Pd(ii)-Complexes of a novel pyridinone based tripeptide conjugate: solution and solid state studies

A novel peptide conjugate (H(L2)) incorporating N-donors of the peptide backbone and an (O,O) donor set of a hydroxypyridinone moiety is synthesized and characterized. This ambidentate chelating ligand is intended to develop Co(iii)/Pt(ii) heterobimetallic multitargeted complexes with anticancer potential. To explore its metal ion binding ability the interaction with Pd(ii) (as a Pt(ii) model but with faster ligand exchange reactions) was studied in aqueous solution by the combined use of pH-potentiometry, NMR and HR MS. In an equimolar solution H(L2) was found to bind Pd(ii) via the terminal amino group and increasing number of peptide nitrogens of the peptide backbone over a wide pH range. Around physiological pH an (N,N) and (O,O) chelated 2 : 2 minor species was also identified. At a 2 : 1 Pd(ii) to ligand ratio the formation of dinuclear species, [Pd2H-x(L2)] (x = 1-4), with high stability and with the involvement of the (O,O) chelating set of the ligand too, was demonstrated. Reaction of H3(L2)2+ with Pd(ii) in the presence of chloride ions at pH ∼ 2.0 afforded [PdH(L2)Cl2]·2H2O (3) in a solid state whose molecular structure was assessed by single crystal X-ray diffraction. The structure of 3 revealed that Pd(ii) is coordinated by a (NH2, Namide) chelate of the ligand in a square planar fashion. It also indicates that under suitable conditions a 2N coordinated Pd(ii) complex can also be obtained even in the presence of four available nitrogen donors in the chelatable position in the ligand most likely due to its neutral charge and the decreased conditional stability of the amide-involved chelate(s) under acidic conditions. Reaction of H(L2) with [Co(tren)]3+ (tren = tris(2-aminoethyl)amine) revealed the exclusive coordination of (L2)-via its (O,O) chelate to the metal core while treatment of the Co-complex with Pd(ii) resulted in the formation of a Co/Pd heterobimetallic complex in solution with (NH2, Namide) chelated Pd(ii). Reaction of 3 with 9-methylguanine indicated the N7 coordination of this simple DNA model to Pd(ii) in a 1 : 1 ratio.

UV-promoted radical formation, and near-IR-induced and spontaneous conformational isomerization in monomeric 9-methylguanine isolated in low-temperature Ar matrices

Three low-energy isomers of 9-methylguanine, the amino-oxo (AO) form and two amino-hydroxy (AH1 and AH2) conformers, were trapped from the gas phase into low-temperature argon matrices. The AH1 and AH2 isomers, differing in the orientation of the OH group, were found to transform into each other upon excitation with near-IR light. The population of the AO form of the compound was not changed upon any near-IR irradiation of the matrix samples. Using monochromatic near-IR light, generated by a frequency-tunable laser source, it was possible to selectively induce the AH1 → AH2 or AH2 → AH1 conversion. Photoreversibility of this conformational transformation was then demonstrated. Exposure of matrix-isolated monomers of 9-methylguanine to broadband near-IR light also led to conformational conversions within the amino-hydroxy tautomeric form; the final stage of this process was always the same photostationary state independent of the initial ratio of AH1 and AH2 populations. Spontaneous conformational conversion, transforming the higher-energy AH2 form into the lower-energy AH1 isomer, was observed for matrix-isolated monomers of 9-methylguanine kept in the dark. The mechanism of this process must rely on quantum tunneling of the light hydrogen atom. Irradiation of matrix-isolated 9-methylguanine with UV laser light at λ = 288 or 285 nm led to a substantial consumption of the two AH forms, while the amount of AO isomer remained unchanged. On the other hand, a decrease in the population of the AO isomer occurred upon excitations at shorter wavelengths, λ = 280 or 275 nm. The spectral changes observed after UV-irradiation suggest the generation (and stabilization in the matrix) of a radical species, resulting from the photocleavage of the O-H or N1-H bonds, in the AH or AO isomer, respectively.

Mononuclear gold(iii) complexes with l-histidine-containing dipeptides: tuning the structural and biological properties by variation of the N-terminal amino acid and counter anion

Gold(iii) complexes with different l-histidine-containing dipeptides, [Au(Gly-l-His-N_A,N_P,N3)Cl]Cl·3H₂O (1a), [Au(Gly-l-His-N_A,N_P,N3)Cl]NO₃·1.25H₂O (1b), [Au(l-Ala-l-His-N_A,N_P,N3)Cl][AuCl₄]·H₂O (2a), [Au(l-Ala-l-His-N_A,N_P,N3)Cl]NO₃·2.5H₂O (2b), [Au(l-Val-l-His-N_A,N_P,N3)Cl]Cl·2H₂O (3), [Au(l-Leu-l-His-N_A,N_P,N3)Cl]Cl (4a) and [Au(l-Leu-l-His-N_A,N_P,N3)Cl][AuCl₄]·H₂O (4b), have been synthesized and structurally characterized by spectroscopic (¹H NMR, IR and UV-vis) and single-crystal X-ray diffraction techniques. The antimicrobial efficiency of these gold(iii) complexes, along with K[AuCl₄] and the corresponding dipeptides, was evaluated against the broad panel of Gram-positive and Gram-negative bacteria and fungi, displaying their moderate inhibiting activity. Moreover, the cytotoxic properties of the investigated complexes were assessed against the normal human lung fibroblast cell line (MRC5) and two human cancer, cervix (HeLa) and lung (A549) cell lines. None of the complexes exerted significant cytotoxic activity; nevertheless complexes that did show selectivity in terms of cancer vs. normal cell lines (2a/b and 4a/b) have been evaluated using zebrafish (Danio rerio) embryos for toxicity and antiangiogenic potential. Although the gold(iii) complexes achieved an antiangiogenic effect comparable to the known angiogenic inhibitors auranofin and sunitinib malate at 30-fold higher concentrations, they had no cardiovascular side effects, which commonly accompany auranofin and sunitinib malate treatment. Finally, binding of the gold(iii) complexes to the active sites of both human and bacterial (Escherichia coli) thioredoxin reductases (TrxRs) was demonstrated by conducting a molecular docking study, suggesting that the mechanism of biological action of these complexes can be associated with their interaction with the TrxR active site.

The challenge of deciphering linkage isomers in mixtures of oligomeric complexes derived from 9-methyladenine and trans-(NH3)2Pt(II) units

Metal coordination to N9-substituted adenines, such as the model nucleobase 9-methyladenine (9MeA), under neutral or weakly acidic pH conditions in water preferably occurs at N1 and/or N7. This leads, not only to mononuclear linkage isomers with N1 or N7 binding, but also to species that involve both N1 and N7 metal binding in the form of dinuclear or oligomeric species. Application of a trans-(NH3)2Pt(II) unit and restriction of metal coordination to the N1 and N7 sites and the size of the oligomer to four metal entities generates over 50 possible isomers, which display different feasible connectivities. Slowly interconverting rotamers are not included in this number. Based on (1)H NMR spectroscopic analysis, a qualitative assessment of the spectroscopic features of N1,N7-bridged species was attempted. By studying the solution behavior of selected isolated and structurally characterized compounds, such as trans-[PtCl(9MeA-N7)(NH3)2]ClO4⋅2H2O or trans,trans-[{PtCl(NH3)2}2(9MeA-N1,N7)][ClO4]2⋅H2O, and also by application of a 9MeA complex with an (NH3)3Pt(II) entity at N7, [Pt(9MeA-N7)(NH3)3][NO3]2, which blocks further cross-link formation at the N7 site, basic NMR spectroscopic signatures of N1,N7-bridged Pt(II) complexes were identified. Among others, the trinuclear complex trans-[Pt(NH3)2{μ-(N1-9MeA-N7)Pt(NH3)3}2][ClO4]6⋅2H2O was crystallized and its rotational isomerism in aqueous solution was studied by NMR spectroscopy and DFT calculations. Interestingly, simultaneous Pt(II) coordination to N1 and N7 acidifies the exocyclic amino group of the two 9MeA ligands sufficiently to permit replacement of one proton each by a bridging heterometal ion, Hg(II) or Cu(II), under mild conditions in water.

Transition metal complexes with oligopeptides: single crystals and crystal structures

The coordination chemistry of short chain peptides with transition metals is described in terms of the available crystal structures. Despite their high interest as synthetic models for metalloproteins and as building blocks for molecular materials based on the tuneable properties of oligopeptides, single crystal X-ray diffraction studies are scarce. A perusal of the most relevant results in this field allows us to define the main characteristics of oligopeptide-metal interactions, the fundamental problems for the crystallization of these complexes, and some hints to identify future promising approaches to advance the development of metallopeptide chemistry.

Reactions and structural characterization of gold(III) complexes with amino acids, peptides and proteins

The present review article highlights recent findings in the field of gold(III) complexes with amino acids, peptides and proteins. The first section of this article provides an overview of the gold(III) reactions with amino acids, such as glycine, alanine, histidine, cysteine and methionine. The second part of the review is mainly focused on the results achieved in the mechanistic studies of the reactions between gold(III) and different peptides and structural characterization of gold(III)-peptide complexes as the final products in these reactions. The last section of this article deals with the reactions of gold(III) complexes with proteins as primary targets for cytotoxic gold compounds. Systematic summaries of these results contribute to the future development of gold(III) complexes as potential antitumor agents and also have importance in relation to the severe toxicity of gold-based drugs.

Thermodynamic and structural characterization of the copper(II) complexes of peptides containing both histidyl and aspartyl residues

Terminally protected pentapeptides with 2 histidines (Ac-HHVGD-NH2 and Ac-HVGDH-NH2) and the terminally free peptides containing both internal aspartyl and C-terminal histidyl residues (FDAH and VIDAH) have been synthesized, and copper(II) complexes studied by potentiometric, UV-Vis, CD, and EPR spectroscopic techniques in solution. Both thermodynamic and spectroscopic data reveal that side chain donor atoms of aspartyl and histidyl residues have a significant contribution to the metal binding affinity of peptide molecules. In the case of terminally protected peptides, the role of the imidazole-N donor functions is reflected in the enhanced stability of the 3N and 4N coordinated copper(II) complexes. The amino and β-carboxylate groups of FDAH and VIDAH create a very effective metal binding site with the (NH2, N−, β-COO−) and (NH2, N−, N−, β-COO−) coordination modes including the N-termini, while the histidine sites are available for the formation of the (Nim, N−, N−) binding mode resulting in the preference of dinuclear complex formation.

Linkage isomerism in the binding of pentapeptide Ac-His(Ala)3His-NH2 to (ethylenediamine)palladium(II): effect of the binding mode on peptide conformation

The reaction of the pentapeptide Ac-His1-Ala2-Ala3-Ala4-His5-NH2 (AcHAAAHNH2) (1) with [Pd(en)(ONO2)2] (en = NH2CH2CH2NH2) in either DMF-d(7) or H2O:D2O (90%:10%) gave three linkage isomers of [Pd(en)(AcHAAAHNH2)](2+) (2), 2a, 2b, and 2c, which differ only in which pair of imidazole nitrogen atoms bind to Pd. In the most abundant isomer, 2a, Pd is bound by N1 from each of the two imidazole rings. In the minor isomers 2b and 2c, Pd is bound by N1(His1) and N3(His5) and by N3(His1) and N1(His5), respectively. The reactions of [Pd(en)(ONO2)2] with the N-methylated peptides Ac-(N3-MeHis)-Ala-Ala-Ala-(N3-MeHis)-NH2 (AcH*AAAH*NH2) (3), Ac-(N3-MeHis)-Ala-Ala-Ala-(N1-MeHis)-NH2 (AcH(*)AAAH(#)NH2) (4), and Ac-(N1-MeHis)-Ala-Ala-Ala-(N3-Me-His)-NH2 (AcH(#)AAAH(*)NH2) (5) each gave a single species [Pd(en)(peptide)](2+) in N,N-dimethylformamide (DMF) or aqueous solution, 7, 8, and 9, respectively, with Pd bound by the two nonmethylated imidazole nitrogen atoms in each case. These complexes were analogous to 2a, 2b, and 2c, respectively. Ac-(N1-MeHis)-Ala-Ala-Ala-(N1-MeHis)-NH2 (AcH(#)AAAH(#)NH2) (6) with [Pd(en)(ONO2)2] in DMF slowly gave a single product, [Pd(en)(AcH(#)AAAH(#)NH2)](2+) (10), in which Pd was bound by the N3 of each imidazole ring. The corresponding linkage isomer of 2 was not observed. Complex 10 was also the major product in aqueous solution, but other species were also present. All compounds were exhaustively characterized in solution by multinuclear 1D ((1)H , (13)C, and, with (15)N-labeled ethylenediamine, (15)N) and 2D (correlation spectroscopy, total correlation spectroscopy, transverse rotating-frame Overhauser effect spectroscopy (T-ROESY), heteronuclear multiple-bond correlation, and heteronuclear single quantum coherence) NMR spectra, circular dichroism (CD) spectra, electrospray mass spectroscopy, and reversed-phase high-performance liquid chromatography. ROESY spectra were used to calculate the structure of 2a, which contained a single turn of a peptide alpha helix in both DMF and water, the helix being better defined in DMF. The Pd(en)(2+) moiety was not used in structure calculations, but its location and coordination by one imidazole N1 from each histidine to form a 22-membered metallocycle were unambiguously established. Convergence of the structures was greatest when calculated with two hydrogen-bond constraints (Ala4 peptide NH...OC acetyl and His5 peptide NH...OC-His1) that were indicated by the low temperature dependence of these NH chemical shifts. Vicinal HN-CHalpha coupling constants and chemical shifts of alpha-H atoms were also consistent with a helical conformation. Similar long-range ROE correlations were observed for [Pd(en)(AcH(*)AAAH(*)NH2)](2+) (7), which displayed a CD spectrum in aqueous solution that suggested the presence of some helicity. Long-range ROE correlations were not observed for 8, 9, or 10, but a combination of NMR data and CD spectroscopy was interpreted in terms of the conformational behavior of the coordinated pentapeptide. Only for the linkage isomer [Pd(en)(AcH(*)AAAH(#)NH2)](2+) (8) was there evidence of a contribution from a helical conformation. The data for 8 were interpreted as interconversion between the helix and random coil conformations. Zn(2+) with peptides gave broad NMR peaks attributed to lability of this metal ion, while reactions of cis-[Pt(NH3)2(ONO2)2] were slow, giving a complex mixture of products rather than the macrochelate ring observed with Pd(en)(2+). In summary, these studies indicate that Pd(en)(2+) coordinates to histidine with similar preference for each of the two imidazole nitrogens, enabling the formation of up to four linkage isomers in its complexes with pentapeptides His-xxx-His. Only the N1-N1 linkage isomer that forms a 22-membered macrochelate ring is able to induce an alpha-helical peptide conformation, whereas the 20- and 21-membered rings of linkage isomers do not. This suggests that linkage isomeric mixtures may compromise histidine coordination to metal ions and reduce alpha-helicity.

Metal ion selectivity of oligopeptides

Metal binding affinity and selectivity of peptides are reviewed with a special emphasis on the high structural variety of peptide complexes. The most common structural type of these complexes is built up by the deprotonation and metal ion coordination of subsequent amide groups in the form of fused five-membered chelate rings. The metal ion selectivity of this process and the role of various anchoring groups are discussed in detail. The highest metal binding affinity of peptides is connected to the presence of two anchoring groups in appropriate location (the "double anchor"): e.g. the NH2-Xaa-Xaa-His/Cys/Asp/Met-Xaa sequence. Among the side chain donor functions, the imidazole of histidyl and thiolate of cysteinyl residues are the most effective ligating groups and their involvement in metal binding results in a great variety of different macrochelate or loop structures and/or formation of various polynuclear complexes. Examples of these structural motifs and their possible applications have been thoroughly discussed.

α-Turn Mimetics: Short Peptide α-Helices Composed of Cyclic Metallopentapeptide Modules

Alpha-Helices are key structural components of proteins and important recognition motifs in biology. Short peptides (<or=15 residues) corresponding to these helical sequences are rarely helical away from their stabilizing protein environments. New techniques for stabilizing short peptide helices could be valuable for studying protein folding, modeling proteins, creating artificial proteins, and may aid the design of inhibitors or mimics of protein function. This study reports the facile incorporation of 3- and 4-alpha turns in 10-15 residue peptides through formation in situ of multiple cyclic metallopeptide modules [Pd(en)(H*XXXH*)](2+). The nonhelical peptides Ac-H*ELTH*H*VTDH*-NH(2) (1), Ac-H*ELTH*AVTDYH*ELTH*-NH(2) (2), and Ac-H*AAAH*HELTH*H*VTDH*-NH(2) (3) (H is histidine-methylated at imidazole-N3) react in N,N-dimethylformamide (DMF) or water with 2, 2, and 3 molar equivalents, respectively, of [Pd(en)(NO(3))(2)] to form exclusively [Pd(2)(en)(2)(Ac-H*ELTH*H*VTDH*-NH(2))](4+) (4), [Pd(2)(en)(2)(Ac-H*ELTH*AVTDYH*ELTH*-NH(2))](4+) (5), and [Pd(3)(en)(3)(Ac-H*AAAH*HELTH*H*VTDH*-NH(2))](6+) (6), characterized by mass spectrometry, 1D and 2D (1)H- and 1D (15)N-NMR spectroscopy. Despite the presence of multiple histidines and other possible metal-binding residues in these peptides, 2D (1)H NMR spectra reveal that Pd(en)(2+) is remarkably specific in coordinating to imidazole-N1 of only (i, i + 4) pairs of histidines (i.e., only those separated by three amino acids), resulting in 4-6 made up of cyclic metallopentapeptide modules ([Pd(en)(H*XXXH*)](2+))(n), n = 2, 2, 3, respectively, each cycle being a 22-membered ring. We have previously shown that a single metallopentapeptide can nucleate alpha-helicity (Kelso et al., Angew. Chem., Int. Ed. 2003, 42, 421-424.). We now demonstrate its use as an alpha-turn-mimicking module for the facile conversion of unstructured short peptides into helices of macrocycles and provide 1D and 2D NMR spectroscopic data, structure calculations via XPLOR and NMR analysis of molecular flexibility in solution (NAMFIS), and CD spectra in support of the alpha-helical nature of these monomeric metallopeptides in solution.

Studies on the interaction of altromycin B and its platinum(II) and palladium(II) metal complexes with calf thymus DNA and nucleotides

The interaction of the anticancer antibiotic altromycin B and its isostructrural Pt(II) and Pd(II) metal complexes with native calf thymus (CT) DNA was studied using UV-thermal denaturation experiments, circular dichroism spectroscopy and temperature controlled spectrophotometric titrations. Altromycin B stabilizes the double helix by raising the T(m), mainly by intercalation of its chromophore between the base pairs and interacting electrostatically via its sugar moieties with the edges of the DNA helix. Moreover, altromycin B induces a B-->A structural transition of CT DNA. The effect on DNA stability and conformation depends on the metal ion. Pt(II) and Pd(II) complexes induce the B-->A structural transition and stabilize the double helix similarly but they present lower final hyperchromicity due to premelting effects which were caused by intra- and interstrand crosslinking. Thus, a synergic effect of the metal ions to altromycin B-CT DNA interaction is observed in both cases. Altromycin B interacts with 5'-GMP, 5'-AMP and 5'-CMP by electrophilic attack of the opened epoxide ring to the N(7)G, N(1)/N(7)A and N(3)C. Thus, covalent binding between these nucleotides and altromycin B takes place and explain the multiple binding mode suggested by the studies of the interaction of altromycin B and its complexes with DNA. The [Pd(II)-altroB] complex dissociates in the presence of the nucleotides, and various species of Pd(II)-nucleotide complexes, especially with 5'-GMP, are formed. The [Pt(II)-altroB] complex dissociates too, but only one or two species of Pt(II)-nucleotide complexes are formed, and in the case of 5'-AMP interaction the formation of a tertiary altroB-Pt(II)-5'AMP complex is proposed. 5'-TMP reacts very weakly in comparison with the other three nucleotides. These interactions were followed by 1H-NMR.

Interplay of terminal amino group and coordinating side chains in directing regioselective cleavage of natural peptides and proteins with palladium(II) complexes

Palladium(II) ions anchored to side chains of histidine and methionine residues in peptides and proteins in weakly acidic aqueous solutions promote hydrolytic cleavage of proximate amide bonds in the backbone. In this study, we determine how attachment of Pd(II) ions to histidine and methionine anchors and also to the terminal amino group in six natural peptides (chains A and B of insulin, segment 11-14 of angiotensinogen, pentagastrin, angiotensin II, and segment 3-8 of angiotensin II) and two proteins (ubiquitin and cytochrome c) affects regioselectivity and rate of backbone cleavage. These Pd(II)-promoted reactions follow a clear pattern of regioselectivity, directed by the anchoring side chains. When the Pd(II) reagent is nonspecifically anchored to the terminal amino group, the ligating site that is present in almost all proteins, the cleavage is fortunately absent. When the reagent is anchored to a residue in positions 1, 2, or 3, cleavage is absent, because the terminal amino group and deprotonated amide nitrogen atom(s) interposed between it and the anchor "lock" the Pd(II) ion in hydrolytically inactive chelate complexes. When the reagent is anchored to residues in positions beyond 3, the second amide bond upstream from the anchor is regioselectively cleaved in all cases when the anchor was "isolated," that is, flanked by noncoordinating side chains. Segment 3-8 of angiotensin II undergoes additional cleavage, which we explain by determining the rate constants for the cleavage, identifying the rate-limiting displacement of ethylenediamine ligand from the Pd(II) ion, and detecting several intermediates. Experiments with cytochrome c demonstrate that the number of cleavage sites can be controlled by adjusting the mole ratio of the Pd(II) reagent to the substrate. Our inorganic peptidases are useful for biochemical applications because their regioselectivity and reactivity set them apart from proteolytic enzymes and organic chemical reagents.

Loss of Hoogsteen pairing ability upon N1 adenine platinum binding

Chloroform- and Freon-soluble mixed thymine, adenine complexes trans-[Pt(MeNH(2))(2)(ChmT-N3)(ChmA-N1)]NO(3) (2) and trans-[Pt(MeNH(2))(2)(ChmT-N3)(TBDMS-ado-N1)]BF(4) (3) (ChmT = anion of 1-cyclohexylmethylthymine ChmTH, ChmA = 9-cyclohexylmethyladenine, TBDMS-ado = 2',3',5'-tri-tert-butyldimethylsilyladenosine) have been prepared and characterized to study their propensity to undergo Hoogsteen and/or reversed Hoogsteen pairing in solution with free ChmTH and free 3',5'-diacetyl-2'-deoxyuridine, respectively. No Hoogsteen or reversed Hoogsteen pairing between 2 and ChmT takes place in CDCl(3). In Freon, partial H bonding between N1 platinated TBDMS-ado and 3',5'-diacetyl-2'-deoxyuridine as well as its [3-(15)N] labeled analogue is unambiguously observed only below 150 K. Comparison of (1)J ((15)N-(1)H) coupling constants of 3',5'-diacetyl-2'-deoxyuridine involved in Hoogsteen pairing with free and N1 platinated adenine suggests that the interaction is inherently weaker in the case of platinated adenine. To better understand the complete absence of hydrogen bonding between the ChmA ligand in 2 and free ChmTH, ab initio calculations (gas phase, 0 K) have been carried out for Hoogsteen pairs involving adenine (A) and thymine (T), as well as simplified analogues of 2 and T, both in the presence and absence of counteranions. The data strongly suggest that reduction of the effective positive charge of the heavy metal ion Pt(2+) by counterions diminishes interaction energies. With regard to mixtures of 2 and ChmTH in chloroform, this implies that ion pair formation between the cation of 2 and NO(3)(-) may be responsible for the lack of any measurable Hoogsteen pairing in this solvent.

Palladium(II) complexes, as synthetic peptidases, regioselectively cleave the second peptide bond "upstream" from methionine and histidine side chains

Palladium(II) complexes promote hydrolysis of natural and synthetic oligopeptides with unprecedented regioselectivity; the only cleavage site is the second peptide bond upstream from a methionine or a histidine side chain, that is, the bond involving the amino group of the residue that precedes this side chain. We investigate this regioselectivity with four N-acetylated peptides as substrates: neurotransmitter methionine enkephalin (Ac-Tyr-Gly-Gly-Phe-Met) and synthetic peptides termed Met-peptide (Ac-Ala-Lys-Tyr-Gly-Gly-Met-Ala-Ala-Arg-Ala), His-peptide (Ac-Val-Lys-Gly-Gly-His-Ala-Lys-Tyr-Gly-Gly-Met(OX)-Ala-Ala-Arg-Ala), in which a Met is oxidized to sulfone, and HisMet-peptide (Ac-Val-Lys-Gly-Gly-His-Ala-Lys-Tyr-Gly-Gly-Met-Ala-Ala-Arg-Ala). While maintaining protein-like properties, these substrates are suitable for quantitative study since their coordination to Pd(II) ion can be determined (by NMR spectroscopy), and the cleavage fragments can be separated (by HPLC methods) and identified (by MALDI mass spectrometry). The only peptide bonds cleaved were the Gly3-Phe4 bond in methionine enkephalin, Gly4-Gly5 bond in Met-peptide, Gly3-Gly4 in His-peptide, and Gly3-Gly4 and Gly9-Gly10 bonds in HisMet-peptide. We explain this consistent regioselectivity of cleavage by studying the modes of Met-peptide coordination to the Pd(II) ion in [Pd(H(2)O)(4)](2+) complex. In acidic solution, the rapid attachment of the Pd(II) complex to the methionine side chain is followed by the interaction of the Pd(II) ion with the peptide backbone upstream from the anchor. In the hydrolytically active complex, Met-peptide is coordinated to Pd(II) ion as a bidentate ligand - via sulfur atom in the methionine side chain and the first peptide nitrogen upstream from this anchor - so that the Pd(II) complex approaches the scissile peptide bond. Because the increased acidity favors this hydrolytically active complex, the rate of cleavage guided by either histidine or methionine anchor increased as pH was lowered from 4.5 to 0.5. The unwanted additional cleavage of the first peptide bond upstream from the anchor is suppressed if pH is kept above 1.2. Four Pd(II) complexes cleave Met-peptide with the same regioselectivity but at somewhat different rates. Complexes in which Pd(II) ion carries labile ligands, such as [Pd(H(2)O)(4)](2+) and [Pd(NH(3))(4)](2+), are more reactive than those containing anionic ligands, such as [PdCl(4)](2)(-), or a bidentate ligand, such as cis-[Pd(en)(H(2)O)(2)](2+). When both methionine and histidine residues are present in the same substrate, as in HisMet-peptide, 1 molar equivalent of the Pd(II) complex distributes itself evenly at both anchors and provides partial cleavage, whereas 2 molar equivalents of the promoter completely cleave the second peptide bond upstream from each of the anchors. The results of this study bode well for growing use of palladium(II) reagents in biochemical and bioanalytical practice.

Interaction of Cu(2+) with His-Val-His and of Zn(2+) with His-Val-Gly-Asp, two peptides surrounding metal ions in Cu,Zn-superoxide dismutase enzyme

His-Val-His and His-Val-Gly-Asp are two naturally occurring peptide sequences, present at the active site of Cu,Zn-superoxide dismutase (Cu,Zn-SOD). The interactions of His-Val-His=A (copper binding site) with Cu(II) and of His-Val-Gly-Asp=B (zinc binding site) with Zn(II) have been studied by using both potentiometric and spectroscopic methods (visible, EPR, NMR). The stoichiometry, stability constants and solution structure of the complexes formed have been determined. The binding modes of the species [CuAH](2+) and [CuA](+) were characterized by histamine type of coordination. [CuA](+) is further stabilized by the formation of a macrochelate with the involvement of the imidazole of the C-terminal histidine. The existence of macrochelate results in a slight distortion of the coordination geometry providing good base for the development of enzyme models. The enhanced stability of the macrochelate suppresses the formation of bis-complexes as well as the amide deprotonation. This process, however, takes place at higher pH resulting in the formation of the 4 N(-) coordinated [NH(2),N(-),N(-),N(im)] species [CuAH(2-)](-). On the other hand, in the case of the Zn(II)-His-Val-Gly-Asp system, coordination takes place at the terminal carboxylate in species [ZnBH(2)](2+). Monodentate binding occurs via the N-terminal imidazole in [ZnBH](+) while histamine type of coordination is possible in [ZnB], [ZnB(2)H](-) and [ZnB(2)](2-) species. Amide deprotonation does not take place in the case of Zn(2+), hydroxo-complexes are formed instead.

Interactions of d(10) metal ions with hippuric acid and cytosine. X-ray structure of the first cadmium (II)-amino acid derivative-nucleobase ternary compound

The interactions of Zn(II), Cd(II) and Hg(II) with hippuric acid (hipH) were studied and several novel compounds were synthesized and studied by NMR. Some new metal-hippuric-cytosine ternary compounds were formed and the structure of the [Cd(hip)(2)(cyt)(H(2)O)](2) ternary complex resolved. Each cadmium (II) atom has a distorted trigonal bipyramid coordination which is linked to a water molecule, a cytosine via N(3), a carboxylic oxygen atom of a hippurate moiety and two bridging dicoordinated hippurates bound through the carboxylic oxygen atoms. To these five main bonds, two longer ancillary interactions can be observed: the second oxygen of the monocoordinated hippurate group and the carboxylic oxygen of the cytosine ligand. The compound is stabilized by an intramolecular stacking between the benzene and cytosine rings and by the hydrogen bonds between the coordinated water molecules and the ligands. This is, to our knowledge, the first structure of a cadmium-amino acid derivative-natural nucleobase compound described so far.

Solution equilibria and structural characterisation of the palladium(II) and mixed metal complexes of peptides containing methionyl residues

Palladium(II) complexes of the peptides GlyMet, GlyMetGly and GlyGlyMet containing methionyl residues were studied by potentiometric and 1H NMR spectroscopic methods. The coordination of terminal amino and deprotonated amide nitrogen and thioether sulfur donor atoms was suggested in the mono complexes of GlyMet and GlyMetGly. The fourth coordination site of these complexes can be occupied by solvent molecule, chloride or hydroxide ions or by another ligand molecule in the bis or mixed ligand complexes. The second ligand coordinates monodentately via the thioether function in acidic media and the amino group under neutral or basic conditions. The stoichiometry of the major species formed in the palladium(II)-GlyGlyMet system is [PdH(-2) L]- and this is coordinated by the amino, two-amide and the thioether donor functions. Thioether bridged mixed metal complexes formed in the reaction of [Pd(dien)]2+ and [Cu(GlyMetH(-1))] or [Ni(GlyMetGlyH(-2))]- also have been detected by spectroscopic techniques.