Chemical and functional characterization of metal-binding pseudotripeptides with different functionalized N-alkyl residues

Abstract

SummaryPseudotripeptide ligands with 4 different N-functionalized glycine residues were qualitatively, semiquantitatively and quantitatively tested for their complexation of the bivalent transition metal ions Zn 2+, Cu 2+, Co 2+, Ni 2+ and Mn 2+. The functional side chains have different length and different groups available for complexation. MALDI-MS and ESI-MS were used for more qualitative or semiquantitative estimation of the complex formation tendencies. The found ranking differs by these two methods only for Zn 2+ and Ni 2+. For one of the pseudotripeptide ligands, the ligand L1, complex formation with certain transition metal was estimated quantitatively by potentiometric titration. The Zn-complex of that ligand polarizes bound water strongly, resulting in a low pKa-value. Complexes of pseudotripeptide ligand L1 with certain metal ions were tested for their hydrolytic activity. The pseudo first order rate constants of the hydrolysis of the substrates 4-nitrophenyl acetate and bis(4-nitrophenyl)phosphate were compared to complexes with the same metal ions formed with a very well studied ligand from the literature, the 1,4,7,10-tetraaza cyclododecane (cyclen). The hydrolysis of the phosphate ester occurs very slowly compared to the acetate ester. No correlation exists between the estimated pKa values of complexes formed from ligand L1 with different metal ions and the phosphate ester hydrolysis. The Ni ions give totally different hydrolytic activities for pseudotripeptide ligand L1 and cyclen. With one exception, the Ni-cyclen complex, all other complexes have only a low or moderate catalytic activity.

Transition metal complexes of a cyclic pseudo hexapeptide: synthesis, complex formation and catalytic activities

“…To confirm the independence of the MALDI results from the ionization method, peptides were additionally analysed by ESI-MS as described in previous publications [22,23]. Sample preparation was the same as above, 15 µl of each sample were diluted with 1 ml of methanol and injected into a Finnigan MAT 95 XL mass spectrometer using nitrogen as sheath gas.…”