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, ANDX-RAY STRUCTURAL DATA

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 sid e 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 henc e comparisons with hemoproteins, C-O vibrational frequencies and X-ray structural data to be made. Negative polar interactions in the bindin g pocket and inhibition of Fe-->CO back-donation or positive distal po lar interactions with amide NH groups appear to have little direct eff ect 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 de spite higher rates of CO desorption being observed for such complexes. On the contrary, C-13 shieldings vary widely and an excellent correla tion 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) a nd (1) Delta(13)C(O-18/O-16) are accurate monitors of the multiple mec hanisms by which steric and electronic interactions are released in su perstructured 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 tha t nu(C-O), delta(C-13) and (1) Delta(13)C(O-18/O-16) measurements refl ect similar interaction which is primarily the modulation of pi back-b onding from the Fe d(x) to the CO pi orbital by the distal pocket pol ar interactions. The lack of correlation between (1) Delta(13)C(O-18/O -16) and crystallographic CO bond lengths (r(C-O)) reflects significan t uncertainties in the X-ray determination of the carbon and oxygen po sitions.