Arthur Gergely

Studies on transition-metal–peptide complexes. Part 9. Copper(II) complexes of tripeptides containing histidine

Copper(II) complexes of the histidine-containing tripeptides glyclyl-L-histidylglycine (GlyHisGly), glycylglycyl-L-histidine (GlyGlyHis), and L-pyroglutamyl-L-histidyl-L-prolinamide (trf), and the dipeptide L-pyroglutamyl-L-histidine methyl ester (pghme), have been studied by pH-metric and spectrophotometric methods. It was found that, similarly as for glycyl-L-histidine (GlyHis), GlyHisGly forms a complex [CuAH–1], but bis complexes are also formed in low concentration. For GlyGlyHis, only the highly stable species [CuAH–2]– is formed. A complex [CuAH–2]– is also formed with trf and pghme which indicates that the prolinamide side-chain in trf does not take part in the co-ordination. The sequence of deprotonation of the N1H group of the imidazole side-chain is as follows: GlyHisGly > GlyHis > pghme > trf > GlyGlyHis.

Thermodynamic study of the parent and mixed complexes of aspartic acid, glutamic acid and glycine with copper(II)

Abstract The equilibrium constants and in part the ΔH and ΔS data for the parent and mixed complexes copper(II)-aspartic acid, copper(II)-glutamic acid, copper(II)-aspartic acid-glycine, copper(II)-glutamic acid-glycine and copper(II)-aspartic acid-glutamic acid were determined pH-metrically and calorimetrically. Protonated complexes are formed in significant concentration in the copper(II)-aspartic acid and copper(II)-glutamic acid systems. The thermodynamic data led to the assumption that in the CuA complex of aspartic acid the β-carboxyl group of the ligand also forms a bond with the metal. The glutamic acid is bound “glycine-like” to the copper(II). There is an appreciable stabilization in the copper(II)-aspartic acid-glycine mixed complex, while in the copper(II)-glutamic acid-glycine system the stability of the mixed complex agrees with the statistical value. The above statements are supported by the dependence of the stability constants on the ionic strength.

Complexes of 3,4-dhydroxyphnyl derivatives, III. Equilibrium study of parent and some mixed ligand complexes of dopamine, alanine and pyrocatechol with nickel(II), copper(II) and zinc(II) ions

Abstract The equilibrium constant of the parent complexes of nickel(II), copper(II) and zinc(II) with dopamine, and of the mixed ligand complexes wiht alanine or pyrocatechol as ligand B, were determined pH-metrically at 25°C and 0.2 M ionic strength. As regards the parent complexes, it was found that only the ortho phenolic hydroxy groups take part in hte coordination. The 1:1 complexes MHA and MA, and the 1:2 complexes MH2A2, MHA2 and MA2 are formed. It was demostrated that the stability relations of the mixed complexes formed with pyrocatechol as ligand B correspond to the statistical case, as a consequence of the values of logK1/K2 of the parent complexes being almost the same. An increased stability compared to the statistical case was observed for mixed complexes with alanine. This was interpreted by the different stability relations of the parent complexes and by the role of neutralization of charge for the complexes of all three metal ions.

Log β, ΔH and ΔS values of mixed complexes of Cu(II) with histamine and some aliphatic aminoacids

Abstract The stability constants of the parent and mixed complexes of Cu(II) with histamine, and glycine, alanine, α-aminobutyric acid or norvaline have been determined potentiometrically. The ΔH and ΔS values have been obtained by calorimetry. From the thermodynamic data it has been found that the stability of the Cu(II)-histamine parent complex is partly determined by back-coordination. The stabilities of the Cu(II)-histamine-aminoacid mixed complexes are much higher than the statistically expected values. From the formational enthalpy values of the mixed complexes it may be concluded that the bond strengths are not equal to the average of the bond strengths in the Cu A 2 and Cu B 2 type parent complexes. The electrostatic characters of the Cu(II)-aminoacid bonds in the mixed complexes are substantially increased compared with those in the parent complexes. The entropy change of the mixed complex formation is higher by about ten e.u. than the statistically expected value. The enthalpy changes ΔH M of the reactions Cu A 2 + Cu B 2 ⇌ 2Cu AB ( A = aminoacid; B = histamine) are positive and depend on the number of carbon atoms in the aminoacid.

Studies on transition-metal–peptide complexes. Part 8. Parent and mixed-ligand complexes of histidine-containing dipeptides

The glycyl-L-histidine (GlyHis), L-histidylglycine (HisGly), and L-carnosine (ligands HA) parent complexes of cobalt(II), nickel(II), and zinc(II) were studied by pH-metry, spectrophotometry, and in part 13C n.m.r. spectroscopy. All three metal ions were found to promote deprotonation of the peptide amide group of GlyHis. HisGly co-ordinates ‘histidine-like’ to the metal ions, i.e. without deprotonation of the peptide amide group. With carnosine, only nickel(II) induces deprotonation of this group. Studies were also made of mixed-ligand systems involving Gly, GlyGly, His, and occasionally 2,2′-bipyridyl as second ligand B, including those containing copper(II). Gly and GlyGly do not hinder the co-ordination of GlyHis via three nitrogens, and mixed-ligand complexes are therefore not present in detectable concentration. With His as ligand B, although mixed-ligand complexes are formed at comparable GlyHis and His concentrations, in the presence of an excess of His the parent complex [M(HisO)2] predominates. A mixed-ligand complex is obtained in significant amount with 2,2′-bipyridyl as ligand B. Appreciable formation of mixed-ligand complexes also occurs in the nickel(II)–carnosine–ligand B systems. For cobalt(II), however, practically only the parent complexes of ligands B are formed. The cobalt(II) complexes of all three dipeptides examined are able to take up molecular oxygen reversibly, the oxygen being released partially or almost completely. For GlyHis it is highly likely that the active complex is [Co(AH–1)], while for HisGly and carnosine the presence of the bis complexes is presumed necessary for oxygen uptake.

Thermodynamic relations of parent and mixed complexes of asparagine and glutamine with copper(II)

A study was made of the equilibrium and, in part, thermodynamic relations of the parent complexes copper(II)-asparagine and copper(II)-glutamine, and of the mixed complexes copper(II)-asparagine-glycine, copper(II)-asparagine-serine, copper(II)-glutamine-glycine and copper(II)-glutamine-serine. The higher stabilities of the parent complexes copper(II)-asparagine compared to the copper(II) complexes of α-amino-butyric acid, the larger enthalpy change accompanying the formation of the complex CuA, and the deprotonation of the complex CuA2 above pH ∼ 9·5, all indicate that the amide group in the asparagine is also coordinated to the copper(II) ion. The results for the complexes copper(II)-glutamine strongly suggest that the glutamine forms bonds to the copper(II) ion only via its amino and carboxyl groups. The results obtained with the mixed ligand complexes support the above findings.

Complexes of sulphur-containing ligands—II: Binary and ternary complexes of D-penicillamine and L-cysteine with nickel(II) and zinc(II) ions

Abstract Complexes of nickel(II) and zinc(II) with L-cysteine and D-penicillamine were investigated by pH-metric, spectrophotometric and magnetic measurements. The formation of polynuclear species M3A4 was proved in the case of L-cysteine. Both ligands can also form protonated complexes. The concentration of mixed species is higher in the systems containing the zinc(II) ion. Mixed ligand complexes were not identified in the nickel(II)-D-penicillamine-glycine system. With L-histidine or histamine as ligand B , the formation of mixed ligand complexes was detected in spite of the different structures of the parent complexes. The stability constants of the mixed complexes of zinc(II) closely corresponded to statistical expectation.

Thermodynamic and NMR studies of some copper(II)-diaminomonocarboxylate equilibrium systems in aqueous solution

Abstract pH-metric calorimetric and NMR studies were carried out on the copper(II) complexes of 2,3-diaminopropionic acid (dapa), 2,4-diaminobutyric acid (daba), 2,5-diaminopentanoic acid (ornithine) and 2,6-diaminohexanoic acid (lysine). It was concluded that lysine is coordinated to the copper(II) ion in a “glycine-like” manner. With dapa, daba and ornithine, however, the ω-amino groups also coordinate in complexes containing the ligand in fully deprotonated form. NMR results show that with the ornithine complex a fast dynamic equilibrium could be assumed between those forms which contain the ω-amino group in coordinated and non-coordinated forms. A similar equilibrium, shifted toward diamine-like coordination, is probable in the daba complex. In practice, dapa behaves like a substituted ethylenediamine in its copper(II) complex.

Effect of mixed-ligand complex formation on the ionization of the pyrrole hydrogens of histamine and histidine

Equilibrium studies have been carried out by pH titrimetry to determine the stability constants of the parent complexes of CuII NiII and ZnII with histamine and histidine, and of the mixed-ligand complexes of these with, glycine, ethylenediamine, 4,5-dihydroxybenzene-1,3-disulphonate, and 2,2′-bipyridyl. From titrations in an alkaline medium it is concluded that nickel(II) and zinc(II) ions do not affect the ionization of the pyrrole hydrogen significantly compared to the free ligand, but CuII decreases the relevant pKvalues in the parent complexes by about three orders of magnitude. As a consequence of charge-transfer processes, the deprotonation can also be in-fluenced significantly by the formation of mixed-ligand complexes.

Complexes of 3,4-dihydroxyphenyl derivatives IV. Equilibrium studies on some transition metal complexes formed with adrenaline and noradrenaline

Abstract The stoichiometries and stability constants of the complexes formed in the manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II)-L-adrenaline and noradrenaline systems were determined pH-metrically at 25 °C and 0.2 mol/dm 3 ionic strength. It was proved that when there is an excess of ligand only the ortho hydroxy groups take part in the coordination, and that the participation of the side-chain in the complex formation is subordinate. From an ESR spectral study of the copper(II)-adrenaline system it was assumed that at a metal ion/ligand ratio of 1:1 and at higher pH (pH > 10) a polynuclear complex may also be formed. In this species the mode of bonding is mixed, with the participation of the side-chain donor groups. It was found that the complex-forming properties of adrenaline and noradrenaline towards the transition metal ions studied are very similar, this being a consequence of the small difference between the structures of these ligands.