Hydrogen and deuterium NMR studies of carboxylate coordination to iron(III) Complexes: Diverse chemical shift values for coordinated carboxyl residues

Isam M. Arafa, Harold M. Goff, Sheila S. David, Bruce P. Murch, Lawrence Que

Research output: Contribution to journalArticle

32 Citations (Scopus)

Abstract

Proton and deuterium NMR signals for coordinated carboxyl residues of high-spin iron(III) and manganese(III) porphyrin complexes have been unambiguously assigned through examination of the deuteriated carboxylate derivatives. Signals are shifted downfield in the region between 15 and 32 ppm for these paramagnetic compounds contained in chlorinated solvents. However, this chemical shift value is attenuated with respect to that observed at 140 ppm for the acetate proton signal of the (N,N′-ethylenebis(salicylideneaminato))(acetato)iron(III) Schiff base complex. Other non-heme iron(III) carboxylate complexes also give coordinated acetate proton signals in the 140 ppm region. The metalloporphyrin acetate signal is highly solvent dependent, and the signal does not follow Curie law behavior in variable-temperature experiments. Specific solvation effects for the carbonyl residue are suggested. These effects and the differing magnitudes for carboxylate chemical shifts in iron(III) porphyrin complexes and non-heme iron(III) complexes suggest monodentate carboxylate coordination to the iron(III) porphyrin and bidentate coordination in the other iron(III) complexes investigated. Given the wide range of carboxyl chemical shift values, identification of amino acid carboxylate binding in metalloproteins by NMR chemical shift values alone is thus highly problematic.

Original languageEnglish (US)
Pages (from-to)2779-2784
Number of pages6
JournalInorganic Chemistry
Volume26
Issue number17
StatePublished - 1987
Externally publishedYes

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Deuterium
Chemical shift
carboxylates
chemical equilibrium
deuterium
Hydrogen
Iron
Nuclear magnetic resonance
iron
nuclear magnetic resonance
hydrogen
porphyrins
Protons
acetates
Acetates
Porphyrins
protons
Metalloporphyrins
Metalloproteins
Schiff Bases

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

Hydrogen and deuterium NMR studies of carboxylate coordination to iron(III) Complexes : Diverse chemical shift values for coordinated carboxyl residues. / Arafa, Isam M.; Goff, Harold M.; David, Sheila S.; Murch, Bruce P.; Que, Lawrence.

In: Inorganic Chemistry, Vol. 26, No. 17, 1987, p. 2779-2784.

Research output: Contribution to journalArticle

Arafa, Isam M. ; Goff, Harold M. ; David, Sheila S. ; Murch, Bruce P. ; Que, Lawrence. / Hydrogen and deuterium NMR studies of carboxylate coordination to iron(III) Complexes : Diverse chemical shift values for coordinated carboxyl residues. In: Inorganic Chemistry. 1987 ; Vol. 26, No. 17. pp. 2779-2784.
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AB - Proton and deuterium NMR signals for coordinated carboxyl residues of high-spin iron(III) and manganese(III) porphyrin complexes have been unambiguously assigned through examination of the deuteriated carboxylate derivatives. Signals are shifted downfield in the region between 15 and 32 ppm for these paramagnetic compounds contained in chlorinated solvents. However, this chemical shift value is attenuated with respect to that observed at 140 ppm for the acetate proton signal of the (N,N′-ethylenebis(salicylideneaminato))(acetato)iron(III) Schiff base complex. Other non-heme iron(III) carboxylate complexes also give coordinated acetate proton signals in the 140 ppm region. The metalloporphyrin acetate signal is highly solvent dependent, and the signal does not follow Curie law behavior in variable-temperature experiments. Specific solvation effects for the carbonyl residue are suggested. These effects and the differing magnitudes for carboxylate chemical shifts in iron(III) porphyrin complexes and non-heme iron(III) complexes suggest monodentate carboxylate coordination to the iron(III) porphyrin and bidentate coordination in the other iron(III) complexes investigated. Given the wide range of carboxyl chemical shift values, identification of amino acid carboxylate binding in metalloproteins by NMR chemical shift values alone is thus highly problematic.

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