Search

The review is mainly concerned with the protonation equilibria of biologically active ligands like amino acids, peptides, DNA constituents, and amino acid esters in nonaqueous media. Equilibrium concentrations of proton-ligand formation as a function of pH were investigated. Also, thermodynamics associated with protonation equilibria were also discussed. [ABSTRACT FROM AUTHOR]

Proton-ligand association constants of imidazole-4-acetic acid (IMA) were determined potentiometrically in aqueous solution at different temperatures in the range 15-35 °C. The stepwise stability constants of IMA with some selected bivalent transition metal ions were also determined in 0.1 mol·dm NaNO. The stability of the complexes follows the trend Cu > Ni > Co > Mn, which is in agreement with the Irving-Williams order of the metal ions. The thermodynamic parameters for Cu(II)-IMA complex formation were derived and discussed. The ternary complexes Cu(IMA)L (IMA = imidazole-4-acetic acid, HL = amino acid, amides or DNA constituents) have been investigated. Ternary complexes of amino acids or amides are formed by a simultaneous mechanism. Amino acids form the complex Cu(IMA)L, whereas amides form two complex species Cu(IMA)L and Cu(IMA)(LH). The DNA constituents form both 1:1 and 1:2 complexes. The stabilities of ternary complexes are quantitatively compared with their corresponding binary complexes. The concentration distribution of the complexes in solution was evaluated as a function of pH. [ABSTRACT FROM AUTHOR]

The protonation constants of some α-amino acids (glycine (Gly), l-alanine (Ala), l-valine (Val), l-serine (Ser), l-leucine (Leu) and l-isoleucine (Ile)) were studied in water and DMSO-water solution mixtures containing 30, 50 and 70 vol-% DMSO; in addition the complex formation equilibria of their copper(II) complexes were studied by potentiometric technique using a combined pH electrode system calibrated in concentration units of the hydrogen ion at 25 ± 0.1 °C under a nitrogen atmosphere, and at an ionic strength of 0.10 mol·dm NaNO. The protonation constants and the overall stability constants of copper(II) complexes were influenced by changes in solvent composition, and their variations are discussed in terms of solvent and structural properties. [ABSTRACT FROM AUTHOR]

Binary and mixed-ligand complexes of zinc(II) involving 2-(aminomethyl)-benzimidazole (AMBI) and amino acids, peptides (HL) or DNA constituents have been investigated. Ternary complexes of amino acids or peptides are formed simultaneously. Amino acids form the complex Zn(AMBI)L, whereas amides form two complex species Zn(AMBI)L and Zn(AMBI)(LH). The ternary complexes of zinc(II) with AMBI and DNA are formed in a stepwise process, whereby binding of zinc(II) to AMBI is followed by ligation of the DNA constituents. The stability of ternary complexes is quantitatively compared with their corresponding binary complexes in terms of the parameters Δlog K, log β and log X. The effect of the side chains of amino acid ligands (Δ) on complex formation is discussed. The values of Δlog K indicated that the ternary complexes containing aromatic amino acids are significantly more stable than the complexes containing alkyl- and hydroxyalkyl-substituted amino acids. This may be taken as evidence for a stacking interaction between the aromatic moiety of AMBI and the aromatic side chains of the bio-active ligands. The concentration distributions of various species formed in solution were also evaluated as a function of the pH. [ABSTRACT FROM AUTHOR]

A comprehensive review of organotin(IV) hydrolysis and complex formation equilibria with amino acids, peptides and DNA constituents is presented with special reference to solution and coordination chemistry studies. The review focuses on results obtained in the author's laboratory and related work of other groups. Also, published studies on these complexes were reviewed with emphasis on their biological aspects. [ABSTRACT FROM AUTHOR]

The hydrolysis of amino acid esters is catalysed by the palladium(II) complex [Pd(Pic)]2+ (Pic = 2-picolylamine). The reaction has been studied by the pH-stat technique. The overall PdII chelate-promoted hydrolysis of the ethyl esters of glycine and alanine may be accounted by inter- and intramolecular mechanisms depicted in Eqns (1) and (2) respectively. [Pd(Pic)(NH2CH(R)CO2R]2+ + OH- k1→[Pd(Pic)(NH2CH(R)CO2]+ + ROH (1) [Pd(Pic)(NH2CH(R)CO2R(OH)]+ + k2→[Pd(Pic)(NH2CH(R)CO2)] + ROH (2) The kinetic data for histidine and methionine methyl esters are fitted assuming that hydrolysis proceeds solely by the intermolecular mechanism [Eqn (1)]. The rate enhancement compared with the free ester is discussed in terms of the structure of the complex. Activation parameters associated with the hydrolysis have been determined and discussed. [ABSTRACT FROM AUTHOR]

Binary and ternary complexes of zinc(II) involving nitrilo-tris(methyl phosphonic acid (H6A) and amino acids, peptides (HL), or DNA constituents have been investigated. The stoichiometry and stability constants for the complexes formed are reported. The results show that ternary complexes are formed in a stepwise manner whereby nitrilo-tris(methylphosphonic acid) binds to zinc(II), which is then followed by coordination of an amino acid, peptide or DNA. Zinc(II) was found to form ZnA and ZnAH n complex species where n=3, 2 or 1. The stabilities of the ternary complexes are compared with the stabilities of their corresponding binary complexes. The concentration distributions of the various complex species have been evaluated. The kinetics of the base hydrolysis of glycine methyl ester in the presence of Zn(II)-NTP complexes was studied in aqueous solution using a pH-stat technique. The p Ka for ionization of the coordinated water molecule is 9.14 as determined from the kinetic results, while direct potentiometric titration of the complex [Zn(NTP)(H2O)] gave 9.98 (±0.02). The rate constant for the intramolecular attack of coordinated hydroxide on the ester is k=(2.65×10−4±0.003) dm3⋅mol−1⋅s−1. [ABSTRACT FROM AUTHOR]

Abstract: The formation equilibria of copper(II) complexes and the ternary complexes Cu(PDC)L (PDC=2,6-bis-(hydroxymethyl)-pyridine, HL=amino acid, amides or DNA constituents) have been investigated. Ternary complexes are formed by a simultaneous mechanism. The results showed the formation of Cu(PDC)L, Cu(PDC, H−1)(L) and Cu(PDC, H−2)(L) complexes. The concentration distribution of the complexes in solution is evaluated as a function of pH. The effect of dioxane as a solvent on the protonation constant of PDC and the formation constants of Cu(II) complexes are discussed. The thermodynamic parameters and calculated from the temperature dependence of the equilibrium constants are investigated. [Copyright &y& Elsevier]

Abstract: Formation equilibria of copper(II) complexes of 2-(aminomethyl)-benzimidazole (AMBI) and the ternary complexes Cu(AMBI)L (L=amino acid, amide, dicarboxylic acid or DNA constituents) have been investigated. Ternary complexes of amino acids or amides are formed by a simultaneous mechanism. Amino acids form the complex Cu(AMBI)L, whereas amides form two complex species Cu(AMBI)L and Cu(AMBI)(LH−1). The ternary complexes of copper(II) with AMBI and dicarboxylic acids or DNA units are formed by a stepwise mechanism, whereby binding of copper(II) to AMBI is followed by ligation of the dicarboxylic acids or DNA components. The values of Δ log K indicate that the ternary complexes containing aromatic amino acids are significantly more stable than the complexes containing alkyl- and hydroxyalkyl-substituted amino acids. This may be taken as an evidence for a stacking interaction between the aromatic moiety of AMBI and the aromatic side chains of the bio-active ligands. The solid complexes Cu(AMBI)L where L=1,1-cyclobutanedicarboxylic acid (CBDCA) and malonic acid were separated and identified by elemental analysis and infrared spectroscopy and magnetic moment. The decomposition course and steps for the isolated complexes were analyzed and the kinetic parameters of the non-isothermal decomposition were calculated. The hydrolysis of glycine methyl ester (MeGly) is catalyzed by the Cu(AMBI)2+ complex. The kinetic data is fitted assuming that the hydrolysis reaction proceeds in two steps. The first step, involving coordination of the amino acid ester by the amino and carbonyl groups, is followed by rate-determining attack by OH− ion. The second step involves the equilibrium formation of the hydroxo-complex Cu(AMBI)(MeGly)(OH) followed by intramolecular OH− attack. [Copyright &y& Elsevier]

The complex formation reactions of [Cu(NTP)(OH 2 )] 4- (NTP  =  nitrilo-tris(methyl phosphonic acid)) with some selected bio-relevant ligands containing different functional groups, are investigated. Stoichiometry and stability constants for the complexes formed are reported. The results show that the ternary complexes are formed in a stepwise mechanism whereby NTP binds to copper(II), followed by coordination of amino acid, peptide or DNA. Copper(II) is found to form Cu(NTP)H n species with n   =  0, 1, 2 or 3. The concentration distribution of the various complex species has been evaluated. The kinetics of base hydrolysis of glycine methyl ester in the presence of copper(II)-NTP complex is studied in aqueous solution at different temperatures. It is proposed that the catalysis of GlyOMe ester occurs by attack of OH - ion on the uncoordinated carbonyl carbon atom of the ester group. Activation parameters for the base hydrolysis of the complex [Cu(NTP)NH 2 CH 2 CO 2 Me] 4- are, ΔH ±   =  9.5  ±  0.3  kJ  mol -1 and ΔS ±   =  -179.3  ±  0.9  J  K -1   mol -1 . These show that catalysis is due to a substantial lowering of ΔH ± . [ABSTRACT FROM AUTHOR]

Binary and ternary complexes of Cu(II) involving imino-bis(methyl phosphonic acid) (IdP) abbreviated as H4A and some selected bio-ligands, amino acids, peptides and DNA constituents (L), were examined. Cu(II) forms CuA and CuAH complexes with IdP. Ternary complexes are formed in a stepwise mechanism whereby iminodiphosphonic acid binds to Cu(II), followed by coordination of amino acid, peptide or DNA. The concentration distribution of the various complex species has been evaluated. The kinetics of base hydrolysis of glycine methyl ester in the presence of Cu(II)-IdP was studied in aqueous solution at different temperatures, and in dioxane-water solutions of different compositions at 25°C. The activation parameters are evaluated and discussed. [ABSTRACT FROM AUTHOR]

Ternary complexes of copper(II) with 2-aminomethylthiophenyl-4-bromosalicylaldehyde (ATS) and some amino acids have been isolated and characterized by elemental analyses, IR, magnetic moment, molar conductance, UV–vis, mass spectra, and ESR. The proposed general formulas of the prepared complexes are [Cu(ATS)(AA)]·nH2O (where AA?=?glycine, alanine, and valine). The low molar conductance values suggest the non-electrolytic nature of the complexes. IR spectra show that ATS is coordinated to copper in a bidentate manner through azomethine-N and phenolic-OH. The amino acids also are monobasic bidentate ligands via amino and ionized carboxylate groups. The magnetic and spectral data indicate the square-planar geometry of Cu(II) complexes. The geometry of the Cu(II) complexes has been fully optimized using parameterized PM3 semiempirical method. The Cu–N bond length is longer than that of Cu–O in the isolated complexes. Also, information is obtained from calculations of molecular parameters for all complexes including net dipole moment of the metal complexes, values of binding energy, and lipophilicity value (logP). The antimicrobial activity studies indicate significant inhibitory activity of complex3against the selected types of bacteria. The mixed ligand complexes have also been studied in solution state. Protonation constants of ATS and amino acids were determined by potentiometric titration in 50% (v/v) DMSO–water solution at ionic strength of 0.1?M NaCl. ATS has two protonation constants. The binary and ternary complexes of copper(II) involving ATS and some selected amino acids (glycine, alanine, and valine) were examined. Copper(II) forms [Cu(ATS)], [Cu(ATS)2], [Cu(AA)], [Cu(AA)2], and [Cu(ATS)(AA)] complexes. The ternary complexes are formed in a simultaneous mechanism. The present paper reports on the synthesis, characterization, and the electronic absorption spectra of Cu(II) ternary complexes involving ATS-Schiff base, and some selected amino acids. The antibacterial, antifungal, and antitumor activities were investigated. The geometry of the studied Cu(II) complexes has been fully optimized using parameterized PM3 semiempirical method. Protonation and complex formation equilibria were investigated. [ABSTRACT FROM PUBLISHER]

The interaction of diethyltin(IV), DET, with selected bioligands having a variety of model functional groups was investigated at 25 °C and ionic strength 0.1 mol·dm −3 NaCl using the potentiometric technique. The hydrolysis constants of the DET cation and the formation constants of the complexes formed in solution were calculated using the MINIQUAD-75 program. The stoichiometry and stability constants for the complexes formed are reported. The results show the formation of 1:1 and 1:2 complexes with amino acids and DNA constituents. The dicarboxylic acids form 1:1 complexes. The peptides form both 110 complexes and the corresponding deprotonated amide species [Et 2Sn(LH −1)] (11-1). The participation of different ligand functional groups in binding to organotin is discussed. The concentration distributions of the various complex species were evaluated as a function of pH. The standard thermodynamic parameters Δ H° and Δ S°, calculated from the temperature dependence of the equilibrium constants, were investigated for the interaction of DET with thymine as a representative example of DNA constituents. The effect of ionic strength and solvent on hydrolysis constants of DET, protonation equilibria of thymine and its complex formation with DET were investigated and discussed. [ABSTRACT FROM AUTHOR]

As a model for forecasting the ligand properties of antibiotics in living organisms , formation equilibria of binary and ternary Cu(II) complex with Cephradine (Cep) antibiotic in presence of α-alanine (α-Ala) and β-alanine (β-Ala) amino acids have been studied using the potentiometric titrations. Many factors were investigated like the formation constant, protonation constant, composition and stabilities of the species formed with copper(II) ions in water and water-ethanol solutions of different compositions. The thermodynamics were calculated and the data clarified. The pH-titrations of the reaction mixtures have been revealed 1:1:1 (Cu:Cep:amino acid (AA)) ternary complex formation. By aid of Δ log 10 K , Δ log 10 β and log 10 X parameters, quantitative matching of the stability of mixed-ligand complexes and the stability of the binary complexes was done. Investigations of the stability of the ternary complexes relative to their corresponding binary complexes indicated that formation of ternary complexes is more preferable than binary metal complexes. The obtained data from protonation of the investigated cephradine antibiotic and β-Ala/α-Ala amino acids and their complex formation equilibria will be a significant contribution to workers carrying out mechanistic studies in biological media. • Studying the Formation equilibria of binary and ternary Cu(II) complex with Cephradine (Cep) antibiotic in presence of amino acids. • (logβ) were determined potentiometrically along a broad range of solvent composition. • The speciation of different species in solution was assessed. • The effect of temperature on protonation of ligands and formation of mixed-ligand complexes was studied. [ABSTRACT FROM AUTHOR]

The stoichiometry and stability constants of the complexes formed between [Pd(MAMP)(HO)] and various biologically relevant ligands containing different functional groups were investigated. The ligands used are amino acids, peptides and DNA constituents. The results show the formation of 1:1 complexes with amino acids and peptides and the corresponding deprotonated amide species. Structural effects of peptides on amide deprotonation were investigated. The purine and pyrimidine bases uracil, uridine, cytosine, inosine, inosine 5′-monophosphate (5′-IMP) and thymine form 1:1 and 1:2 complexes. The concentration distribution of the various complex species was calculated as a function of pH. The effect of chloride ion concentration on the formation constant of CBDCA with Pd(MAMP) was also reported. The results show ring opening of CBDCA and monodentate complexation of the DNA constituent with the formation of [Pd(MAMP)(CBDCA-O)DNA], where (CBDCA-O) represents cyclobutane dicarboxylate coordinated by one carboxylate oxygen. The equilibrium constant of the displacement reaction of coordinated inosine, as a typical DNA constituent, by SMC and/or methionine was calculated. The results are expected to contribute to the chemistry of antitumor agents. The calculated parameters of the optimized complexes support the measured formation constants. [ABSTRACT FROM AUTHOR]