13C shieldings, 18O isotope effects on 13C shieldings, and 57Fe-13C spin couplings of the Fe-C-O unit in superstructured hemoprotein models: Comparison with hemoproteins, C-O vibrational frequencies, and X-ray structural data

¹³C NMR spectra of several carbon monoxide (99.7% ¹³C and 11.8% ¹⁸O enriched) hemoprotein models with varying polar and steric effects of the distal organic superstructure and constraints of the proximal side are reported. This enables the ⁵⁷Fe-¹³C(O) coupling constants ( ), ¹³C shieldings ((¹³C)), and ¹⁸O isotope effects on¹³ C shieldings (¹¹³C(¹⁸O/¹⁶O)) to be measured and hence comparisons with hemoproteins, C-O vibrational frequencies and X-ray structural data to be made. Negative polar interactions in the binding pocket and inhibition of Fe//CO back-donation or positive distal polar interactions with amide NH groups appear to have little direct effect on couplings. Similarly, the axial hindered base 1,2-dimethylimidazole has a minor effect on the values despite higher rates of CO desorption being observed for such complexes. On the contrary,¹³ C shieldings vary widely and an excellent correlation was found between the infrared C-O vibrational frequencies ((C-O)) and¹³ C shieldings and a reasonable correlation with¹⁸ O isotope effects on ¹³C shieldings. This suggests that (¹³C), (C-O) and^{1 13} C(¹⁸O/¹⁶O) are accurate monitors of the multiple mechanisms by which steric and electronic interactions are released in superstructured heme model compounds. The ¹³C 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 pH values are included. The latter were found to obey a similar linear (¹³ (¹³C) versus (C-O) relationship, which proves that both heme models and heme proteins are homogeneous from the structural and electronic viewpoint. Our results suggest that (C-O), (¹³C) and ¹¹³C(¹⁸O/¹⁶O) measurements reflect similar interaction which is primarily the modulation of back-bonding from the Fe d to the CO * orbital by the distal pocket polar interactions. The lack of correlation between^{1 13} C(¹⁸O/¹⁶O) and crystallographic CO bond lengths (r(C-O)) reflects significant uncertainties in the X-ray determination of the carbon and oxygen positions.