A synthetic analogue for the active site of plant‐type ferredoxin: Two different coordination isomers by a four‐Cys‐containing [20]‐peptide

The (Fe 2 S 2 ) 2+ complex of an artificial 20‐peptide ligand, Ac‐Pro‐Tyr‐Ser‐ Cys ‐Arg‐Ala‐Gly‐Ala‐ Cys ‐Ser‐Thr‐ Cys ‐Ala‐Gly‐Pro‐Leu‐Leu‐Thr‐Cys‐Val‐NH 2 , containing an invariant Cys‐A‐B‐C‐D‐Cys‐X‐Y‐Cys (A, B, C, D, X, Y = amino acid residues) fragment of plant‐type ferredoxins was synthesized by a ligand exchange method with [Fe 2 S 2 (S‐ t ‐Bu) 4 ] 2− . 1 H‐nmr spectroscopic and electrochemical data of the complex indicate the presence of two coordination isomers. One of them having a Cys‐X‐Y‐Cys bridging coordination to the two Fe(III) ions, has the (Fe 2 S 2 ) 2+ core environment similar to those of the denatured plant‐type ferredoxins and exhibits a positive shifted redox potential at −0.64 V vs saturated colonel electrode (SCE) in N,N ‐dimethylformamide (DMF). Another isomer with the Cys‐A‐B‐C‐D‐Cys bridging coordination shows a negative redox potential at −0.96 V vs SCE in DMF. © 1992 John Wiley & Sons, Inc.