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 and 57Fe Mossbauer and infrared spectroscopies

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), Fe-57 Mossbauer, and infrared spectroscopic techniques, combined with density functional theoretical calculations to analyze the spectroscopic results. The results of C-13 NMR isotropic chemical shift, C-13 NMR chemical shift anisotropy, O-17 NMR isotropic chemical shift, O-17 nuclear quadrupole coupling constant, Fe-57 MMR isotropic chemical shift, Fe-57 Mossbauer quadrupolar splitting, and infrared measurements indicate that CO binds to Fe in a close to linear fashion in all conformational substates. The C-13-isotropic shift and shift anisotropy for an A(0) substate model compound: Fe(5,10,15,20-tetraphenylporphyrin)(CO)(N-methylimidazole), as well as the O-17 chemical shift, and the O-17 nuclear quadrupole coupling constant (NQCC) are virtually the same as those found in the A(0) substate of Physeter catodon CO myoglobin and lead to most probable ligand tilt (tau) and bend (beta) angles of 0 degrees and 1 degrees when using a Bayesian probability or Z surface method fur structure determination. The infrared vco for the model compound of 1969 cm(-1) is also that found for A(0) proteins. Results for the A(1) substate (including the Fe-57 NMR chemical shift and Mossbauer quadrupole splitting) are also consistent with close to linear and untilted Fe-C-O geometries (tau = 4 degrees, P = 7 degrees), with the small changes in ligand spectroscopic parameters being attributed to electrostatic field effects. When taken together, the C-13 shift, C-13 shift anisotropy, O-17 shift, O-17 NQCC, Fe-57 shift, Fe-57 Mossbauer quadrupole splitting, and nu(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.