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C-13, O-17 and Fe-57 NMR spectra of several carbonmonoxy hemoprotein models with varying polar and steric effects of the distal organic superstructure, constraints of the proximal side, and porphyrin ruffling are reported. Both heme models and heme proteins obey a similar excellent linear delta(C-13) versus nu(C-O) relationship which is primarily due to modulation of pi-back-bonding from the Fe d(pi) to CO pi* orbital by the distal pocket polar interactions. The lack of correlation between delta(C-13) and delta(O-17) suggests that the two probes do not reflect a similar type of electronic and structural perturbation. delta(O-17) is not primarily influenced by the local distal field interactions and does not correlate with any single structural property of the Fe-C-O unit; however, atropisomerism and deformation of the porphyrin geometry appear to play a significant role. Fe-57 shieldings Vary by nearly 900 ppm among various hemes and an excellent correlation was found between delta(Fe-57) and the absolute crystallographic average displacement of the meso carbon atoms, \C-m\ relative to the porphyrin core mean plane. The excellent correlation between iron-57 shieldings and the average shieldings of the meso carbons of the porphyrin skeleton of TPP derivatives suggests that the two probes reflect a similar type of electronic and structural perturbation which is primarily porphyrin ruffling. (C) 2000 Elsevier Science Inc. All rights reserved. J Inorg Biochem

C-13 shieldings, delta(C-13) O-17 shieldings, delta(O-17), and O-18 isotope effects on C-13 shieldings, (1)Delta(13)C(O-18/O-16), Of carbon monoxide (99.7% C-13, 0.9% O-17, and 11.8% O-18 enriched) in a variety of solvents and of the Fe-C-O unit of several carbonmonoxy hemoprotein models with varying polar and steric effects of the distal organic superstructure and constraints of the proximal side are reported. This enables, first, comparisons with hemoproteins, C-O vibrational frequencies, nu(C-O), and X-ray structural data to be made; second, to investigate whether polarizable CO is an adequate model for distal ligand effects in carbonmonoxy heme proteins and synthetic model compounds; third, to investigate the effect of electronic perturbation within the heme pocket and pocket deformation on delta(C-13), delta(O-17), (1)Delta(13)C(O-18/O-16), and nu(C-O). A variety of solvents with varying dielectric constants and solvation abilities appears to have negligible effect on delta(O-17), delta(C-13), and (1)Delta(13)C(O-18/O-16) and little direct effect on nu(CO) of dissolved carbon monoxide. On the contrary, C-13 and O-17 shieldings of several carbonmonoxy hemoprotein models vary widely and an excellent correlation was found between the infrared C-O vibrational frequencies and C-13 shieldings and a reasonable correlation with O-18 isotope effects on C-13 shieldings. The C-13 shieldings of heme models cover a 4.0 ppm range which is extended to 7.0 ppm when several HbCO and MbCO species at different pHs are included. The latter were found to obey a similar linear delta(C-13) vs nu(C-O) relationship. nu(C-O), delta(C-13), and (1)Delta(13)C(O-18/O-16) parameters of heme,model compounds reflect similar interaction which is primarily the modulation of rr back-bonding from Fe d(pi) to CO pi* orbital by the distal pocket polar interactions. Our results suggest that, contrary to earlier claims, polarizable carbon monoxide is not an adequate model for distal ligand effects in carbonmonoxy hemoproteins and synthetic model compounds. Very probably this is caused by the large effect of the electric field on the back-bonding and the large polarizability of the Jr: subsystem of the Fe-C-O unit. The O-17 shieldings of heme models cover a range of 17 ppm which is extended to 24 ppm when selected heme proteins are included. The lack of correlation between delta(C-13) and delta(O-17) suggests that the two probes do not reflect a similar type of electronic and structural perturbation. delta(O-17) is not primarily influenced by the local distal field interactions and does not correlate with any single structural property of the Fe-C-O unit; however, atropisomerism and deformation of the porphyrin geometry appear to play a significant role. Inorg Chem

Fe-57 NMR spectra of several carbonmonoxy hemoprotein models with varying polar and steric effects of the distal organic superstructure, constraints of the proximal side, and porphyrin ruffling are reported. Fe-57 shieldings, delta(Fe-57), vary by nearly 900 ppm among various hemes, and an excellent correlation was found between delta(Fe-57) and the absolute crystallographic average displacement of the meso carbon atoms, \C-m\, relative to the porphyrin core mean plane. The great variation of delta(Fe-57) as a function of \C-m\ (similar to 140 ppm/0.1 Angstrom) demonstrates that iron-57 shieldings can be used in structure refinement protocols for the extraction of more accurate structures for heme rings in heme model compounds. The excellent correlation between iron-57 shieldings and the average shieldings of the meso carbons of the porphyrin skeleton of TPP derivatives suggests that the two probes reflect similar types of electronic and structural perturbations, which are primarily due to porphyrin ruffling. The present findings also emphasize the value in predicting 57Fe shieldings in superstructured metalloporphyrins from C-13 shieldings of the meso carbons J Am Chem Soc