJournal of the American Chemical Society
Volume107
Issue number8
StatePublished - 1985

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Fences
X ray crystallography
Hemoglobin
Porphyrins
X ray absorption
Bond length
Absorption spectra
Fourier transforms
Hemoglobins
Ethanol
Iron
Ligands
Atoms
Pyrroles
X Ray Crystallography
Fourier Analysis
X-Rays
imidazole
1,2-dimethylimidazole
deoxyhemoglobin

ASJC Scopus subject areas

  • Chemistry(all)

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Alternative Fe-O2 bond lengths in O2 adducts of iron porphyrins : Implications for hemoglobin cooperativity. / Woolery, Geoffrey L.; Walters, Marc; Suslick, Kenneth S.; Powers, Linda S.; Spiro, Thomas G.

In: Journal of the American Chemical Society, Vol. 107, No. 8, 1985, p. 2370-2373.

Research output: Contribution to journalArticle

Woolery, Geoffrey L. ; Walters, Marc ; Suslick, Kenneth S. ; Powers, Linda S. ; Spiro, Thomas G. / Alternative Fe-O2 bond lengths in O2 adducts of iron porphyrins : Implications for hemoglobin cooperativity. In: Journal of the American Chemical Society. 1985 ; Vol. 107, No. 8. pp. 2370-2373.
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title = "Alternative Fe-O2 bond lengths in O2 adducts of iron porphyrins: Implications for hemoglobin cooperativity",
abstract = "The extended fine structure (EXAFS) of the Fe K edge X-ray absorption spectrum has been analyzed for O2 adducts of {"}picket-fence{"} iron(II) porphyrin complexes with hindered bases: Fe(O2)(TpivPP)(2-MeIm)·C2H5OH and Fe(O2)(TpivPP)(1,2-Me2Im) (TpivPP = 5,10,15,20-(α,α,α,α)-(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 {\AA}, in agreement with X-ray crystallography. For the 1,2-Me2Im complex, however, the Fe-O distance was 1.77 {\AA}, the same as the crystallographically determined distance for the O2 adduct with the unhindered base, 1-MeIm (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-NIm 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.",
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T1 - Alternative Fe-O2 bond lengths in O2 adducts of iron porphyrins

T2 - Implications for hemoglobin cooperativity

AU - Woolery, Geoffrey L.

AU - Walters, Marc

AU - Suslick, Kenneth S.

AU - Powers, Linda S.

AU - Spiro, Thomas G.

PY - 1985

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N2 - The extended fine structure (EXAFS) of the Fe K edge X-ray absorption spectrum has been analyzed for O2 adducts of "picket-fence" iron(II) porphyrin complexes with hindered bases: Fe(O2)(TpivPP)(2-MeIm)·C2H5OH and Fe(O2)(TpivPP)(1,2-Me2Im) (TpivPP = 5,10,15,20-(α,α,α,α)-(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 Å, in agreement with X-ray crystallography. For the 1,2-Me2Im complex, however, the Fe-O distance was 1.77 Å, the same as the crystallographically determined distance for the O2 adduct with the unhindered base, 1-MeIm (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-NIm 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.

AB - The extended fine structure (EXAFS) of the Fe K edge X-ray absorption spectrum has been analyzed for O2 adducts of "picket-fence" iron(II) porphyrin complexes with hindered bases: Fe(O2)(TpivPP)(2-MeIm)·C2H5OH and Fe(O2)(TpivPP)(1,2-Me2Im) (TpivPP = 5,10,15,20-(α,α,α,α)-(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 Å, in agreement with X-ray crystallography. For the 1,2-Me2Im complex, however, the Fe-O distance was 1.77 Å, the same as the crystallographically determined distance for the O2 adduct with the unhindered base, 1-MeIm (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-NIm 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.

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