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

Charalampos Kalodimos, Ioannis P. Gerothanassis, Anastasios Troganis, Bernard Loock, Michel Momenteau

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4 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)423-435
Number of pages13
JournalJournal of Biomolecular NMR
Volume11
Issue number4
DOIs
StatePublished - Jan 1 1998

Keywords

  • C shieldings
  • Hemoproteins
  • Isotope effects
  • Model compounds

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