TY - JOUR
T1 - 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
T2 - Comparison with hemoproteins, C-O vibrational frequencies, and X-ray structural data
AU - Kalodimos, Charalampos
AU - Gerothanassis, Ioannis P.
AU - Troganis, Anastasios
AU - Loock, Bernard
AU - Momenteau, Michel
PY - 1998/1/1
Y1 - 1998/1/1
N2 - 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.
AB - 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.
KW - C shieldings
KW - Hemoproteins
KW - Isotope effects
KW - Model compounds
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U2 - 10.1023/A:1008253901009
DO - 10.1023/A:1008253901009
M3 - Article
AN - SCOPUS:0002163471
VL - 11
SP - 423
EP - 435
JO - Journal of Biomolecular NMR
JF - Journal of Biomolecular NMR
SN - 0925-2738
IS - 4
ER -