H-1, H-2 AND F-19 NMR-SPECTROSCOPY OF A NOVEL IRON(II) TETRADENTATE CYSTEINE-PEPTIDE COMPLEX HAVING ONLY ONE DELTA-ISOMER - DETECTION OF 2 ISOMERS IN COORDINATION OF CYS-X-Y-CYS BIDENTATE PEPTIDE LIGANDS TO IRON(II) ION

An Fe(II) complex with Z-Ala-Cys-Pro-Leu-Cys-Gly-NH-C6H4-m-F (Z = benz oxylcarbonyl) having an invariant bidentate peptide ligand, Cys-X-Y-Cy s, in rubredoxins was synthesized. The contact-shifted H-1, H-2 and F- 19 NMR signals of )(2)[Fe(Z-Ala-cys-Pro-Leu-cys-Gly-NH-C6H4-m-F)(2)] ( 1) in acetonitrile-d(3) or acetonitrile at 30 degrees C indicate the e xistence of two isomers (delta and lambda) in the solution of bidentat e complex 1 with different orientations of the bidentate peptide ligan d to a tetrahedral Fe(II) ion. A novel tetradentate cysteine-containin g peptide ligand, xylene(CO-Ala-Cys-Pro-Leu-Cys-Gly-NH-C6H4-m-F)(2), i s designed by energy minimum calculations and synthesized. Its Fe(II) complex, (Et(4)N)(2)[Fe(cis-1,2- lene(CO-Ala-cys-Pro-Leu-cys-Gly-NH-C6 H4-m-F)(2))], showed only set of contact-shifted H-1, H-2 and F-19 NMR signals of Cys CbetaH2 due to the preferable formation of the delta i somer.