THE INFLUENCE OF ASPARTIC OR GLUTAMIC-ACID RESIDUES IN TETRAPEPTIDES ON THE FORMATION OF COMPLEXES WITH NICKEL(II) AND ZINC(II)

The formation of the complexes formed by Ni-II and Zn-II with Asp-Asp- Asp and a series of tetrapeptides containing one or two Asp residues o r one Glu residue are reported. Stability constants were measured pH-m etrically. The particular species and the metal ion binding sites were determined using H-1 NMR, UV-vis and CD spectroscopy. The beta-carbox ylate group of the Asp residue stabilizes the complexes significantly, particularly when present as the N-terminal residue. As a result the tendency for Ni-II to deprotonate and bind to amide-nitrogen atoms, fo rming planar diamagnetic complexes, is reduced and their formation del ayed to a significantly higher pH when compared to other peptides. The side chain of the Glu residue has a much smaller effect. As anticipat ed, Zn-II was unable to deprotonate and bind to peptide nitrogens.