Copper(II), Nickel(II) and Zinc(II) Complexes of Peptide Fragments of Tau Protein.

Copper(II), nickel(II) and zinc(II) complexes of various peptide fragments of tau protein were studied by potentiometric and spectroscopic techniques. All peptides contained one histidyl residue and represented the sequences of tau(91-97) (Ac-AQPHTEI-NH2), tau(385-390) (Ac-KTDHGA-NH2) and tau(404-409) (Ac-SPRHLS-NH2). Imidazole-N donors of histidine were the primary metal binding sites for all peptides and all metal ions, but in the case of copper(II) and nickel(II), the deprotonated amide groups were also involved in metal binding by increasing pH. The most stable complexes were formed with copper(II) ions, but the presence of prolyl residues resulted in significant changes in the thermodynamic stability and speciation of the systems. It was also demonstrated that nickel(II) and especially zinc(II) complexes have relatively low thermodynamic stability with these peptides. The copper(II)-catalyzed oxidation of the peptides was also studied. In the presence of H2O2, the fragmentation of peptides was detected in all cases. In the simultaneous presence of H2O2 and ascorbic acid, the fragmentation of the peptide is less preferred, and the formation of 2-oxo-histidine also occurs.

Fluorine-based Zn salan complexes.

We synthesised and characterised the racemic and chiral versions of two Zn salan fluorine-based complexes from commercially available materials. The complexes are susceptible to absorbing H2O from the atmosphere. In solution (DMSO-H2O) and at the millimolar level, experimental and theoretical studies identify that these complexes exist in a dimeric-monomeric equilibrium. We also investigated their ability to sense amines via19F NMR. In CDCl3 or d6-DMSO, strongly coordinating molecules (H2O or DMSO) are the limiting factor in using these easy-to-make complexes as chemosensory platforms since their exchange with analytes requires an extreme excess of the latter.

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.

Full Equilibrium Picture in Aqueous Binary and Ternary Systems Involving Copper(II), 1-Methylimidazole-Containing Hydrazonic Ligands, and the 103-112 Human Prion Protein Fragment.

In this work, we describe two novel 1-methylimidazole N-acylhydyrazonic ligands and their interaction with copper(II) in solution. Binary systems constituted by each of these hydrazones and the metal ion were studied by potentiometric titrations. The magnitude of their affinities for zinc(II) was also determined for the sake of comparison. Additionally, a full evaluation of the copper(II) chelation profile of the new ligands in ternary systems containing a human prion protein fragment was performed. Mixed ligand complexes comprising the HuPrP103-112 fragment, copper(II) ions, and an N-acylhydrazone were characterized by potentiometry, ultraviolet-visible spectroscopy, and circular dichroism. Some of these species were also identified by electrospray ionization mass spectrometry and unequivocally assigned through their isotopic distribution pattern. To the best of our knowledge, this is the first report concerning the stability of ternary complexes involving a hydrazonic metal-protein interaction modulator, copper, and a peptide. The ability of N-acylhydrazones to prevent peptide oxidation was also examined. Both ligands can partially prevent the formation of the doubly oxidized product, a process mediated by copper(II) ions. Oxidative stress is considered an important hallmark of neurodegenerative diseases such as prion-related spongiform encephalopathies. In this context, active intervention with respect to the deleterious copper-catalyzed methionine oxidation could represent an interesting therapeutic approach.

Salpyran: A Cu(II) Selective Chelator with Therapeutic Potential.

We report the rational design of a tunable Cu(II) chelating scaffold, 2-(((2-((pyridin-2-ylmethyl)amino)ethyl)amino)methyl)phenol, Salpyran (HL). This tetradentate ligand is predicated to have suitable permeation, has an extremely high affinity for Cu compared to clioquinol (pCu7.4 = 10.65 vs 5.91), and exhibits excellent selectivity for Cu(II) over Zn(II) in aqueous media. Solid and solution studies corroborate the formation of a stable [Cu(II)L]+ monocationic species at physiological pH values (7.4). Its action as an antioxidant was tested in ascorbate, tau, and human prion protein assays, which reveal that Salpyran prevents the formation of reactive oxygen species from the binary Cu(II)/H2O2 system, demonstrating its potential use as a therapeutic small molecule metal chelator.

Ambivalent role of ascorbic acid in the metal-catalyzed oxidation of oligopeptides.

The effect of ascorbic acid on the metal-catalyzed oxidation of a human prion protein model peptide has been studied. The complex formation of the peptide was clarified first. The studied model peptide contains a methionine and a histidine amino acids which are important both as binding sites for metal ions and sensitive parts of the protein for oxidation. pH-potentiometric, UV-Vis and circular dichroism spectroscopic techniques were applied to study the stoichiometry, stability and structure of the copper(II) complexes, while HPLC-MS and MS/MS were used for identifying the products of metal-catalyzed oxidation. 3N and 4N complexes with (Nim,N-,N-,S) and (Nim,N-,N-,N-) coordination modes are formed at pH 7.4, where the oxidation was studied. Singly, doubly and triply oxidized products are formed in which the methionine and/or the histidine side chain is oxidized. The oxidation was carried out with hydrogen peroxide solution by the addition of metal ions, namely copper(II) and iron(III) and/or ascorbic acid.

Complex formation processes and metal ion catalyzed oxidation of model peptides related to the metal binding site of the human prion protein.

Interaction of copper(II) and nickel(II) ions with the Ac-PHAAAGTHSMKHM-NH2 tridecapeptide containing the His85, His96 and His111 binding sites of human prion protein has been studied by various techniques. pH-potentiometry, UV-Vis and circular dichroism spectroscopy were applied to study the stoichiometry, stability and structure of the copper(II) and nickel(II) complexes, while HPLC-MS and MS/MS were used for identifying the products of copper(II) catalyzed oxidation. The copper binding ability of shorter fragments, namely the nonapeptide Ac-PHAAAGTHS-NH2 and pentapeptide Ac-PHAAA-NH2 have also been studied. The tridecapeptide is able to bind three equivalent of copper(II) ion, since the histidine residues behave as independent metal binding sites. Nevertheless, the metal binding ability of histidine residue mimicking the octarepeat domain (His85) is decreased, while the other parts of the peptide mimicking the histidines outside the octarepeat domain bind the copper ions in comparable concentration. On the other hand, this peptide is able to coordinate only two equivalents of nickel ion on the domains outside the octarepeat region. Furthermore the His96 binding site is more effective for the nickel ions. Both histidine and methionine residues are sensitive for oxidation, the oxidation of these residues are proved, and in the case of the histidine residues follows the order His96 > His85 ≫ His111.

Impact of pyridine-2-carboxaldehyde-derived aroylhydrazones on the copper-catalyzed oxidation of the M112A PrP103-112 mutant fragment.

Misfolded prion protein (PrPSc) is known for its role in fatal neurodegenerative conditions, such as Creutzfeldt-Jakob disease. PrP fragments and their mutants represent important tools in the investigation of the neurotoxic mechanisms and in the evaluation of new compounds that can interfere with the processes involved in neuronal death. Metal-catalyzed oxidation of PrP has been implicated as a trigger for the conformational changes in protein structure, which, in turn, lead to misfolding. Targeting redox-active biometals copper and iron is relevant in the context of protection against the oxidation of biomolecules and the generation of oxidative stress, observed in several conditions and considered an event that might promote sporadic prion diseases as well as other neurodegenerative disorders. In this context, ortho-pyridine aroylhydrazones are of interest, as they can act as moderate tridentate ligands towards divalent metal ions such as copper(II). In the present work, we explore the potentiality of this chemical class as peptide protecting agents against the deleterious metal-catalyzed oxidation in the M112A mutant fragment of human PrP, which mimics relevant structural features that may play an important role in the neurotoxicity observed in prion pathologies. The compounds inhere studied, especially HPCFur, showed an improved stability in aqueous solution compared to our patented lead hydrazone INHHQ, displaying a very interesting protective effect toward the oxidation of methionine and histidine, processes that are related to both physiological and pathological aging.

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 %).

Copper(ii)-benzotriazole coordination compounds in click chemistry: a diagnostic reactivity study.

This diagnostic study aims to shed light on the catalytic activity of a library of Cu(ii) based coordination compounds with benzotriazole-based ligands. We report herein the synthesis and characterization of five new coordination compounds formulated as [CuII(L4)(MeCN)2(CF3SO3)2] (1), [CuII(L5)2(CF3SO3)2] (2), [CuII(L6)2(MeCN)(CF3SO3)]·(CF3SO3) (3), [CuII(L6)2(H2O)(CF3SO3)]·(CF3SO3)·2(Me2CO) (4), and [Cu(L1)2(L1')2(CF3SO3)2]2·4(CF3SO3)·8(Me2CO) (5), derived from similar nitrogen-based ligands. The homogeneous catalytic activity of these compounds along with our previously reported coordination compounds (6-13), derived from similar ligands, is tested against the well-known Cu(i)-catalysed azide-alkyne cycloaddition reaction. The optimal catalyst [CuII(L1)2(CF3SO3)2] (10) activates the reaction to afford 1,4-disubstituted 1,2,3-triazoles with yields up to 98% and without requiring a reducing agent. Various control experiments are performed to optimize the method and examine parameters such as ligand variation, metal coordination geometry and environment, in order to elucidate the behaviour of the catalytic system.

Copper(II) interaction with the Human Prion 103-112 fragment - Coordination and oxidation.

The prion protein (PrP) is a membrane-anchored cell surface glycoprotein containing 231 amino acids. It has been associated with a group of neurodegenerative disorders. Copper(II) interaction with the Human Prion 103-112 fragment and its mutants has been studied with various techniques. The studied human prion fragment contains both histidine and methionine residues, while methionine residues are systematically replaced or displaced in the studied mutants. pH-potentiometric, UV-Vis and circular dicroism spectroscopic techniques were applied to study the stoichiometry, stability and structure of the copper(II) complexes, while HPLC-MS and MS/MS were used for identifying the products of copper(II) catalyzed oxidation. The complex formation reactions of the studied ligands are rather similar; only 1:1 complexes are formed, where the imidazole nitrogen of the histidine residue is the main binding site beside the amide nitrogens of the peptide chain. The only difference is, that in the peptides which contain methionine in position 109, in addition to the (Nim,N-,N-) coordination mode, a weak interaction of thioether sulfur atoms can be supposed. The mutant peptide which does not contain methionine did not undergo oxidation, only the fragmentation of the peptide chain was perceived. However, in the case of methionine containing peptides, the peptide chain was not cleaved; but the oxidation of methionine to methionine sulfoxide occurred.

Potentiometric and spectroscopic studies on the copper(II) complexes of rat amylin fragments. The anchoring ability of specific non-coordinating side chains.

Copper(ii) complexes of peptides modelling the sequence of the 17-22 residues of rat amylin have been studied by potentiometric, UV-Vis, CD and ESR spectroscopic methods. The peptides were synthesized in N-terminally free forms, NH2-VRSSNN-NH2, NH2-VRSSAA-NH2, NH2-VRAANN-NH2, NH2-VRSS-NH2, NH2-SSNN-NH2, NH2-SSNA-NH2 and NH2-AANN-NH2, providing a possibility for the comparison of the metal binding abilities of the amino terminus and the -SSNN- domain. The amino terminus was the primary ligating site in all cases and the formation of only mononuclear complexes was obtained for the tetrapeptides. The thermodynamic stability of the (NH2, N(-), N(-)) coordinated complexes was, however, enhanced by the asparaginyl moiety in the case of NH2-SSNN-NH2, NH2-SSNA-NH2 and NH2-AANN-NH2. Among the hexapeptides the formation of dinuclear complexes was characteristic for NH2-VRSSNN-NH2 demonstrating the anchoring ability of the -SSNN- (SerSerAsnAsn) domain. The complexes of the heptapeptide NH2-GGHSSNN-NH2 were also studied and the data supported the above mentioned anchoring ability of the -SSNN- site.

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.

Affinity, speciation, and molecular features of copper(II) complexes with a prion tetraoctarepeat domain in aqueous solution: insights into old and new results.

Characterization of the copper(II) complexes formed with the tetraoctarepeat peptide at low and high metal-to-ligand ratios and in a large pH range, would provide a breakthrough in the interpretation of biological relevance of the different metal complexes of copper(II)-tetraoctarepeat system. In the present work, the potentiometric, UV/Vis, circular dichroism (CD), and electron paramagnetic resonance (EPR) studies were carried out on copper(II) complexes with a PEG-ylated derivative of the tetraoctarepeats peptide sequence (Ac-PEG27 -(PHGGGWGQ)4 -NH2 ) and the peptide Ac-(PHGGGWGQ)2 -NH2 . Conjugation of tetraoctarepeat peptide sequence with polyethyleneglycol improved the solubility of the copper(II) complexes. The results enable a straightforward explanation of the conflicting results originated from the underestimation of all metal-ligand equilibria and the ensuing speciation. A complete and reliable speciation is therefore obtained with the released affinity and binding details of the main complexes species formed in aqueous solution. The results contribute to clarify the discrepancies of several studies in which the authors ascribe the redox activity of copper(II)-tetraoctarepeat system considering only the average effects of several coexisting species with very different stoichiometries and binding modes.

Mixed metal copper(II)-nickel(II) and copper(II)-zinc(II) complexes of multihistidine peptide fragments of human prion protein.

Mixed metal copper(II)-nickel(II) and copper(II)-zinc(II) complexes of four peptide fragments of human prion protein have been studied by potentiometric, UV-vis and circular dichroism spectroscopic techniques. One peptide contained three histidyl residues: HuPrP(84-114) with H85 inside and H96, H111 outside the octarepeat domain. The other three peptides contained two histidyl residues; H96 and H111 for HuPrP(91-115) and HuPrP(84-114)H85A while HuPrP(84-114)H96A contained the histidyl residues at positions 85 and 111. It was found that both histidines of the latter peptides can simultaneously bind copper(II) and nickel(II) ions and dinuclear mixed metal complexes can exist in slightly alkaline solution. One molecule of the peptide with three histidyl residues can bind two copper(II) and one nickel(II) ions. H85 and H111 were identified as the major copper(II) and H96 as the preferred nickel(II) binding sites in mixed metal species. The studies on the zinc(II)-PrP peptide binary systems revealed that zinc(II) ions can coordinate to the 31-mer PrP peptide fragments in the form of macrochelates with two or three coordinated imidazol-nitrogens but the low stability of these complexes cannot prevent the hydrolysis of the metal ion in slightly alkaline solution. These data provide further support for the outstanding affinity of copper(II) ions towards the peptide fragments of prion protein but the binding of nickel(II) can significantly modify the distribution of copper(II) among the available metal binding sites.

Copper(II) complexes of rat amylin fragments.

The fragments of rat amylin rIAPP(17-29) (Ac-VRSSNNLGPVLPP-NH(2)), rIAPP(17-22) (Ac-VRSSNN-NH(2)), rIAPP(19-22) (Ac-SSNN-NH(2)) and rIAPP(17-20) (Ac-VRSS-NH(2)) together with the related mutant peptides (Ac-VASS-NH(2) and Ac-VRAA-NH(2)) have been synthesized and their copper(II) complexes studied by potentiometric, UV-Vis, CD and EPR spectroscopic methods. Despite the lack of any common strongly coordinating donor functions some of these fragments are able to bind copper(II) ions in the physiological pH range. The longest fragment rat amylin(17-29) keeps one equivalent copper(II) ion in solution in the whole pH range, while two other peptides Ac-VRSSNN-NH(2) and Ac-SSNN-NH(2) are also able to interact with copper(II) ions in the slightly alkaline pH range. According to the spectral parameters of the complexes, the peptides can be classified into two different categories: (i) the tetrapeptides Ac-VRSS-NH(2), Ac-VASS-NH(2) and Ac-VRAA-NH(2) can interact with copper(II) only under strongly alkaline conditions (pH > 10.0) and the formation of only one species with four amide nitrogen coordination can be detected; (ii) the peptides Ac-VRSSNNLGPVLPP-NH(2), Ac-VRSSNN-NH(2) and Ac-SSNN-NH(2) can form complexes above pH 6.0 with the major stoichiometries [CuH(-2)L], [CuH(-3)L](-) and [CuH(-4)L](2-). These data support that rIAPP(17-29) can interact with copper(II) ions under physiological conditions and the SSNN tetrapeptide fragment can be considered as the shortest sequence responsible for metal binding. Density functional theory (DFT) calculations provide some information on the possible coordination modes of Ac-SSNN-NH(2) towards the copper(II) ion and suggest that for [CuH(-2)L], [CuH(-3)L](-) and [CuH(-4)L](2-), the binding of two, three and four deprotonated amide nitrogens, with NH(-) of the side chain of asparagine as anchoring group, is probable. Moreover, these data reveal that peptides can be effective metal binding ligands even in the absence of anchoring groups, if more polar side chains are present in a specific sequence.

Nickel(II) and mixed metal complexes of amyloid-beta N-terminus.

Nickel(II) complexes of Abeta(1-16)Y10A and its smaller fragments including Abeta(1-4), Abeta(1-6), Ac-Abeta(1-6) and Ac-Abeta(8-16)Y10A have been studied by potentiometric, UV-Vis and circular dichroism spectroscopic measurements. The formation of mixed metal complexes and the distribution of metal ions among the possible coordination sites in the Cu(II)-Ni(II)-Abeta(1-16)Y10A and Cu(II)-Ni(II)-Zn(ii)-Abeta(1-16)Y10A systems have also been evaluated. It was found that the hexadecapeptide and its fragments are effective nickel(II) binding ligands and complex formation processes of nickel(II) ions are quite similar to those of copper(II). Formation of mono- and di-nuclear complexes was detected in the nickel(II)-Abeta(1-16)Y10A system suggesting the existence of two separated metal binding motifs: the N-terminus and internal histidyl residues. The preference for the coordination at the N-terminus was supported by the spectroscopic measurements but in equilibrium with the metal binding at the internal histidyl sites. Neither zinc(II) nor nickel(II) can, however, substitute copper(II) in the mixed metal complexes of Abeta(1-16)Y10A, but both metal ions are able to alter the distribution of copper(II) ions among the various binding sites. Both N-terminus (amino and His6) and internal histidyl residues (His13 and His14) can work as dinuclear binding motifs, preferably accommodating copper(II) and zinc(II), respectively, while nickel(II) can occupy the remaining free coordination sites.

Nickel(II) complexes of the multihistidine peptide fragments of human prion protein.

Nickel(II) complexes of the peptide fragments of human prion protein containing histidyl residues both inside and outside the octarepeat domain have been studied by the combined application of potentiometric, UV-visible and circular dichroism spectroscopic methods. The imidazole-N donor atoms of histidyl residues are the exclusive metal binding sites below pH 7.5, but the formation of stable macrochelates was characteristic only for the peptide HuPrP(76-114) containing four histidyl residues. Yellow colored square planar complexes were obtained above pH 7.5-8 with the cooperative deprotonation of three amide nitrogens in the [N(im),N(-),N(-),N(-)] coordination mode. It was found that the peptides can bind as many nickel(II) ions as the number of independent histidyl residues. All data supported that the complex formation processes of nickel(II) are very similar to those of copper(II), but with a significantly reduced stability for nickel(II), which shifts the complex formation reactions into the slightly alkaline pH range. The formation of coordination isomers was characteristic of the mononuclear complexes with a significant preference for the nickel(II) binding at the histidyl sites outside the octarepeat domain. The results obtained for the two-histidine fragments of the protein, HuPrP(91-115), HuPrP(76-114)H85A and HuPrP(84-114)H96A, made it possible to compare the binding ability of the His96 and His111 sites. These data reveal a significant difference in the nickel(II) and copper(II) binding sites of the peptides: His96 was found to predominate almost completely for nickel(II) ions, while the opposite order, but with comparable concentrations, was reported for copper(II).

A study of the reactions of a methionine- and histidine-containing tetrapeptide with different Pd(II) and Pt(II) complexes: selective cleavage of the amide bond by platination of the peptide and steric modification of the catalyst.

(1)H NMR spectroscopy was applied to the study of the reactions of [M(en)(H(2)O)(2)](2+) complexes (M = Pd(ii) and Pt(ii)) with the N-acetylated methionyl-glycyl-histidyl-glycineamide, MeCOMet-Gly-His-GlyNH(2). All reactions were performed in the pH range 1.5-2.0 with equimolar amounts of the [M(en)(H(2)O)(2)](2+) complex and the tetrapeptide at 60 degrees C. In all these reactions, a metal(ii) complex bound to a methionine residue affects the regioselective cleavage of the amide bond involving the carboxylic group of methionine. The priority in the cleavage of the Met-Gly amide bond in relation to the other amide bonds in this peptide is due to the high affinity of Pt(ii) and Pd(ii) ions for the sulfur donor atom. The mechanism of these hydrolytic reactions is discussed and, for its clarification, the reaction of the [Pd(en)(H(2)O)(2)](2+) complex with MeCOMet-Gly-His-GlyNH(2) was additionally investigated by potentiometric titration. The steric effects of the various palladium(ii) complexes of the type [Pd(L)(H(2)O)(2)](2+), in which L is a chelating diamine (ethylenediamine, en, 2-picolylamine, pic, or 2,2-dipyridylamine, dpa) on the hydrolytic cleavage of the amide bond involving the carboxylic group of histidine in the MeCOMet-Gly-His-GlyNH(2) tetrapeptide were also studied by (1)H NMR spectroscopy. All reactions were performed under the above-mentioned conditions and in the initial stage of these reactions, the MeCOMet-Gly-His-GlyNH(2) was reacted with an equimolar amount of the [Pt(dien)Cl](+) complex (dien is diethylenetriamine) and then the monoplatinated [Pt(dien)(MeCOMet-Gly-His-GlyNH(2)-S)](2+) complex was treated with an equimolar amount of [Pd(L)(H(2)O)(2)](2+). It was found that the rate of hydrolysis of the His-GlyNH(2) amide bond in [Pt(dien)(MeCOMet-Gly-His-GlyNH(2)-S)](2+) decreased from the en to the pic complex, with finally a total inhibition of this reaction with [Pd(dpa)(H(2)O)(2)](2+). These results are an important step in the study of the regioselective cleavage of peptides and proteins and in the development of new palladium(ii) complexes as artificial metallopeptidases.