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The characterization of the copper(II) complex species formed with the PrP106-126 peptide bearing both C- and N-termini protected by acetylation and amidation, respectively (HuPrPAc106-126NH2) is reported. The results concerning the copper(II) complexes with Hu-PrPAc106-126NH2 and the related fragments allow a reliable comparison between the metal ions affinities of the two binding sites located within the disordered region, showing the coordination features of the main species formed in the physiological pH range.

A microequilibrium analysis of a series of amino acid and peptide derivatives containing the chelating bis(imidazol-2-yl)methyl group is presented and measurement were performed in D(sub 2)O to follow the deprotonation steps. The main advantage of the method is that it does not require the synthesis and NMR microequilibrium analysis of substances modeling the individual parts of the target ligand, in contrast to the method used by others.

The compound MeRe(S)(mtp)(PPh3) where mtpH2 is 2-(mercaptomethyl) thiophenol was used to catalyse the reaction between pyridine N-oxides, PyO and triphenylphosphine. A chain mechanism with three steady-state thiorhenium species as chain carriers is implicated.

Complex formation processes between the 39-mer residue peptide fragment of human prion protein, HuPrP(76-114), and copper(ll) ions have been studied by potentiometric, UV-vis, circular dichroism (CD), electron paramagnetic resonance, and electrospray ionization mass spectrometry methods. This peptide consists of 39 amino acid residues and contains two histidines (His77 and His85) belonging to the octarepeat domain and two histidines (His96 and His111) outside this domain. It was found that HuPrP(76-114) is able to bind 4 equiv of metal ions and all histidyl residues are independent, except nonequivalent metal binding sites in the oligonuclear species. Imidazole nitrogen donor atoms are the primary and exclusive metal binding sites below pH 5.5 in the form of various macrochelates. The macrochelation slightly suppresses, but cannot prevent, the deprotonation and metal ion coordination of amide functions, resulting in the formation of (Nim,N-), (Nim,N-,N-), and (Nim,N-,N-,N-)-coordinated copper(ll) complexes in the pH range from 5.5 to 9. CD spectroscopy results gave clear evidence for the differences in the metal binding affinity of the histidyl sites according to the following order: His111 > His96 ⪢ His77 ∼ His85. Among the oligonuclear complexes, the formation of di- and tetranuclear species seems to be favored over the trinuclear ones, at pH values beyond the physiological ones. This phenomenon was not observed in the complex formation reactions of HuPrP(84-114), a peptide fragment containing only one histidyl residue from the octarepeat. As a consequence, the data support the existence of cooperativity in the metal binding ability of this peptide probably due to the presence of two octarepeat sequences of the dimeric octarepeat domain of HuPrP(76-114) at basic pH values. [ABSTRACT FROM AUTHOR]

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), cobalt(ii) and cadmium(ii) complexes of terminally protected multihistidine peptides including Ac-HGH-OH, Ac-HGH-NHMe, Ac-HHGH-OH, Ac-HAHVH-NH(2), Ac-HVHGH-NH(2), Ac-HGHVH-NH(2) and Ac-(His-Sar)(n)-His-NH(2) (n = 1, 2 or 3) were studied by potentiometric, UV-Vis, CD and (1)H NMR spectroscopic techniques. It was found that the complexes in which the histidine imidazole nitrogens coordinate with ML stoichiometry are the main species in the physiological pH-range in all cases. The stability of these complexes is determined by the number of bound imidazole rings, the presence of the carboxylate group and the quality of the metal ion centre. The larger the number of coordinated imidazole-N donor atoms, the higher the stability of the complex. The stability constants of the ML complexes follow the Ni(ii) > Co(ii) approximately Cd(ii) order. Cobalt(ii) and cadmium(ii) are not, but nickel(ii) is able to promote the deprotonation and the coordination of amide nitrogens and NiH(-2)L and NiH(-3)L (Ni(2)H(-4)L) species predominate in basic solutions. For the pentapeptides with the exception of the sarcosine containing ligand the presence of coordination isomers is supported by spectroscopic methods. These data reveal that the favoured isomers are coordinated on the C-termini, but the ratio of isomers depends on the sequence of peptides.

Macroscopic and microscopic protonation processes and zinc(II) complexes of a series of multihistidine peptides (Ac-HGH-OH, Ac-HGH-NHMe, Ac-HHGH-OH, Ac-HHGH-NHMe, Ac-HVGDH-NH(2), Ac-HHVGD-NH(2), Ac-HVHAH-NH(2), Ac-HAHVH-NH(2), Ac-HPHAH-NH(2) and Ac-HAHPH-NH(2)) were studied by potentiometric, NMR and ESI-MS spectroscopic techniques. Protonations of histidyl imidazole-N donor functions were not much affected by the number and location of histidyl residues, but the presence of C-terminal carboxylate groups had a significant impact on the basicities of the neighbouring histidyl sites. The formation of 2N(im) and 3N(im) macrochelates with the stoichiometry of [ZnL] was the major process in the complexation reactions of all peptides followed by the formation of hydroxo or amide bonded species. Thermodynamic stabilities of the zinc(II) complexes were primarily determined by the number of histidyl residues, but the presence of C-terminal carboxylate functions has also a significant contribution to metal binding. The stabilizing effect of the aspartyl beta-carboxylate group was also observed, but its extent is much weaker than that of the C-terminal carboxylate with a neighbouring histidyl residue. Zinc(II) promoted peptide amide deprotonation and co-ordination was observed only in the zinc(II)-Ac-HHVGD-NH(2) system above pH 8.

A 31-mer polypeptide, which encompasses residues 84-114 of human prion protein HuPrP(84-114) and contains three histidyl residues, namely one from the octarepeat (His85) and two histidyl residues from outside the octarepeat region (His96 and His111), and its mutants with two histidyl residues HuPrP(84-114)His85Ala, HuPrP(84-114) His96Ala, HuPrP(84-114)His111Ala and HuPrP(91-115) have been synthesised and their Cu2+ complexes studied by potentiometric and spectroscopic (UV/Vis, CD, EPR, ESI-MS) techniques. The results revealed a high Cu2+-binding affinity of all peptides, and the spectroscopic studies made it possible to clarify the coordination mode of the peptides in the different complex species. The imidazole nitrogen donor atoms of histidyl residues are the exclusive metal-binding sites below pH 5.5, and they have a preference for macrochelate structure formation. The deprotonation and metal-ion coordination of amide functions take place by increasing the pH; all of the histidines can be considered to be independent metal-binding sites in these species. As a consequence, di- and trinuclear complexes can be present even in equimolar samples of the metal ion and peptides, but the ratios of polynuclear species do not exceed the statistically expected ones; this excludes the possibility of cooperative Cu2+ binding. The species with a (N(im),N,N)-binding mode are favoured around pH 7, and their stability is enhanced by the macrochelation from another histidyl residue in the mononuclear complexes. The independence of the histidyl sites results in the existence of coordination isomers and the preference for metal binding follows the order of: His111>His96>His85. Deprotonation and metal-ion coordination of the third amide functions were detected in slightly alkaline solutions at each of the metal-binding sites; all had a (N(im),N,N,N)-coordination mode. Spectroscopic measurements also made it clear that the four lysyl amino groups of the peptides are not metal-binding sites in any cases.

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.