Circular Dichroism of Palladium(II) Complexes of Amino Acids and Peptides

Palladium(II) induces the ionization of peptide hydrogens near pH 3.5 to yield tetragonal complexes with amide nitrogen donor atoms similar to copper(II) and nickel(II) peptide complexes. Promotion of peptide hydrogen ionization increases in the series Co(II) < Ni(II) < Cu(II) < Pd(II). As previously demonstrated for the analogous copper(II) and nickel(II) complexes, the circular dichroism results of sign identity and magnitude additivity in a series of tripeptide complexes of palladium(II) rule out the applicability of any octant or quadrant rules as descriptions of optical activity in ligand field bands of these tetragonal complexes. Several lines of evidence suggest that a hexadecant (Dm) rule best describes the optical activity in the case of small side chains. A homologous series of potentially tridentate 1 alpha-am-diaminocarboxylates are only bidentate with palladium(II). 2,3-Diaminopropionate and 2,4-diaminobutyrate chelate primarily through two nitrogen donor atoms and lysine chelates through the five-membered chelate ring a-amino and carboxylate groups, while ornithine chelates by a mixture of both binding modes despite the seven-membered chelate ring required for binding through the two nitrogen donor atoms. Vicinal effects of substituents rather than chelate ring puckering appear to be the primary source of optical activity in these complexes.