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

Abstract

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 (\(^{\text{1}} {\text{J}}_{{\text{57}}_{{\text{Fe}}} \_13_{\text{C}} }\)), 13C shieldings (δ(13C)), and 18O isotope effects on13 C shieldings (1Δ13C(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 \(^{\text{1}} {\text{J}}_{{\text{57}}_{{\text{Fe}}} \_13_{\text{C}} }\) couplings. Similarly, the axial hindered base 1,2-dimethylimidazole has a minor effect on the \(^{\text{1}} {\text{J}}_{{\text{57}}_{{\text{Fe}}} \_13_{\text{C}} }\) 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 (ν(C-O)) and13 C shieldings and a reasonable correlation with18 O isotope effects on 13C shieldings. This suggests that δ(13C), ν(C-O) and1 Δ13 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 δ(13 (13C) versus ν(C-O) relationship, which proves that both heme models and heme proteins are homogeneous from the structural and electronic viewpoint. Our results suggest that ν(C-O), δ(13C) and 1Δ13C(18O/16O) measurements reflect similar interaction which is primarily the modulation of π back-bonding from the Fe dπ to the CO π* orbital by the distal pocket polar interactions. The lack of correlation between1 Δ13 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.