Peptide ligands chelating effects

III. Chelating Effects of Peptide Ligands A. Rubredoxin Peptide Model Complexes... 

The chelation effect of Cys-X-Y-Cys was also found for the MCD spectra of Fe(III) complexes of Z-Cys-Pro-Leu-Cys-OMe or Z-Cys-Ala-Ala-Cys-OMe, which exhibited a characteristic ligand-metal charge transfer (LMCT) at 350 nm, but not for those of the Fe(III) complex of Z-Ala-Cys-OMe, Z-Cys-Ala-Cys-OMe, or S2-o-xyl (28). The MCD spectral differences in the region of 300-400 nm are related to the difference in the electronic states of the Fe(III) singly occupied metal t2 orbitals affected by the lone pair on the sulfur atom. The possibility of a spectroscopic splitting by a specific Cys-thiolate orientation relative to other Fe-S bondings has been predicted theoretically by Bair and Goddard (29). The orientation of n orbitals of the sulfur lone pair is now found to be determined by the peptide conformation which dictates the steric disposition of the S-C bond. 

The selectivity of peptide motifs for certain metals comes from the coordinating contribution from amino acid side chains, the common coordination number of the metal, hardness/softness of the metal ion, ligand field stabilisation effects and the hardness/softness of any coordinating side chains of the amino acid sequence. An example of the influence of side chains and the importance of the position of the side chain comes from the tripeptides Gly-Gly-His, also known as copper binding peptide. The side chain imidazole ring of the His residue has a very efficient nitrogen donor (the imidazole N), which can form a tetradentate chelate ring for coordination as in Scheme 10.3. 

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