1H, 2H and 19F NMR spectroscopy of a novel iron(II) tetradentate cysteine‐peptide complex having only one δ isomer. Detection of two isomers in coordination of CysXYCys bidentate peptide ligands to iron(II) ion

Abstract

AbstractAn Fe(II) complex with Z‐Ala‐CysProLeuCysGlyNHC6H4mF (Z = benzoxylcarbonyl) having an invariant bidentate peptide ligand, CysKYCys, in rubredoxins was synthesized. The contact‐shifted 1H, 2H and 19F NMR signals of (Et4N)2[Fe(ZAlacysProLeucysGlyNHC6H4mF)2] (1) in acetonitrile‐d3 or acetonitrile at 30°C indicate the existence of two isomers (δ and λ) in the solution of bidentate complex 1 with different orientations of the bidentate peptide ligand to a tetrahedral Fe(II) ion. A novel tetradentate cysteine‐containing peptide ligand, cis‐1,2‐cyclohexylene(COAlaCysProLeuCysGlyNHC6H4mF)2, is designed by energy minimum calculations and synthesized. Its Fe(II) complex, (Et4N)2[Fe(cis‐1,2‐cyclohexylene(CO‐Ala‐cys‐Pro‐Leu‐cys‐Gly‐NH‐C6H4‐m‐F)2)], showed only set of contact‐shifted 1H, 2H and 19F NMR signals of Cys CβH2 due to the preferable formation of the δ isomer.