Complex Formation and Hydrolytic Processes of Protected Peptides Containing the -SXH- Motif in the Presence of Nickel(II) Ion.

Interactions between metal ions and proteins are considered reversible, such as the coordination of a metal ion to a protein or enzyme, but irreversible processes like the oxidative reactions, aggregation or hydrolytic processes may occur. In the presence of Ni(II)-ions selective hydrolysis of the peptides containing the -SXH- or -TXH- motif was observed. Since the side chain of histidine serves as the metal ion binding site for many native proteins, and very often histidine is present in a -SXH- or -TXH- sequence, to study the complex formation and hydrolytic processes in presence of nickel(II) ion four peptides were synthesised: Ac-SKHM-NH2, A3SSH-NH2, A4SSH-NH2, AAAeKSH-NH2. The Ni(II)-induced hydrolysis of Ac-SKHM-NH2 peptide occurs rapidly in alkaline medium already at room temperature. In two peptides containing -SSH- sequence on the C-termini, the N-terminal part is the major binding site for the nickel(II) ion, but the formation of dinuclear complexes was also observed. In the [Ni2LH-6]2- complex of hexapeptide, the coordination sphere of the metal ions is saturated with deprotonated Ser-O-, which does not result in hydrolysis of the peptide. For A4SSH-NH2, both Ni(II) ions fulfill the conditions for hydrolysis, which was confirmed by HPLC analyses at pH ~ 8.2 and 25 °C.

Interactions of Copper(II) and Zinc(II) Ions with the Peptide Fragments of Proteins Related to Neurodegenerative Disorders: Similarities and Differences.

Metal binding ability and coordination modes of the copper(II) and zinc(II) complexes of various peptide fragments of prion, amyloid-ß, and tau proteins, are summarized in this review. Imidazole-N donors are the primary metal binding sites of all three proteins, but the difference in the location of these residues and the presence or absence of other coordinating side chains result in significant differences in the complex formation processes. The presence of macrochelates and the possibility of forming multicopper complexes are the most important characteristic of prion fragments. Amyloid-ß can form highly stable complexes with both copper(II) and zinc(II) ions, but the preferred binding sites are different for the two metal ions. Similar observations are obtained for the tau fragments, but the metal ion selectivity of the various fragments is even more pronounced. In addition to the complex formation, copper(II) ions can play an important role in the various oxidative reactions of peptides. Results of the metal ion-catalyzed oxidation of peptide fragments of prion, amyloid-ß, and tau proteins are also summarized. Amino acid side chain oxidation (mostly methionine, histidine and aspartic acid) and protein fragmentations are the most common consequences of this process.

Copper(II) Coordination Abilities of the Tau Protein's N-Terminus Peptide Fragments: A Combined Potentiometric, Spectroscopic and Mass Spectrometric Study.

Copper(II) complexes of the N-terminal peptide fragments of tau protein have been studied by potentiometric and various spectroscopic techniques (UV-vis, CD, ESR and ESI-MS). The octapeptide Tau(9-16) (Ac-EVMEDHAG-NH2 ) contains the H14 residue of the native protein, while Tau(26-33) (Ac-QGGYTMHQ-NH2 ) and its mutants Tau(Q26K-Q33K) (Ac-KGGYTMHK-NH2 ) and Tau(Q26K-Y29A-Q33K) (Ac-KGGATMHK-NH2 ) include the H32 residue. To compare the binding ability of H14 and H32 in a single molecule the decapeptide Ac-EDHAGTMHQD-NH2 (Tau(12-16)(30-34)) has also been synthesized and studied. The histidyl residue is the primary metal binding site for metal ions in all the peptide models studied. In the case of Tau(9-16) the side chain carboxylate functions enhance the stability of the M-Nim coordinated complexes compared to Tau(26-33) (logK(Cu-Nim )=5.04 and 3.78, respectively). Deprotonation and metal ion coordination of amide groups occur around the physiological pH range for copper(II). The formation of the imidazole- and amide-coordinated species changes the metal ion preference and the complexes formed with the peptides containing the H32 residue predominate over those of H14 at physiological pH values (90 %-10 %) and in alkaline samples (96 %-4 %).

The influence of penicillamine/cysteine mutation on the metal complexes of peptides.

N-Terminally free but C-terminally amidated peptides Pen-SSACS-NH2 and CSSA-Pen-S-NH2 containing l-penicillamine (Pen) in sequence have been synthesized and their nickel(ii), zinc(ii) and cadmium(ii) complexes were studied by potentiometric and spectroscopic measurements. This study is the first example of the synthesis and metal complexes of peptides containing penicillamine in a peptide sequence constructed from natural amino acids. The data were compared to those of the two cysteine counterparts, CSSACS-NH2. It was found that the replacement of l-cysteine with l-penicillamine has a significant impact on the complex formation processes with nickel(ii) ions. The major differences include the suppression of polynuclear complex formation, the enhanced metal binding affinity of the amino terminus and the increased tendency for the formation of amide bonded species. The tridentate (NH2,S-,S-) coordination was characteristic of the zinc(ii) and cadmium(ii) complexes in the case of all three peptides containing two thiolate functions.

Copper(II) and nickel(II) binding sites of peptide containing adjacent histidyl residues.

Copper(II) and nickel(II) complexes of the terminally protected nonapeptide Ac-SGAEGHHQK-NH2 modeling the metal binding sites of the (8-16) domain of amyloid-ß have been studied by potentiometric, UV-vis, CD and ESR spectroscopic methods. The studies on the mutants containing only one of the histidyl residues (Ac-SGAEGAHQK-NH2, Ac-SGAEGHAQK-NH2) have also been performed. The formation of imidazole and amide coordinated mononuclear complexes is characteristic of all systems with a preference of nickel(II) binding to the His14 site, while the involvement of both histidines in metal binding is suggested in the corresponding copper(II) complexes. The formation of bis(ligand) and dinuclear complexes has also been observed in the copper(II)-Ac-SGAEGHHQK-NH2 system. The results provide further support for the copper(II) binding ability of the (8-16) domain of amyloid-ß and support the previous assumptions that via the bis(ligand) complex formation copper(II) ions may promote the formation of the oligomers of amyloid-ß.

Copper(II), nickel(II) and zinc(II) complexes of the N-terminal nonapeptide fragment of amyloid-ß and its derivatives.

Copper(II), nickel(II) and zinc(II) complexes of the nonapeptide fragment of amyloid-ß Aß(1-9) (NH2-DAEFRHDSG-NH2) and its two derivatives: NH2-DAAAAHAAA-NH2 and NH2-DAAAAAHAA-NH2 have been studied by potentiometric, UV-visible and CD spectroscopic methods. The results reveal the primary role of the amino terminus of peptides in copper(II) and nickel(II) binding. The formation of dinuclear complexes was also possible in the copper(II) containing systems but only the first six amino acids from the amino terminus were involved in metal binding in the physiologically relevant pH range. The coordination chemistry of the two alanine mutated peptides is almost the same as that of the native nonapeptide, but the thermodynamic stability of the copper(II) complexes of the mutants is significantly reduced. This difference probably comes from the secondary interactions of the polar side chains of Asp, Glu, Ser and Arg residues present in the native peptide. Moreover, this difference reveals that the amino acid sequence of the N-terminal domains of amyloid peptides is especially well suited for the complexation with copper(II) ions.

Binary and ternary mixed metal complexes of terminally free peptides containing two different histidyl binding sites.

Copper(II), nickel(II) and zinc(II) complexes of the terminally free peptides AHAAAHG and AAHAAAHG have been studied by combined applications of potentiometric and various spectroscopic techniques, including UV-visible, CD and EPR for copper(II) and UV-visible, CD and NMR for nickel(II). It was found that the octapeptide AAHAAAHG can easily bind two equivalents of copper(II) or nickel(II) ions and the amino terminus was identified as the primary ligating site of the molecule. On the other hand, this peptide has a relatively low zinc(II) binding affinity. Mono- and di-nuclear copper(II) and nickel(II) complexes were also formed with the heptapeptide AHAAAHG but this peptide can effectively bind one equivalent of zinc(II) ions, too, with the involvement of the deprotonated amide nitrogen in zinc(II) binding. The enhanced stability of the [MH-1L] species of AHAAAHG was explained by the tridentate (NH2,N(-),Nim) coordination of the amino terminus supported by the macrochelation of the internal histidyl residue. Mixed metal copper(II)-nickel(II) complexes were also formed with both peptides and copper(II) ions were coordinated to the amino terminal, while nickel(II) ions to the internal histidyl sites.