AN EXPERIMENTAL AND QUANTUM-CHEMICAL INVESTIGATION OF CO BINDING TO HEME-PROTEINS AND MODEL SYSTEMS - A UNIFIED MODEL-BASED ON C-13, O-17, AND FE-57 NUCLEAR-MAGNETIC-RESONANCE AND FE-57 MOSSBAUER AND INFRARED SPECTROSCOPIES

We have investigated the question of how CO ligands bind to iron in me talloporphyrins 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 isotropi c chemical shift, C-13 NMR chemical shift anisotropy, O-17 NMR isotrop ic chemical shift, O-17 nuclear quadrupole coupling constant, Fe-57 MM R isotropic chemical shift, Fe-57 Mossbauer quadrupolar splitting, and infrared measurements indicate that CO binds to Fe in a close to line ar fashion in all conformational substates. The C-13-isotropic shift a nd shift anisotropy for an A(0) substate model compound: 5,20-tetraphe nylporphyrin)(CO)(N-methylimidazole), as well as the O-17 chemical shi ft, and the O-17 nuclear quadrupole coupling constant (NQCC) are virtu ally 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 (bet a) 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) protein s. Results for the A(1) substate (including the Fe-57 NMR chemical shi ft and Mossbauer quadrupole splitting) are also consistent with close to linear and untilted Fe-C-O geometries (tau = 4 degrees, P = 7 degre es), with the small changes in ligand spectroscopic parameters being a ttributed to electrostatic field effects. When taken together, the C-1 3 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 v ery close to linear and untilted Fe-C-O geometries for all carbonmonox yheme proteins. These results represent the first detailed quantum che mical analysis of metal-ligand geometries in metalloproteins using up to seven different spectroscopic observables from three types of spect roscopy and suggest a generalized approach to structure determination.