Coordination of copper(II) ions by the 11-20 and 11-28 fragments of human and mouse beta-amyloid peptide

A potentiometric and spectroscopic (UV-vis, CD and EPR) study of Cu(II) binding to the (11-20), (11-28), (Ac-11-20H) and (Ac-11-28) fragments of human (H) and mouse (M) beta-amyloid peptide was carried out. The values of the protonation constants of the two lysine side chain amino groups for the (11-28) and (Ac-11-28) fragments of beta-amyloid peptide differ noticeably suggesting considerable interactions between the two residues. The N-terminal amino acid sequence Xaa-Yaa-His for the (11-20H) and (11-28H) fragments determines the coordination ability of the fragments studied to copper(II) ions. Addition of the (17-20) and (17-28) sequences to the (11-16) fragment of human and mouse beta-amyloid peptide does not change the coordination mode, and the stabilities of the complexes formed are comparable to those of the (11-16) peptide, although 1N complexes of the (11-28) fragments are stabilized by about one order of magnitude compared to those of the (11-16) peptides. The (Ac-11-28) peptides form complexes with the same coordination mode as those for the (Ac-11-16) fragments. The stability of the complexes for the (Ac-11-28H) fragment is one or two orders of magnitude higher compared to those of the (Ac-11-16H) fragment. This stabilization may result from structural organization of a peptide in copper(II) complexes.     

Cu(II) interaction with N-terminal fragments of human and mouse beta-amyloid peptide

The stoichiometry, stability constants and solution structure of the complexes formed in the reaction of copper(II) with N-terminal fragments of human and mouse beta-amyloid peptide, 1-6, 1-9, 1-10 have been determined by potentiometric, UV/VIS, CD and EPR spectroscopic methods. The fragments 1-9 and 1-10 form complexes with the same coordination modes as the fragments 1-6. The coordination of the metal ion for human and mouse fragments starts from the N-terminal Asp residue which stabilizes significantly the 1N complex as a result of chelation through the beta-carboxylate group. In a wide pH range of 4-10, the imidazole nitrogen of His(6) is coordinated to form a macrochelate. Results show that, in the pH range 5-9 the human fragments form the complex with different coordination mode compared to that of the mouse fragments. The low pK(1)(amide) values (approximately 5) obtained for the mouse fragments may suggest the coordination of the amide nitrogen of His(6) while in case of the human fragments the coordination of the amide nitrogen of Ala(2) is suggested. The replacement of glycine by the arginine residue in the fifth position of the beta-amyloid peptide sequence changes the coordination modes of a peptide to metal ion in the physiological pH range. In a wide pH (including physiological) range the mouse fragments of beta-amyloid peptide are much more effective in Cu(II) binding than the human fragments.     

Products of Cu(II)-catalyzed oxidation in the presence of hydrogen peroxide of the 1-10, 1-16 fragments of human and mouse beta-amyloid peptide

The interactions of proteins with reactive oxygen species (ROS) may result in covalent modifications of amino acid residues in proteins, formation of protein-protein cross-linkages, and oxidation of the protein backbone resulting in protein fragmentation. In an attempt to elucidate the products of the metal-catalyzed oxidation of the human (H) and mouse (M) (1-10H), (1-10M), (1-16H) and (1-16M) fragments of beta-amyloid peptide, the high performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) methods and Cu(II)/H(2)O(2) as a model oxidizing system were employed. Peptide solution (0.50 mM) was incubated at 37 degrees C for 24 h with metal:peptide:H(2)O(2) molar ratio 1:1:1 for the (1-16H), (1-16M) fragments, and 1:1:2 for the (1-10H), (1-10M) peptides in phosphate buffer, pH 7.4. Oxidation targets for all peptide studied are the histidine residues coordinated to the metal ions. For the (1-16H) peptide are likely His(13) and/or His(14), and for the (1-16M) fragment His(6) and/or His(14), which are converted to 2-oxo-His. Metal-binding residue, the aspartic acid (D(1)) undergoes the oxidative decarboxylation and deamination to pyruvate. The cleavages of the peptide bonds by either the diamide or alpha-amidation pathways were also observed.     

Acid-base properties and copper(II) complexes of dipeptides containing histidine and additional chelating bis(imidazol-2-yl) residues

Copper(II) complexes of dipeptides of histidine containing additional chelating bis(imidazol-2-yl) agent at the C-termini (PheHis-BIMA [N-phenylalanyl-histidyl-bis(imidazol-2-yl)methylamine] and HisPhe-BIMA [N-histidyl-phenylalanyl-bis(imidazol-2-yl)methylamine]) were studied by potentiometric, UV-Visible and Electron Paramagnetic Resonance (EPR) techniques. The imidazole nitrogen donor atoms of the bis(imidazol-2-yl)methyl group are described as the primary metal binding sites forming stable mono- and bis(ligand) complexes at acidic pH. The formation of a ligand-bridged dinuclear complex [Cu2L2]4+ is detected in equimolar solutions of copper(II) and HisPhe-BIMA. The coordination isomers of the dinuclear complex are described via the metal binding of the bis(imidazol-2-yl)methyl, amino-carbonyl and amino-imidazole(His) functions. In the case of the copper(II)-PheHis-BIMA system the [NH2, N-(amide), N(Im)] tridentate coordination of the ligand is favoured and results in the formation of di- and trinuclear complexes [Cu2H(-1)L]3+ and [Cu3H(-2)L2]4+ in equimolar solutions. The presence of these coordination modes shifts the formation of "tripeptide-like" ([NH2, N-, N-, N(Im)]-coordinated) [CuH(-2)L] complexes into alkaline pH range as compared to other dipeptide derivatives of bis(imidazol-2-yl) ligands. Although there are different types of imidazoles in these ligands, the deprotonation and coordination of the pyrrole-type N(1)H groups does not occur below pH 10.     

Coordination abilities of piperyd-1-yl-methane-1,1-diphosphonic acids towards zinc(II), magnesium(II) and calcium(II): potentiometric and NMR studies

The combination of the pH-metric and NMR studies is used to examine the stabilities and coordination modes as well as related structural aspects of zinc(II), magnesium(II) and calcium(II) complexation to piperyd-1-yl-methane-1,1-diphosphonic acid (1) and its derivatives containing a topologically modified piperidine ring (2-7). The studied compounds coordinate metal ions exclusively via the phosphonate functions with a nitrogen atom remaining protonated over the whole range of studied pH. Compounds 1-6 readily form soluble multinuclear complexes of type [M(3)(HL)(2)] and [M(3)(HL)(3)](3-) with Zn(2+) or [M(2)(H(2)L)(2)] with Ca(2+) and Mg(2+). These species are formed based on dimers consisting of two head-to-head arranged molecules linked by strong symmetrical hydrogen bonds. The placement of the two methyl groups at 2- and 6-positions on the piperidine ring precludes the molecular recognition via similar hydrogen bonds and accounts for different complexation properties of 7 compared to 1-6. The role that the metal coordination plays on conformation dynamics in 1-7 is also discussed.     

Coordination abilities of N-terminal fragments of alpha-synuclein towards copper(II) ions: a combined potentiometric and spectroscopic study

Copper(II) complexes of the 1-17 (MDVFMKGLSKAKEGVVA-NH(2)), 1-28 (MDVFMKGLSKAKEGVVAAAEKTKQGVAE-NH(2)), 1-39 (MDVFMKGLSKAKEGVVAAAEKTKQGVAEAPGKTKEGVLY-NH(2)) and 1-39 (A30P) fragments of alpha-synuclein were studied by potentiometric, UV-Vis (UV-visible), CD (circular dichroism) and EPR (electron paramagnetic resonance) spectroscopic methods to determine the stoichiometry, stability constants and coordination modes of the complexes formed. The beta-carboxylate group of Asp residue in second position of the peptide chain coordinates strongly to Cu(II) ion over the pH range 4-9.5 to give unusually stable 2N complex with {NH(2), N(-), beta-COO(-), H(2)O} coordination mode. At pH above 7 the results suggest the formation of 2N, 3N, 4N complexes (in equatorial plane) and the involvement of the lateral NH(2) group of Lys residue in the axial coordination of Cu(II) ion. In CD spectra sigma (epsilon-NH(2)-Lys)-->Cu(II) charge transfer transition is observed. Addition of the 18-28 and 18-39 fragments to the 1-17 peptide does not change the coordination mode and the 1-39 fragment forms the Cu(II) complexes with higher stabilities compared to those of the 1-17, 1-28 and 1-39(A30P) fragments of alpha-synuclein.     

N,N-bis(2-mercaptoethyl)methylamine: a new coligand for Tc-99m labeling of hydrazinonicotinamide peptides

Hydrazinonicotinamide (HYNIC) forms stable coordination complexes with Tc-99m when reacted with Tc(V)oxo species such as Tc-mannitol or other Tc-polyhydric complexes. However, radio-HPLC of [Tc-For-MLFK-HYNIC] labeled via Tc-polyhydric ligands demonstrated multiple radiochemical species each with unique biodistribution patterns. This is likely due to the fact that Tc can bind to the hydrazino moiety, as well as polyhydric ligands, in a variety of coordination geometries. Tridentate ligands, such as bis(mercaptoethyl)methylamine (NS2), may constrain the possible coordination geometries and improve overall stability. To investigate this, we synthesized NS2, converted the [Tc-mannitol-For-MLFK-HYNIC] to the corresponding NS2-containing complex [Tc-NS2-For-MLFK-HYNIC], and compared its infection imaging and biodistribution properties with [Tc-mannitol-For-MLFK-HYNIC]. Conversion to the NS2 complex was confirmed by HPLC which showed a single unique hydrophobic species with retention time greater than the [Tc-mannitol-For-MLFK-HYNIC] complex. Imaging experiments with both preparations were performed in rabbits with E. coli infections in the left thigh. Tissue radioactivity measurements demonstrated that compared to Tc-mannitol-peptide, accumulation of Tc-NS2-peptide was lower in blood, heart, and normal muscle and higher in spleen, infected muscle, and pus (p < 0.01). These results indicate that the Tc-NS2-peptide complex is chemically more homogeneous and exhibits improved infection localization and biodistribution properties. In an effort to model the interactions of the metal-HYNIC core with NS2 and related ligand types, the reactions of [ReCl3(NNC5H4NH)(NHNC5H4N)] and [99TcCl3(NNC5H4NH)(NHNC5H4N)], effective structural analogues for the [M(NNC5H4NH(x))2] core, with NS2, C5H3N-2,6-(CH2SH)2, O(CH2CH2SH)2, and S(CH2CH2SH)2 were investigated and the compounds [M[CH3N(CH2CH2S)2](NNC5H4N)(NHNC5H4N] (M = 99Tc (5a), Re (5b)), [Re[C5H3N-2,6-(CH2S)2](NNC5H4N)(NHNC5H4N)].CH2Cl2.0.5MeOH (7), [Re[SCH2CH2)2O] (NNC5H4N)(NHNC5H4N)] (8), and [Re[(SCH2CH2)2S](NNC5H4NH)(NHNC5H4N)]Cl (9) were isolated. Similarly, the reaction of [ReCl3(NNC5H4NH)(NHNC5H4N)] with the bidentate ligands pyridine-2-methanethiol and 3-(trimethlysilyl)pyridine-2-thiol led to the isolation of [ReCl(C5H4N-2-CH2S) (NNC5H4N)(NHNC5H4N)] (10) and [Re(2-SC5H3N-3-SiMe3)2 (NNC5H4N)(NHNC5H4N)] (11), respectively, while reaction with N-methylimidazole-2-thiol yielded the binuclear complex [Re(OH)Cl(SC3H2N2CH3)2(NNC5H4N)2 (NHNC5H4N)2] (12). The analogous metal-(HYNIC-OH) precursor, [ReCl3[NNC5H3NH(CO2R)] [NHNC5H3N(CO2R)]] (R = H, 13a; R = CH3, 13b) has been prepared and coupled to lysine to provide [RCl3[NNC5H3NH(CONHCH2CH2CH2CH2CH(NH2)CO2H)] [NHNC5H3NH(CONHCH2CH2CH2CH2CH(NH2)CO2H)]].2HCl (14.2HCl), while the reaction of the methyl ester 13b with 2-mercaptopyridine yields [Re(2-SC5H4N)2[NNC5H3N(CO2Me)][NHNC5H3N(CO2Me)]] (15). While the chemical studies confirm the robustness of the M-HYNIC core (M = Tc, Re) and its persistence in ligand substitution reactions at adjacent coordination sites of the metal, the isolation of oligomeric structures and the insolubility of the peptide conjugates of 13, 14, and 15 underscore the difficulty of characterizing these materials on the macroscopic scale, an observation relevant to the persistent concerns with reagent purity and identity on the tracer level.     

Synthesis,molecular structure,solution equilibrium,and antiproliferative activity of thioxotriazoline and thioxotriazole complexes of copper II and palladium II

Preparations of copper(II) and palladium(II) complexes of 4-amino-5-methylthio-3-(2-pyridyl)-1,2,4-triazole (L(1)) and the copper(II) complex of 1,4-dihydro-4-amino-3-(2-pyridyl)-5-thioxo-1,2,4-triazole (HL) are described. These complexes have been characterized by means of spectroscopy and microanalysis. Molecular structures of HL (1), [CuCl(2)(H(2)L)]Cl.2H(2)O (2a), cis-[CuCl(2)(L(1))] (3), and cis-[PdCl(2)(L(1))] (4) have been determined by single-crystal X-ray diffraction. The HL ligand acts as a N,S bidentate through the thioxo moiety and the exo-amino group whilst the ligand L(1) forms N,N coordination complexes through the pyridine and triazole nitrogen atoms. Speciation in solution of the systems Cu/HL and Cu/L(1) have been determined by means of potentiometry and spectrophotometry as well as for the Cu/L(1)/A (HA=glycine) system in order to determine species present at physiological pH. Antiproliferative activity of these complexes and their ligands was evaluated, using the AlamarBlue Assay, on normal human fibroblasts (HF) and human fibrosarcoma tumor (HT1080) cells. The copper compounds cis-[CuCl(2)(H(2)L)]Cl and cis-[CuCl(2)(L(1))] exerted significant antiproliferative activity of both normal and neoplastic cells; although dose-response experiments revealed that the HT1080 cell line was more sensitive to the tested drugs than normal fibroblasts.     

The influence of methionine-containing peptides on the reaction of carboplatin with 5'-guanosine monophosphate: a comparison with cisplatin

The pH- and time-dependent reaction of the anticancer drug carboplatin, [Pt(cbdca-kappa(2)O,O')(NH(3))(2)] (cbdca=cyclobutane-1,1-dicarboxylate), with the tripeptides H-glyglymet-OH (glycylglycyl-L-methionine) and Ac-glyglymet-OH at 313 K was investigated by high-performance liquid chromatography, NMR and mass spectrometry. The relative stability of the initial ring-opened kappaS complex [Pt(cbdca-kappaO)(Ac-glyglymet-OH-kappaS)(NH(3))(2)] leads to increased formation of the kinetically favoured kappaS:kappaS' bis-adduct [Pt(Ac-glyglymet-OH-kappaS)(2)(NH(3))(2)](2+) in comparison with cisplatin. As a result a second 1:2 reaction pathway kappaS-->kappaS:kappaS'-->kappa(2)N(M), S:kappaS'-->kappa(3)N(G2),N(M), S:kappaS', where N(M) and N(G2) represent, respectively, metallated methionine and glycine nitrogen atoms, competes with the 1:1 route kappaS-->kappa(2)N(M), S-->kappa(3)N(G2),N(M), S also observed for cisplatin. Cleavage of N-acetylglycine at the backbone C(O)-N bond to the second gly residue (G2) is observed after 100 h for the respective tridentate complexes [Pt(Ac-glyglyH(-1)metH(-1)-OH-kappa(3)N(G2),N(M), S) (Ac-glyglymet-OH-kappaS)] and [Pt(Ac-glyglyH(-1)metH(-1)-OH-kappa(3)N(G2),N(M), S)(NH(3))] at pH <5.2. The longevity of the initial kappaS complex leads to about an eight-fold increase in the rate of formation of the kappaN7:kappaN7' bis-adduct [Pt(5'-GMP-kappaN7)(2)(NH(3))(2)](2-) for the reaction of carboplatin with 5'-GMP(2-) at pH 7 in the presence of Ac-glyglymet-OH. A mixed-ligand kappaS:kappaN7 species [Pt(5'-GMP-kappaN7)(Ac-glyglymet-OH-kappaS)(NH(3))(2)] provides the major precursor for this 1:2 nucleotide complex and kappaN7 coordination of 5'-GMP(2-) is also observed in the kappa(2)N(M),S:kappaN7 complex [Pt(5'-GMP-kappaN7)(Ac-glyglymetH(-1)-OH-kappa(2)N(M),S)(NH(3))(2)](-) formed by substitution of the ammine ligand trans to the methionine sulphur. As the intermediate kappaS:kappaN7 species is formed rapidly within the first 10 h of reaction, these results suggest that the transfer reaction pathway kappaS-->kappaS:kappaN7-->kappaN7:kappaN7' involving kappaS platinated peptides could play an important role in accelerating the rate of DNA binding for carboplatin.     

Synthesis, structure, and reactivity of monofunctional platinum(II) and palladium(II) complexes containing the sterically hindered ligand 6-(methylpyridin-2-yl)acetate

Three complexes containing the novel, sterically hindered ligand 6-(methylpyridin-2-yl)acetate (PICAC) have been synthesized and characterized: [Pt(NH3)2(PICAC-N,O)]NO3 (1), [Pt(en)(PICAC-N,O)]NO3 (2), and [Pd(en)(PICAC-N,O)]NO3 (3) (en = ethane-1,2-diamine). The crystal structures of 2 and 3 have been determined. The two complexes are isostructural and exhibit a mixed [N3O] coordination. In both cases, PICAC forms a sterically crowded six-membered chelate. Signal multiplicities in 1H NMR spectra of 1-3 indicate that the N,O chelates are conformationally rigid on the NMR timescale as a result of the steric bulk of the pyridine derivative. Complex 2 undergoes facile ring opening in 0.1M NaCl solution at neutral pH, resulting in a zwitterionic species in which carboxylate oxygen has been replaced with chloride. The complex was identified by X-ray crystallography as [PtCl(en)(PICAC-N)] x H2O (4), which contains a "dangling" carboxylate group. In 4, the pyridine moiety adopts an almost perpendicular orientation relative to the platinum coordination plane. Likewise, complex 2 reacts rapidly with 5'-guanosine monophosphate (5'-GMP) to form the monofunctional adduct [Pt(en)(PICAC)(5'-GMP)] (5) (NMR, 25 degrees C, t(1/2) approximately 24 min). 2-D nuclear Overhauser enhancement spectroscopy (NOESY) and double quantum-filtered correlated spectroscopy (dqf-COSY) experiments (500 MHz) and variable temperature NMR spectroscopy confirm that adduct 5 exists as a 1:1 mixture of rotamers in solution as a result of the mutual repulsion between the cis-oriented pyridine and guanine bases. While 2 readily reacts with DNA nitrogen, its monofunctional adducts show no significant effect on the conformation of native DNA. Circular dichroism (CD) spectra recorded of platinum-modified calf-thymus DNA suggest that the structural damage produced by complex 2 does not mimic that produced by the clinical agent. Both the unusual reactivity and the inability to induce cisplatin-like DNA conformational changes are proposed to be responsible for the marginal biological activity of the new complexes.     

The binding of Ni(II) ions to hexahistidine as a model system of the interaction between nickel and His-tagged proteins

The aim of this work is to study the binding of nickel ions to hexahistidine (His(6)) combining potentiometric titrations and spectroscopic (UV-Vis and circular dichroism) determinations in order to establish the species distribution as a function of the pH, their stoichiometry, stability and geometry. For comparative purposes, the same procedure was applied to the Ni-histidine (His) system. His behaves as a tridentate ligand, coordinating the carboxyl group, the imidazole and the amino nitrogen atoms to Ni(II) ions in an octahedral coordination and a bis(histidine) complex is formed at pH higher than 5. For the Ni-His(6) system, the complex formation starts at pH 4 and five different species (Ni(His(6))H, Ni(His(6)), Ni(n)(His(6))(n), Ni(n)(His(6))(n)H(-n/2), Ni(n)(His(6))(n)H(-n)) are formed as a function of the pH. Ni(His(6))H involves the coordination of the imidazole nitrogen and a deprotonated amide nitrogen (N(Im), N(-)) resulting in an octahedral geometry. In Ni(His(6)), an imidazole nitrogen is deprotonated and coordinated (2N(Im), N(-)) to the metal ion with a square planar geometry. The aggregated forms result from the extra Ni-N(Im) coordination, resulting in a 4N square planar geometry that is stabilized by inter/intramolecular hydrogen bonds. This coordination mode is not altered during the deprotonation steps from Ni(n)(His(6))(n).     

The interaction of beta-amyloid protein fragment (12-28) with lipid environments.

The neurotoxicity of beta-amyloid protein (beta AP) fragments may be a result of their solution conformation, which is very sensitive to solution conditions. In this work we describe NMR and CD studies of the conformation of beta AP(12-28) in lipid (micelle) environments as a function of pH and lipid type. The interaction of beta AP(12-28) with zwitterionic dodecylphosphocholine (DPC) micelles is weak and alters the conformation when compared to water solution alone. By contrast, the interaction of the peptide with anionic sodium dodecylsulfate (SDS) micelles is strong: beta AP(12-28) is mostly bound, is alpha-helical from K16 to V24, and aggregates slowly. The pH-dependent conformation changes of beta AP(12-28) in solution occur in the pH range at which the side-chain groups of E22, D23, H13, and H14 are deprotonated (pKas ca. 4 and 6.5); the interaction of beta AP(12-28) with SDS micelles alters the pH-dependent conformational transitions of the peptide whereas the weak interaction with DPC micelles causes little change.     

New unsymmetric dinuclear Cu(II)Cu(II) complexes and their relevance to copper(II) containing metalloenzymes and DNA cleavage

The new homodinuclear complexes, [Cu(2)(II)(HLdtb)(mu-OCH(3))](ClO(4))(2) (1) and [Cu(2)(II)(Ldtb)(mu-OCH(3))](BPh(4)) (2), with the unsymmetrical N(5)O(2) donor ligand (H(2)Ldtb) - {2-[N,N-Bis(2-pyridylmethyl)aminomethyl]-6-[N',N'-(3,5-di-tert-butylbenzyl-2-hydroxy)(2-pyridylmethyl)]aminomethyl}-4-methylphenol have been synthesized and characterized in the solid state by X-ray crystallography.In both cases the structure reveals that the complexes have a common {Cu(II)(mu-phenoxo)(mu-OCH(3))Cu(II)} structural unit.Magnetic susceptibility studies of 1 and 2 reveal J values of -38.3 cm(-1) and -2.02 cm(-1), respectively, and that the degree of antiferromagnetic coupling is strongly dependent on the coordination geometries of the copper centers within the dinuclear {Cu(II)(mu-OCH(3))(mu-phenolate)Cu(II)} structural unit.Solution studies in dichloromethane, using UV-Visible spectroscopy and electrochemistry, indicate that under these experimental conditions the first coordination spheres of the Cu(II) centers are maintained as observed in the solid state structures, and that both forms can be brought into equilibrium ([Cu(2)(HLdtb)(mu-OCH(3))](2+)=[Cu(2)(Ldtb)(mu-OCH(3))](+)+H(+)) by adjusting the pH with Et(3)N (Ldtb(2-) is the deprotonated form of the ligand).On the other hand, potentiometric titration studies of 1 in an ethanol/water mixture (70:30 V/V; I=0.1M KCl) show three titrable protons, indicating the dissociation of the bridging CH(3)O(-) group.The catecholase activity of 1 and 2 in methanol/water buffer (30:1 V/V) demonstrates that the deprotonated form is the active species in the oxidation of 3,5-di-tert-butylcatechol and that the reaction follows Michaelis-Menten behavior with k(cat)=5.33 x 10(-3)s(-1) and K(M)=3.96 x 10(-3)M. Interestingly, 2 can be electrochemically oxidized with E(1/2)=0.27 V vs.Fc(+)/Fc (Fc(+)/Fc is the redox pair ferrocinium/ferrocene), a redox potential which is believed to be related to the formation of a phenoxyl radical.Since these complexes are redox active species, we analyzed their activity toward the nucleic acid DNA, a macromolecule prone to oxidative damage.Interestingly these complexes promoted DNA cleavage following an oxygen dependent pathway.     

Synthesis, characterization, and biological evaluation of neutral nitrido technetium(V) mixed ligand complexes containing dithiolates and aminodiphosphines. A novel system for linking technetium to biomolecules

A new biomolecule labeling method that utilizes the [(99m)Tc(N)(PNP)](2+) metal fragment is presented. Thus, a series of nitrido mixed-ligand M(V) complexes (M = (99m)Tc, (99g)Tc, Re), [M(N)(Ln)(PNP)], where Ln is the dianionic form of a dithiolate or substituted-dithiolate ligand and PNP is an aminodiphosphine, is described. (99m)Tc complexes can be prepared using either a two-step or a three-step procedure starting from generator-eluted pertechnetate through a prereduced mixture of [(99m)Tc(N)]-containing species, followed by sequential or contemporary addition of the relevant dithiolate and aminodiphosphine. The reactions of 2,3-dimercaptopropionic acid (H(2)L1) with [Tc(N)(PNP)](2+) were investigated in detail. It was found that this bidentate ligand coordinated the metal fragment through the [S(-),S(-)] donor atom pair, to yield neutral mixed-ligand complexes [(99m)Tc(N)(L1)(PNP)] in high specific activity. The additional carboxylic functional group was not involved in metal coordination, thus remaining available for conjugation to target-specific molecules. Dithiolates incorporating pendant functional group(s) gave rise to a 1:1 diastereoisomeric mixture of syn-[M(N)(Ln)(PNP)] and anti-[M(N)(Ln)(PNP)] derivatives, depending on the relative orientation of the dithiolate substituent(s) with respect to the terminal nitrido group, and no isomeric conversion was detected. (99m)Tc species had been proven to be identical with the (99g)Tc complexes prepared at the macroscopic level by comparison of the corresponding radiometric and UV/vis HPLC profiles. Challenge experiments with cysteine or glutathione indicated that these physiological agents had no effect on the stability of this class of mixed-ligand (99m)Tc-complexes. Biodistribution studies in rats of selected (99m)Tc-complexes showed a rapid clearance from the blood and tissues after 60 min pi.     

The hydrolysis of the anti-cancer ruthenium complex NAMI-A affects its DNA binding and antimetastatic activity: an NMR evaluation

The coordination of the antimetastatic agent NAMI-A, [H(2)im][trans-RuCl(4)(dmso-S)(Him)], (Him=imidazole; dmso=dimethyl sulfoxide), to the DNA model base 9-methyladenine (9-MeAde) was investigated in water. NMR spectroscopy was first applied for the study of the molecular stability and hydrolysis of NAMI-A in aqueous solution over a range of pH (3.0-7.4) and chloride ion concentrations (0-1 M) at 37.0 degrees C. In physiological conditions (phosphate buffer, pH 7.4) NAMI-A disappears from the solution in 15 min due to chloride and dmso hydrolysis, leading to uncharacterised poly-oxo Ru species. Conversely, at lower pH (3.0-6.0) and in water (pH approximately 5.5), only a partial dmso hydrolysis occurs, slowly forming the [trans-RuCl(4)(H(2)O)(Him)](-) complex. This latter species coordinates to 9-MeAde (via the N7 of 9-MeAde), forming the [trans-RuCl(4)(9-MeAde)(Him)](-) complex. NAMI-A and [trans-RuCl(4)(H(2)O)(Him)](-) give comparable intracellular ruthenium concentrations and accumulate in KB cells (human mouth carcinoma) and accumulate these at the G(2)/M phase, while poly-oxo Ru species do not, and their cell uptake is reduced to 50%. On the contrary, G(2)/M arrest and protein content in the murine metastatic cell line metGM, are not influenced by NAMI-A hydrolysis. Hydrolysed NAMI-A species apparently are easier taken up by the metGM cells, showing intracellular ruthenium concentrations one order of magnitude greater than those of intact NAMI-A. Therefore, it is proposed that the selective antimetastatic activity of NAMI-A during in vivo experiments can be attributed to its hydrolysed species.     

A novel approach to the high-specific-activity labeling of small peptides with the technetium-99m fragment [99mTc(N)(PXP)]2+ (PXP = diphosphine ligand).

A new labeling approach for incorporating bioactive peptides into a technetium-99m coordination complex is described. This method exploits the chemical properties of the novel metal-nitrido fragment [99mTc(N)(PXP)]2+, composed of a terminal Tc[triple bond] N multiple bond bound to an ancillary diphosphine ligand (PXP). It will be shown that this basic, molecular building block easily forms in solution as the dichloride derivative [99mTc(N)(PXP)Cl2], and that this latter complex selectively reacts with monoanionic and dianionic, bidentate ligands (YZ) having soft, pi-donor coordinating atoms to afford asymmetrical nitrido heterocomplexes of the type [99mTc(N)(PXP)(YZ)]0/+ without removal of the basic motif [99mTc(N)(PXP)]2+. The reactions of the amino acid cysteine was studied in detail. It was found that cysteine readily coordinates to the metal fragment [99mTc(N)(PXP)]2+ either through the [NH2, S-] pair of donor atoms or, alternatively, through the [O-, S-] pair, to yield the corresponding asymmetrical complexes in very high specific activity. Thus, these results were conveniently employed to devise a new, efficient procedure for labeling short peptide sequences having a terminal cysteine group available for coordination to the [99mTc(N)(PXP)]2+ fragment. Examples of the application of this novel approach to the labeling of the short peptide ligand H-Arg-Gly-Asp-Cys-OH (H(2)1) and of the peptidomimetic derivative H-Cys-Val-2-Nal-Met-OH (H2) will be discussed.     

Solution equilibrium studies on metal complexes of 2,3-dihydroxy-phenylalanine-hydroxamic acid (Dopaha) and models: catecholate versus hydroxamate coordination in iron(III)-, aluminium(III)- and molybdenum(VI)-Dopaha complexes

Equilibrium results based on pH potentiometric, spectrophotometric and (1)H NMR measurements for the complexes of Fe(III), Al(III) and Mo(VI) with 2,3-dihydroxy-phenylalanine-hydroxamic acid (Dopaha) as well as for binary model systems Fe(III)-, Al(III)-, Mo(VI)-acetohydroxamic acid (Aha), -alpha-alaninehydroxamic acid (alpha-Alaha) and -1,2-dihydroxy-3,5-benzene-disulphonate (Tiron) and ternary model systems Fe(III)-, Al(III)-, Mo(VI)-Tiron-Aha, are summarized in this paper. The amine-type coordination mode is not detectable with these metal ions at all. Precipitation occurs at pH <5.5 with Fe(III) and Al(III) even at a Dopaha-to-metal ion ratio of 10:1. Hydroxamate-type coordination was demonstrated with both metals below the pH range of precipitation but, after dissolution, catecholate-type coordination was exclusively found. The hydroxamate-type coordination mode occurs only in the very acidic pH range for Mo(VI) complexes and the crossover from hydroxamate to catecholate binding occurs at pH >3. A ligand-bridged dinuclear species, [(MoO(2))(2)(Dopaha)(2)](2+), involving mixed-type (catecholate and hydroxamate) coordination modes is formed in the pH range 2.5-5.5. [MoO(2)A(2)H(2)], with catecholate-type coordination, forms above pH 3. On increasing the pH further, deprotonation of the coordinated Dopaha and hydrolytic processes result in the formation of catecholate-coordinated [MoO(3)AH] and [MoO(3)A]. MoO(4)(2-) and free Dopaha exist above pH 10.     

Strong protective action of Copper(II) N-substituted sulfonamide complexes against reactive oxygen species

Copper(II) complexes of N-benzothiazolsulfonamides, [Cu(N-2-(5,6-dimethylbenzothiazole)toluenesulfonamidate)(2)(dmso)(2)] (1), [Cu(N-2-(6-chlorobenzothiazole)benzenesulfonamidate)(2)(dmso)(2)] (2) and [Cu(N-2-(6-chlorobenzothiazole)toluenesulfonamidate)(2)(dmso)(2)] (3) with interesting protective properties against superoxide radicals have been prepared. The compounds have been characterized by X-ray diffraction and their chemical properties have been studied by spectroscopic methods. The crystal structure of 1 shows that the copper(II) is surrounded by two benzothiazole N atoms from the sulfonamide ligands and two O atoms from the dimethylsulfoxide molecules in a square planar arrangement. The coordination polyhedron around copper(II) in 2 and 3 is distorted square pyramidal being the metal ion linked to benzothiazole N and sulfonamidate O atoms of the ligand and to two dimethylsulfoxide O atoms. The three complexes have a strong protective action over Delta sod1 mutant of Saccharomyces cerevisiae against reactive oxygen radicals derived from respiration and against those generated by hydrogen peroxide and menadione.     

The interaction of 5-fluoroorotic acid with transition metals: synthesis and characterisation of Ni(II), Cu(II) and Zn(II) complexes

5-Fluoroorotic acid (H(3)FOro) is a potent inhibitor for some metalloproteins such as dihydroorotase and dihydroorotate dehydrogenase and for thymidylate synthase (nonmetalloprotein) in the human malaria parasite Plasmodium falciparum. To study the coordination chemistry of H(3)Foro, the ammonium salt [NH(4)(+)][H(2)FOro(-)].1H(2)O (1) and the first coordination compounds of H(3)FOro with transition metals [Ni(HFOro(2-))(H(2)O)(4)].1H(2)O (2), [Cu(HFOro(2-))(NH(3))(H(2)O)](n) (3) and [Cu(3)(FOro(3-))(2)(NH(3))(6)(H(2)O)(2)] (4) have been synthesised and characterised by single-crystal X-ray diffraction, IR spectroscopy and by thermogravimetry. Three different coordination modes of 5-fluoroorotic acid have been established. In all cases the ligand is chelated to the metal via an amido-nitrogen and a carboxylate-oxygen but for (3), there is also a carboxylate oxygen from another HFOro(2-) ligand resulting in a polymeric structure and for (4), the second amido-nitrogen in the ororotic acid ring coordinates to give a trinuclear complex. The metal coordination polyhedra are octahedral in (2), square-pyramidal in (3) and square-planar and approximately square-pyramidal in (4). An octahedral coordination geometry including a N(1)/O(61)-chelating HFOro(2-) ligand with four aqua ligands is proposed for the Zn complex [Zn(HFOro(2-)) (H(2)O)(4)].0.5H(2)O (5), based on IR and thermogravimetric data. Extensive hydrogen bonded networks and some ring-ring stacking interactions are observed in each of the structures.     

Catechol oxidase-like oxidation chemistry of the 1-20 and 1-16 fragments of Alzheimer's disease-related beta-amyloid peptide: their structure-activity correlation and the fate of hydrogen peroxide

The Cu2+ complexes of the 1-16 and the 1-20 fragments of the Alzheimer's disease-related beta-amyloid peptide (CuAbeta) show significant oxidative activities toward a catechol-like substrate trihydroxylbenzene and plasmid DNA cleavage. The latter reflects possible oxidative stress to biological macromolecules, yielding supporting data to the pathological role of these soluble Abeta fragments. The former exhibits enzyme-like kinetics and is dependent on [H2O2], exhibiting k(cat) of 0.066 s-1 (6000-fold higher than the reaction without CuAbeta) and k(cat)/Km of 37.2 m-1s-1 under saturating [H2O2] of approximately 0.24%. This kinetic profile is consistent with metal-centered redox chemistry for the action of CuAbeta. A mechanism is proposed by the use of the catalytic cycle of dinuclear catechol oxidase as a working model. Trihydroxylbenzene is also oxidized by CuAbeta aerobically without H2O2, affording rate constants of 6.50x10(-3) s-1 and 3.25 m-1s-1. This activity is also consistent with catechol oxidase action in the absence of H2O2, wherein the substrate binds and reduces the Cu2+ center first, followed by O2 binding to afford the mu-eta2:eta2-peroxo intermediate, which oxidizes a second substrate to complete the catalytic cycle. A tetragonally distorted octahedral metal coordination sphere with three coordinated His side chains and some specific H-bonding interactions is concluded from the electronic spectrum of CuAbeta, hyperfine-shifted 1H NMR spectrum of CoAbeta, and molecular mechanics calculations. The results presented here are expected to add further insight into the chemistry of metallo-Abeta, which may assist better understanding of the neuropathology of Alzheimer's disease.