1 H, 2 H and 19 F 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

An Fe(II) complex with Z‐Ala‐CysProLeuCysGlyNHC 6 H 4 m F (Z = benzoxylcarbonyl) having an invariant bidentate peptide ligand, CysKYCys, in rubredoxins was synthesized. The contact‐shifted 1 H, 2 H and 19 F NMR signals of (Et 4 N) 2 [Fe(ZAlacysProLeucysGlyNHC 6 H 4 m F) 2 ] (1) in acetonitrile‐ d 3 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(COAlaCysProLeuCysGlyNHC 6 H 4 m F) 2 , is designed by energy minimum calculations and synthesized. Its Fe(II) complex, (Et 4 N) 2 [Fe( cis ‐1,2‐cyclohexylene(CO‐Ala‐cys‐Pro‐Leu‐cys‐Gly‐NH‐C 6 H 4 ‐ m ‐F) 2 )], showed only set of contact‐shifted 1 H, 2 H and 19 F NMR signals of Cys C β H 2 due to the preferable formation of the δ isomer.