Ternary Complexes In Solution Research Articles

Dynamics and Energetics of Elongation Factor SelB in the Ternary Complex and the Ribosome

SelB is an elongation factor specialized to deliver the selenocysteine (Sec) tRNA to the ribosome by recoding the UGA stop codon on the mRNA. Initially the tRNA is in complex with selB and GTP forming the ternary complex (TC). High-resolution cryo-EM structures of intermediates of the Sec incorporation pathway uncover large-scale conformational changes of the ribosome and the TC. To complement the structural information with energetics and rapid dynamics, we performed extensive all-atom molecular dynamics simulations of the ribosome with bound TC as well as of the free TC in solution. The simulations of the free TC were started after extracting the TC from the ribosome-bound cryo-EM structures. The TC was found to rapidly interconvert between the different conformations allowing us to construct the free-energy landscape of the involved motions. This free-energy landscape indicates that the intrinsic large-scale conformational changes of the tRNA and SelB during the delivery to the ribosome are not rate-limiting to the process. In simulations of the free TC started from the GTPase-activated ribosome-bound conformation, the TC rapidly transitions into an inactivated conformation, showing that the GTPase-activated state is strongly stabilized by the ribosome. The simulations of the full ribosome with bound TC in the intermediate states allow us to identify the motions that are rate-limiting to the process of tRNA delivery and to identify the molecular mechanism of the domain closure of small ribosomal subunit upon tRNA decoding.

Equilibrium studies of ternary systems containing some selected transition metal ions, triazoles and aromatic carboxylic acids

Solution equilibria of the binary and ternary complex systems of the divalent transition metal ions Cu2+, Ni2+, Zn2+, and Co2+ with 1,2,4-triazole (TRZ), 3-mercapto-1,2,4-triazole (TRZSH), and 3-amino-1,2,4-triazole (TRZAM) and aromatic carboxylic acids (phthalic, anthranilic, salicylic, and 5-sulfosalicylic acid) have been studied pH-metrically at (25.0±0.1) °C, and a constant ionic strength I=1×10−1 mol L−1 NaNO3 in an aqueous medium. The potentiometric titration curves show that binary and ternary complexes of these ligands are formed in solution. The stability constants of the different binary and ternary complexes formed were calculated on the basis of computer analysis of the titration data. The relative stability of the different ternary complex species is expressed in terms of Δ log K values, log X and R. S.% parameters. The effect of temperature of the medium on both the proton-ligand equilibria for TRZAM and phthalic acid and their metal-ligand equilibria with Cu2+, Ni2+, and Co2+ has been studied along with the corresponding thermodynamic parameters. The complexation behavior of ternary complexes is ascertained using conductivity measurements. In addition, the formation of ternary complexes in solution has been confirmed by using UV-visible spectrophotometry.

Solution Equilibria and Thermodynamic Studies of Complexation of Divalent Transition Metal Ions with Some Triazoles and Biologically Important Aliphatic Dicarboxylic Acids in Aqueous Media

The formation of binary and ternary complexes of the divalent transition metal ions CuII, NiII, ZnII, and CoII with some triazoles [1,2,4-triazole (TRZ), 3-mercapto-1,2,4-triazole, and 3-amino-1,2,4-triazole], and the biologically important aliphatic dicarboxylic acids adipic, succinic, malic, malonic, maleic, tartaric, and oxalic acid, was investigated in aqueous solutions using the potentiometric technique at 25 °C and I = 0.10 mol·dm−3 NaNO3. The formation of 1:1 and 1:2 binary complexes and 1:1:1 ternary complexes was inferred from the corresponding titration curves. The formation of ternary complexes occurs in a stepwise manner with the carboxylic acids acting as primary ligands. The ionization constants (pKa) of the investigated ligands were redetermined and used for determining the stability constants of the binary and ternary complexes formed in solution. The order of stability of the ternary complexes was investigated in terms of the nature of the triazole, carboxylic acid and metal ion used. The ∆log10K values, percent relative stabilization, and log10X for the ternary complexes have been evaluated and discussed. The concentration distributions of the various species formed in solution were evaluated. The ionization constants of TRZ and malic acid and stability constants of their binary and ternary complexes with CuII, NiII, and CoII metal ions were studied at four different temperatures (15, 25, 35, and 45 °C) and the corresponding thermodynamic parameters have been evaluated and discussed. The complexation behavior of ternary complexes was ascertained using conductivity measurements. In addition, the formation of ternary complexes in solution has been confirmed by using UV–visible spectrophotometry.

Mitogen-activated Protein Kinase (MAPK) Phosphatase 3-mediated Cross-talk between MAPKs ERK2 and p38α

MAPK phosphatase 3 (MKP3) is highly specific for ERK1/2 inactivation via dephosphorylation of both phosphotyrosine and phosphothreonine critical for enzymatic activation. Here, we show that MKP3 is able to effectively dephosphorylate the phosphotyrosine, but not phosphothreonine, in the activation loop of p38α in vitro and in intact cells. The catalytic constant of the MKP3 reaction for p38α is comparable with that for ERK2. Remarkably, MKP3, ERK2, and phosphorylated p38α can form a stable ternary complex in solution, and the phosphatase activity of MKP3 toward p38α substrate is allosterically regulated by ERK2-MKP3 interaction. This suggests that MKP3 not only controls the activities of ERK2 and p38α but also mediates cross-talk between these two MAPK pathways. The crystal structure of bisphosphorylated p38α has been determined at 2.1 Å resolution. Comparisons between the phosphorylated MAPK structures reveal the molecular basis of MKP3 substrate specificity.

Effect of substituent of β-diketones on the synergistic extraction of lanthanoids with linear polyether

The synergistic extraction of 14 trivalent lanthanoids (Ln3+) into 1,2-dichloroethane with a linear polyether (DEO6), HO(C2H4O)6C12H25, and β-diketones (HA) having different substituents was investigated at 25.0°C. The HAs used were trifluoroacetylacetone (Htfa), thenoyltrifluoroacetone (Htta), benzoyltrifluoroacetone (Hbta), naphthoyltrifluoroacetone (Hnta), and pivaloyltrifluoroacetone (Hpta). By the extraction of Ln3+ with β-diketone alone, the extraction constants of the neutral LnA3 complex, Kex=[LnA3]org[H+]aq3/[Ln3+]aq[HA]org3, were determined. The intrinsic extraction constants, Kex*=[LnA3]org/[Ln3+]aq[A−]aq3, were evaluated by employing the regular solution theory. Results indicate that the extractability of LnA3 is dependent on the lipophilicity of the ligand, and the planar aromatic rings do not cause steric hindrance in the formation of the binary complex. Addition of DEO6 significantly enhanced the extraction of Ln3+ by the formation of LnA3(DEO6). The ternary complex formation constants, βadd, were determined for all the Ln3+ and HA. The βadd of bta− and nta− complexes is similar with those of tfa− complexes, indicating that planar aromatic rings do not sterically hinder even the formation of the ternary complex. The higher values of βadd for the complexes of tta−, which has a slightly dipolar thenoyl moiety, can be accounted for the presence of ligand–ligand interaction. The formation constants of the ternary complexes of pta− were lower compared to complexes of other β-diketones because of steric hindrance due to the bulky t-butyl moiety. The detailed structures of the ternary complexes in solution were elucidated by NMR spectroscopy. Estimated structures sufficiently explain the variation in stability constants of LnA3(DEO6) among HAs and across the series of Ln3+. The structures thus obtained were ascertained by the molecular models created by MM2 calculation.

Palladium(II) Complexes Containing Dipicolinic Acid (DPA), Iminodiacetic Acid (IDA), and Various Biologically Important Ligands

Ternary 1:1:1 complexes of Pd(II) with dipicolinic acid (DPA) or iminodiacetic acid (IDA) as primary ligands and some selected mono- and dicarboxylic amino acids (glycine, α-alanine, leucine, valine, phenylalanine, tryptophan, methionine, histadiene aspartic acid, and glutamic acid), aromatic carboxylic acids (salicylic and phthalic), and aliphatic carboxylic acids (succinic, oxalic, malic, maleic, malonic, and tartaric) as secondary ligands have been investigated by using the potentiometric technique at T = 30 °C and I = 0.1 mol·dm−3. The ternary complexes are formed in a stepwise mechanism. Confirmation of the ternary complexes in solution has been carried out using conductometric measurements.

Influence of decreasing solvent polarity (1,4-dioxane/water mixtures) on the stability and structure of complexes formed by copper(II), 2,2′-bipyridine or 1,10-phenanthroline and guanosine 5′-diphosphate: evaluation of isomeric equilibria†‡

The stability constants of the 1 : 1 complexes formed between Cu(Arm)2+, where Arm = 2,2′-bipyridine or 1,10-phenanthroline, and guanosine 5′-diphosphate (GDP)3− or its monoprotonated form H(GDP)2− were determined by potentiometric pH titrations in water and in water containing 30 or 50% (v/v) 1,4-dioxane (25°C; I = 0.1 M, NaNO3). The stability of the binary Cu(GDP)− complex is enhanced due to macrochelate formation of the diphosphate-coordinated Cu2+ with N7 of the guanine residue as previously shown. In Cu(Arm)(GDP)− the N7 is released from Cu2+ and the stability enhancement of more than one log unit in aqueous solution is clearly attributable to intramolecular stack formation between the aromatic rings of Arm and the guanine moiety. Indeed, stacked isomers occur to more than 90% in equilibrium with open unstacked forms. Surprisingly, the same formation degrees of the stacks are observed for Cu(Arm)(dGMP) complexes, where dGMP2− = 2′-deoxyguanosine 5′-monophosphate, despite the fact that the overall stability of the latter species is by about 2.7 log units lower. In 1,4-dioxane–water mixtures stack formation is drastically reduced, probably due to hydrophobic solvation of the aromatic rings by the ethylene bridges of 1,4-dioxane. The relevance of these results regarding biological systems is indicated. †This study is dedicated to Professor Dr Alfredo Mederos on the occasion of his retirement from the University of La Laguna (Spain) with the very best wishes for all of his future endeavors. ‡This is part 70 of the series Ternary Complexes in Solution; for parts 69 and 68 see 14 and 15, respectively.

New Insights into M(II)−Hydroxamate Interactions: The Electro-Analytical Behavior of Metal(II) Complexes Involving Monohydroxamic Acids and Diamines in an Aqueous Medium

The binary and ternary systems of Cu(II), Co(II), Ni(II), Zn(II), Cd(II), Mn(II), Mg(II), and Ca(II) metal ions with the hydroxamic acids, acetohydroxamic acid (Aha), benzohydroxamic acid (Bha), and salicylhydroxamic acid (Sham), and diamines, ethylenediamine (en), 1,3-diaminopropane (dapr), and o-phenylenediamine (o-pda), were investigated potentiometrically at 25 °C and I = 0.10 mol·dm−3 (NaNO3) in aqueous solution. The acid−base properties of the ligands were investigated. The potentiometric titration curves show that the ternary complexes are formed in a stepwise manner. The stability constants of the binary and ternary complexes were calculated on the basis of computer analysis of titration data. The order of stability of the binary and ternary complexes in terms of the nature of hydroxamic acids, diamines, and metal ions was investigated and discussed. The relative stability of ternary complex species is expressed in terms of Δlog K and % R.S. parameters. The distribution of binary and ternary complexes in solution was evaluated. Evaluation of the effect of temperature of the medium on both the ionization process of the ligands and complex formation for the M(II)−en−Sham (M(II) = Cu(II), Co(II), and Ni(II)) ternary systems has been studied along with the corresponding thermodynamic parameters. The complexation behavior of ternary complexes is ascertained using conductivity measurements. In addition, confirmation of the formation of ternary complexes in solution has been performed using cyclic voltammetry measurements.

Thermodynamics and the structure of binary and ternary complexation of Am3+ Cm3+ and Eu3+ with DTPA and DTPA + IDA

Abstract The formation, their stability and the thermodynamic parameters of the binary complexation of Am3+, Cm3+ and Eu3+ with DTPA and of the ternary complexation of DTPA + IDA were studied in aqueous solutions of I = 6.60 m (NaClO4) and temperatures of 0–60 °C by solvent extraction technique. The stability constants of these complexes increased with increasing temperature. Positive enthalpy and the large entropy values for the binary and the ternary complexation indicated significant effect of cation dehydration at such high ionic strengths. TRLFS and NMR (1H and 13C) data provided insight into the structure of the ternary complexes in solution. In the binary [M(DTPA)]2− complex, M3+ binds via five carboxylates and three nitrogens, while in the ternary complex [M(DTPA)(IDA)]4−, DTPA binds by the same eight coordination sites and IDA binds via one of its carboxylate anion groups.

Complexation thermodynamics and the structure of the binary and the ternary complexes of Am 3+, Cm 3+ and Eu 3+ with CDTA and CDTA + IDA

The stability constants and the thermodynamic parameters of the formation of the binary complexes of trivalent Am 3+, Cm 3+ and Eu 3+ with CDTA and of their ternary complexes with CDTA + IDA were determined by solvent extraction measurements in aqueous solutions of I = 6.60 m (NaClO 4) at temperatures of 0–60 °C. The endothermic enthalpy and the positive entropy values reflect the significant effects of cation dehydration and of the rigidity of the ligand structure in the formation of these complexes. TRLFS and NMR ( 1H and 13C) data provided information on the structure of the ternary complexes in solution. The size and rigidity of CDTA affect the binding mode of IDA in the complexation of M(CDTA)(IDA)(H 2O) 3− and M(CDTA)(IDA) 3− in which IDA has a bidentate coordination mode in the former and a tridentate coordination mode in the latter.

Complexation thermodynamics and the structure of the binary and the ternary complexes of Am 3+, Cm 3+ and Eu 3+ with IDA and EDTA + IDA

The stability and the associated thermodynamic parameters of the binary and the ternary complexes of trivalent Am, Cm, and Eu with IDA and with EDTA + IDA, were determined by using a solvent extraction technique for aqueous solutions of I = 6.60 m (NaClO 4) at temperatures of 0–60 °C. The endothermic enthalpy and the positive entropy reflect the significant effect of dehydration in the formation of these complexes at high ionic strength. TRLFS and NMR ( 1H and 13C) data helped to establish the structure of the ternary complexes in solution. In the ternary complex M(EDTA)(IDA) 3−, EDTA binds via four carboxylates and two nitrogens, and IDA via two carboxylates and one nitrogen to the central Eu 3+.

A Dinuclear Lanthanide Complex for the Recognition of Bis(carboxylates): Formation of Terbium(III) Luminescent Self-Assembly Ternary Complexes in Aqueous Solution

The synthesis and photophysical properties of a coordinatively unsaturated cationic dinuclear terbium complex, 2.Tb(2), that can detect the presence of mono- or bis(carboxylates) in buffered aqueous solution at physiological pH is described. Full ligand synthesis and structural characterization of 2.Na(2) are also described. Spectroscopic measurements determined that each Tb(III) metal center has two metal-bound water molecules (q = 2). The recognition or sensing of N,N-dimethylaminocarboxylic acid, 4, and the bis(carboxylate) terephthalic acid, 5, which can also function as sensitizing antennae, was found to occur through the binding of these carboxylates to the metal center via the displacement of the metal bound water molecules. This gave rise to the formation of luminescent ternary complexes in solution in 2:1 or 1:1 (ion:2.Tb(2)) stoichiometry, respectively. Aliphatic bis(carboxylates) also bind to 2.Tb(2) where the selectivity for the ion recognition and stoichiometry was dictated by the structure of the anion, being most selective for pimelic acid, 6. Binding of either l- or d-tartaric acid gave rise to the formation ternary complex formation, with 2:1 stoichiometry, where the ion recognition resulted in quenching of the lanthanide emission.

Metal Ion−Buffer Interactions. Complex Formation of N,N-bis(2-Hydroxyethyl)glycine (Bicine) with Various Biologically Relevant Ligands

Ternary 1:1:1 complexes of some metal ions Cu(II), Ni(II), and Co(II) with N,N-bis(2-hydroxyethyl)glycine (bicine) and some aliphatic carboxylic acids (succinic, oxalic, malic, maleic, and tartaric), aromatic carboxylic acids (5-sulfosalicylic, salicylic, and phthalic), and hydroxamic acids (benzohydroxamic acid and salicylhydroxamic acid) are described. The formation constants of the 1:1:1 ternary complexes were determined potentiometrically at (298.15 ± 0.1) K and 0.10 mol·dm-3 (NaNO3) ionic strength. The ternary complex formation was found to take place in a stepwise manner. The stability of the ternary complex is also discussed in relation to that of the binary complexes of secondary ligands. An evaluation of the effects of temperature and ionic strength of the medium on the stability of the ternary system Cu(II) + bicine + ligands has been studied. The thermodynamic parameters (ΔH°, ΔG°, and ΔS°) were calculated and discussed. The stability constant of the above-mentioned ternary system has been investigated in a dioxane−water solution. Confirmation of the ternary complexes in solution has been carried out using differential pulse polarography, square wave voltammetry, conductometric measurements, and V-visible spectroscopic measurements.

Differential and Simultaneous Adenosine Di- and Triphosphate Binding by MutS

The roles of ATP binding and hydrolysis in the function of MutS in mismatch repair are poorly understood. As one means of addressing this question, we have determined the affinities and number of adenosine di- and triphosphate binding sites within MutS. Nitrocellulose filter binding assay and equilibrium fluorescence anisotropy measurements have demonstrated that MutS has one high affinity binding site for ADP and one high affinity site for nonhydrolyzable ATP analogues per dimer equivalent. Low concentrations of 5'-adenylylimidodiphosphate (AMPPNP) promote ADP binding and a large excess of AMPPNP is required to displace ADP from the protein. Fluorescence energy transfer and filter binding assays indicate that ADP and nonhydrolyzable ATP analogues can bind simultaneously to adjacent subunits within the MutS oligomer with affinities in the low micromolar range. These findings suggest that the protein exists primarily as the ATP.MutS.ADP ternary complex in solution and that this may be the form of the protein that is involved in DNA encounters in vivo.

Catalysis and electron transfer in protein crystals: the binary and ternary complexes of methylamine dehydrogenase with electron acceptors

Polarized absorption microspectrophotometry has been used to detect catalysis and intermolecular electron transfer in single crystals of two multiprotein complexes: (1) the binary complex between Paracoccus denitrificans methylamine dehydrogenase, which contains tryptophan–tryptophylquinone (TTQ) as a cofactor, and its redox partner, the blue copper protein amicyanin; (2) the ternary complex between the same two proteins and cytochrome c-551i. Continuous wave electron paramagnetic resonance has been used to compare the state of copper in polycrystalline powders of the two systems. While catalysis and intermolecular electron transfer from reduced TTQ to copper are too fast to be accessible to our measurements, heme reduction occurs over a period of several minutes. The observed rate constant is about four orders of magnitude lower than in solution. The analysis of the temperature dependence of this apparent constant provides values for the parameters H AB, related to electronic coupling between the two centers, and λ, the reorganizational energy, that are compatible with electron transfer being the rate-determining step. From these parameters and the known distance between copper and heme, it is possible to calculate the parameter β, which depends on the nature of the intervening medium, obtaining a value typical of electron transfer across a protein matrix. These findings suggest that the ternary complex in solution might achieve a higher efficiency than the rigid crystal structure thanks to an as yet unidentified role of protein dynamics.

Functional mutagenesis of AML1/RUNX1 and PEBP2β/CBFβ define distinct, non-overlapping sites for DNA recognition and heterodimerization by the runt domain

The Runt domain family of transcription factors play key roles in transcriptional regulation of definitive hematopoiesis and osteogenesis. This transcription factor family is characterized by a DNA-binding α-subunit harboring the Runt domain and a secondary subunit, β, which binds to the Runt domain and enhances its interaction with DNA. Missense mutations in the Runt domain from either the blood or bone-related gene product are associated with the onset of acute human leukemia as well as a disease of skeletal patterning known as cleidocranial dysplasia. NMR “footprinting” analysis of Runt domain/β/DNA ternary complexes in solution previously identified the likely residues that form the heterodimerization and DNA-binding surfaces of the Runt domain. Functional mutagenesis at 37 positions in the Runt domain or β confirms the original identification of these interaction surfaces and reveals that the heterodimerization and DNA-binding surfaces of the Runt domain occur at distinct, non-overlapping sites within the domain. The analysis of an additional 21 disease-related missense mutations identified from patients with either blood or bone disease demonstrates that the primary defect in these patients is a failure in DNA-recognition by the Runt domain. The molecular basis for the DNA-binding defect is analyzed in the context of the three-dimensional structure of the Runt domain in binary and ternary protein/DNA complexes.

Ternary Complexes in Solution. Comparison of the Coordination Tendency of Some Biologically Important Zwitterionic Buffers toward the Binary Complexes of Some Transition Metal Ions and Some Amino Acids

Potentiometric equilibrium measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol dm-3 (KNO3) for the interaction of glycine (aminoethanoic acid), serine (2-amino-3-hydroxypropanoic acid), methionine (2-amino-4-(methylthio)butanoic acid), aspartic acid (aminobutanedioic acid), glutamic acid (2-aminopropanedioic acid), and histidine (α-amino-1H-imidazole-4-propanoic acid) and Cu(II), Co(II), Ni(II), Mn(II), and Zn(II) with the biologically important secondary ligand zwitterionic buffers β-hydroxy-4-morpholinepropanesulfonic acid (MOPSO), 4-morpholinepropanesulfonic acid (MOPS), 3-[bis(2-hydroxyethyl)amino]-2-hydroxy-1-propanesulfonic acid (DIPSO), and 3-[N-tris(hydroxymethyl)methyl)amino]-2-hydroxypropanesulfonic acid (TAPSO) in 1:1:1 and 1:1:2 ratios, and the formation of various 1:1:1 ternary complexes and 1:1:2 quaternary complex species was inferred from the potentiometric pH titration curves. Initial estimates of the formation constants of the resulting species and the acid...

Ternary complexes in solution. Intramolecular stacking interactions in mixed ligand complexes formed by copper(II), 2,2′-bipyridyl or 1,10-phenanthroline and a pyrimidine-nucleoside 5′-diphosphate (CDP 3−, UDP 3−, dTDP 3−)

The stability constants of the 1:1 complexes formed between Cu 2+ or Cu(Arm) 2+, where Arm = 2,2′-bipyridyl (Bpy) or 1,10-phenanthroline (Phen), and pyrimidine-nucleoside 5′-diphosphates (NDP 3−, i.e. CDP 3−, UDP 3− or dTDP 3−), were determined by potentiometric pH titrations in aqueous solution (25°C; I = 0.1 M, NaNO 3). It is shown that the stability of the binary Cu(NDP) − complexes is determined solely by the basicity of the diphosphate group; this is different for the ternary Cu(Arm)(NDP) − complexes. It is demonstrated that the equilibrium, Cu(Arm) 2+ + Cu(NDP) −Cu(Arm)(NDP) − + Cu 2+, is displaced considerably to the right-hand side. Part of this displacement is due to the well known experience that mixed ligand complexes formed by a divalent 3d ion, a heteroaromatic N base and an O donor ligand possess increased stability. This increased stability is now accounted for by the results obtained recently for the Cu 2+/Arm/methylphosphonylphosphate (CH 3-P(O) 2 −-O-PO 3 2−) systems (B. Song et al., Inorg. Chim. Acta, 273 (1998) 101). The other part of this displacement, which amounts on average to an increased stability of the mixed ligand Cu(Arm)(NDP) − complexes of about 0.35 log unit, is due to an intramolecular ligand—ligand stacking between the nucleobase residues and the aromatic rings of Bpy or Phen. The formation degree of these intramolecular stacks in the Cu(Arm)(NDP) − complexes corresponds to approximately 55%, thus leaving about 45% of the complexes in an unstacked or open form in this intramolecular isomeric equilibrium. The relevance of the results regarding biological systems is indicated briefly.

Ternary complexes in solution. Intramolecular hydophobic and stacking interactions in mixed ligand complexes formed by copper (II), 2,2'-bipyridyl or 1,10-phenanthroline, and n-butyl diphosphate (BuDP3-) or phenyl diphosphate (PhDP3-)

Ternary complexes in solution. Intramolecular hydophobic and stacking interactions in mixed ligand complexes formed by copper (II), 2,2'-bipyridyl or 1,10-phenanthroline, and n-butyl diphosphate (BuDP3-) or phenyl diphosphate (PhDP3-)

Stereoselective Formation of Ternary Copper(II) Complexes of (S)‐Amino‐acid Amides and (R)‐ or (S)‐Histidine and (R)‐ or (S)‐Tyrosine in Aqueous Solution

AbstractFormation constants of ternary complexes of CuII with (S)‐amino‐acid amides ((S)‐phenylalaninamide, (S)‐prolinamide, and (S)‐tryptophanamide) and (R)‐ or (S)‐histidine and (R)‐ or (S)‐tyrosine were determined potentiometrically in aqueous solution. Significant stereoselectivity was presented by all three amides towards histidine, the diastereoisomeric complexes with ‘heterochiral’ ligands being more stable than those with ‘homochiral’ ligands (see Table 3). The stereoselectivity observed with (S)‐phenylalaninamide and (S)‐tryptophanamide may be explained on the basis of hydrophobic stacking interactions between 1H‐imidazole and the aromatic side chain, favoured by the terdentate behaviour of histidine (see Fig.2), whereas repulsive effects seem to be prevalent with (S)‐prolinamide. Only (S)‐prolinamide and (S)‐phenylalaninamide show appreciable stereoselectivity with tyrosine, which is bidentate, probably on account of repulsive interactions. The present results on the stability of ternary complexes in solution allow to draw some conclusions on the mechanism of chiral discrimination performed by CuII complexes of (S)‐amino‐acid amides added to the mobile phase in HPLC (reversed phase).