AN ELECTRONIC INFLUENCE OF A DISTANT AROMATIC RING IN REDUCED RUBREDOXIN MODELS - IRON(II) COMPLEXES WITH Z-CYS-PRO-LEU-CYS-GLY-X (X = NHCH2C6H4-P-F, NHCH2CH2C6H4-P-F, AND PHE-OME)

Cysteine-containing oligopeptides with a distal aromatic ring, Z-Cys-Pro-Leu-Cys-Gly-X (X = NHCH2C6H4-p-F, NHCH2CH2C6H4-p-F and Phe-OMe), and their Fe(II) complexes were synthesized. The absorption, CD and H-1-NMR spectral data indicate that mononuclear Fe(II) complexes were formed through the chelate coordination of the four cysteine thiolate residues. In particular, the complex with a phenylalanine residue, [Fe(Z-cys-Pro-Leu-cys-Gly-Phe-OMe)2]2-, exhibits absorption maxima at 312 (8240) and 332 nm (6750) and CD transitions at 316 (-27.0) and 334 nm (16.1) in acetonitrile solution which are very similar to those of reduced rubredoxin in aqueous solution. The observation of isotropically shifted F-19-NMR signals in both [Fe(Z-cys-Pro-Leu-cys-Gly-NHCH2C6H4-p-F)2]2- and [Fe(Z-cys-Pro-Leu-cys-Gly-NHCH2CH2C6H4-p-F)2]2- indicates that there are some pi-pi interactions between the aromatic ring and the sulfur atom of the cysteine residue. Such interactions have already been presumed in native rubredoxin.