C-13 shieldings, O-18 isotope effects on C-13 shieldings, and Fe-57-C-13 spin couplings of the Fe-C-O unit in superstructured hemoprotein models: Comparison with hemoproteins, C-O vibrational frequencies, and X-ray structural data
C-13 shieldings, O-18 isotope effects on C-13 shieldings, and Fe-57-C-13 spin couplings of the Fe-C-O unit in superstructured hemoprotein models: Comparison with hemoproteins, C-O vibrational frequencies, and X-ray structural data
(EN)
Kalodimos, C. G.
(EN)
Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
(EL)
Kalodimos, C. G.
(EN)
C-13 NMR spectra of several carbon monoxide (99.7% C-13 and 11.8% O-18 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 Fe-57-C-13(O) coupling constants ((1)J(57Fe-13C)), C-13 shieldings (delta(C-13)), and O-18 isotope effects on C-13 shieldings ((1) Delta(13)C(O-18/O-16)) 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, C-13 shieldings vary widely and an excellent correlation was found between the infrared C-O vibrational frequencies (nu(C-O)) and C-13 shieldings and a reasonable correlation with O-18 isotope effects on C-13 shieldings. This suggests that delta(C-13), nu(C-O) and (1) Delta(13)C(O-18/O-16) are accurate monitors of the multiple mechanisms by which steric and electronic interactions are released in superstructured heme model compounds. 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 pH values are included. The latter were found to obey a similar linear delta(C-13) versus nu(C-O) relationship, which proves that both heme models and heme proteins are homogeneous from the structural and electronic viewpoint. Our results suggest that nu(C-O), delta(C-13) and (1) Delta(13)C(O-18/O-16) measurements reflect similar interaction which is primarily the modulation of pi back-bonding from the Fe d(x) to the CO pi* orbital by the distal pocket polar interactions. The lack of correlation between (1) Delta(13)C(O-18/O-16) and crystallographic CO bond lengths (r(C-O)) reflects significant uncertainties in the X-ray determination of the carbon and oxygen positions.
(EN)
