An experimental and quantum chemical investigation of CO binding to heme proteins and model systems: A unified model based on 13C, 17O, and 57Fe nuclear magnetic resonance and57Fe mossbauer and infrared spectroscopies

Michael T. McMahon, Angel C. DeDios, Nathalie Godbout, Renzo Salzmann, David D. Laws, Hongbiao Le, Robert H. Havlin, Eric Oldfield

Research output: Contribution to journalArticlepeer-review


We have investigated the question of how CO ligands bind to iron in metalloporphyrins and metalloproteins by using a combination of nuclear magnetic resonance (NMR), 57Fe Mossbauer, and infrared spectroscopic techniques, combined with density functional theoretical calculations to analyze the spectroscopic results. The results of 13C NMR isotropic chemical shift, 13C NMR chemical shift anisotropy, 17O NMR isotropic chemical shift, 17O nuclear quadrupole coupling constant, 57Fe NMR isotropic chemical shift, 57Fe Mossbauer quadrupolar splitting, and infrared measurements indicate that CO binds to Fe in a close to linear fashion in all conformational substates. The 13C-isotropic shift and shift anisotropy for an A(o) substate model compound: Fe(5,10,15,20-tetraphenylporphyrin)(CO)(N-methylimidazole), as well as the 17O chemical shift, and the 17O nuclear quadrupole coupling constant (NQCC) are virtually the same as those found in the A(o) substate of Physeter catodon CO myoglobin and lead to most probable ligand tilt (τ) and bend (β) angles of 0°and 1°when using a Bayesian probability or Z surface method for structure determination. The infrared V(co) for the model compound of 1969 cm-1 is also that found for A(o) proteins. Results for the A1 substate (including the 57Fe NMR chemical shift and Mossbauer quadrupole splitting) are also consistent with close to linear and untilted Fe-C-O geometries (τ = 4°, β = 7°), with the small changes in ligand spectroscopic parameters being attributed to electrostatic field effects. When taken together, the 13C shift, 13C shift anisotropy, 17O shift, 17O NQCC, 57Fe shift, 57Fe Mossbauer quadrupole splitting, and v(co) all strongly indicate very close to linear and untilted Fe-C-O geometries for all carbonmonoxyheme proteins. These results represent the first detailed quantum chemical analysis of metal-ligand geometries in metalloproteins using up to seven different spectroscopic observables from three types of spectroscopy and suggest a generalized approach to structure determination.

Original languageEnglish (US)
Pages (from-to)4784-4797
Number of pages14
JournalJournal of the American Chemical Society
Issue number19
StatePublished - May 20 1998

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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