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

¹³ C NMR spectra of several carbon monoxide (99.7% ¹³ C and 11.8% ¹⁸ O 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 ⁵⁷ Fe- ¹³ C(O) coupling constants ( ¹ J _57Fe-13C ), ¹³ C shieldings (δ( ¹³ C)), and ¹⁸ O isotope effects on ¹³ C shieldings ( ¹ Δ ¹³ C( ¹⁸ C/ ¹⁶ O)) 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 ¹ J _57Fe-13C couplings. Similarly, the axial hindered base 1,2-dimethylimidazole has a minor effect on the ¹ J _57Fe-13C 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 (υ(C-O)) and ¹³ C shieldings and a reasonable correlation with ¹⁸ O isotope effects on ¹³ C shieldings. This suggests that δ( ¹³ C), υ(C-O) and 1Δ ¹³ C( ¹⁸ O/ ¹⁶ O) are accurate monitors of the multiple mechanisms by which steric and electronic interactions are released in superstructured heme model compounds. The ¹³ C 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 δ( ¹³ C) versus υ(C-O) relationship, which proves that both heme models and heme proteins are homogeneous from the structural and electronic viewpoint. Our results suggest that υ(C-O), δ( ¹³ C) and ¹ Δ ¹³ C( ¹⁸ O/ ¹⁶ O) measurements reflect similar interaction which is primarily the modulation of π back-bonding from the Fe d _π to the CO π* orbital by the distal pocket polar interactions. The lack of correlation between 1Δ ¹³ C( ¹⁸ O/ ¹⁶ O) and crystallographic CO bond lengths (r(C-O)) reflects significant uncertainties in the X-ray determination of the carbon and oxygen positions.