Mixed-Ligand Complexation of Zinc and Cobalt(II) Complexonates with Amino Acids in an Aqueous Solution

The formation of mixed-ligand complexes in the M(II)–Nta and Ida–L systems (M = Co, Zn; L = His, Orn, Lys, Gly, Im, en), where Ida and Nta are the residues of iminodiacetic and nitrilotriacetic acids, was studied by pH-metry, calorimetry, and NMR spectroscopy. The thermodynamic parameters (logK, Δ _r G⁰, Δ _r H, Δ _r S) of formation for these complexes were determined at 298.15 K and an ionic strength I = 0.5 (KNO3). The most probable pattern of coordination between a complexone and an amino acid in mixed-ligand complexes was revealed.     

Thermodynamics of the complexation of arabinogalactan with salicylic and <Emphasis Type="Italic">p</Emphasis>-aminobenzoic acids in aqueous solutions

The thermodynamics of complexation of arabinogalactan with salicylic and p-aminobenzoic acids in aqueous solutions is studied by means spectroscopy. The standard thermodynamic characteristics (ΔH°; ΔG°; ΔS°) of complexation are calculated.     

Synthesis of amino acid conjugates of glycyrrhizic acid using <Emphasis Type="Italic">N</Emphasis>-hydroxyphthalimide and <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>'-dicyclohexylcarbodiimide

Synthesis of amino acid conjugates of glycyrrhizic acid with the use of N-hydroxyphthalimide, N,N'-dicyclohexylcarbodiimide, and tert-butyl esters of L-amino acids (valine, isoleucine, phenylalanine, and methionine) was performed followed by deprotection with trifluoroacetic acid. The target amino acid conjugates were isolated by column chromatography on silica gel in 40–45% yield. The structure of the prepared compounds was confirmed by IR and ¹³C NMR spectroscopy.     

Enthalpic characteristics of the sublimation of linear and branched amino acids

Linear correlation relationships are suggested for estimating Δ_subl H of crystalline amino acids with different spatial configurations of side chains. It is shown that, for the same number of carbon atoms, the “solidity” coefficients (K _sold) are somewhat larger for crystals containing amino acids with branched side radicals than with linear ones, despite the fact that the packing density (D _pack) of the latter is higher.     

High-Performance Liquid Chromatographic Separation of Stereoisomers of <Emphasis Type="Bold">β</Emphasis>-Amino Acids and a Comparison of Separation Efficiencies on Chirobiotic T and TAG Columns

Direct and indirect reversed-phase high-performance liquid chromatographic methods were developed for the separation of enantiomers of seventeen unnatural β-amino acids, including several β-3-homo amino acids. The direct separations of the underivatized analytes were performed on chiral stationary phases containing macrocyclic glycopeptide antibiotic teicoplanin (Chirobiotic T column) and teicoplanin aglycone (Chirobiotic TAG column). The indirect method involved pre-column derivatization with two new chiral derivatizing agents, (1S,2S)-1,3-diacetoxy-1-(4-nitrophenyl)-2-propylisothiocyanate, (S,S)-DANI and (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester, (S)-NIFE. The different methods were compared in systematic chromatographic examinations. The effects of organic modifier, mobile phase composition, pH and flow rate on the separation were investigated.     

Formation of Mixed-Ligand Complexes of Metals(II) with Monoamine Complexones and Amino Acids in Solution

The formation of mixed-ligand complexes in the M(II)–Nta, Ida–L (M = Cu(II), Ni, Zn, Co(II), L = Ser, Thr, Asp, Arg, Asn) systems, where Ida and Nta are the residues of iminodiacetic and nitrilotriacetic acids, respectively, is studied using pH measurements, calorimetry and spectrophotometry. The thermodynamic parameters (logK, Δ_rG⁰, Δ_rH, Δ_rS) of their formation at 298.15 K and ionic strength I = 0.5 (KNO₃) are determined. The most likely scenario of amino acid residue coordination in the composition of mixed complexes is discussed.     

Synthesis of 6-Acyl-7-aryl-4,7-dihydrotetrazolo[1,5-<Emphasis Type="Italic">a</Emphasis>]pyrimidine-5-carboxylic acids and their methyl esters

6-Acyl-7-aryl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-5-carboxylic acids and methyl 6-acyl-7-aryl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-5-carboxylates were synthesized by fusion of 4-aryl-2,4-dioxobutanoic acids and their methyl esters, respectively, with 1H-tetrazol-5-amine and aromatic aldehydes. The reaction of methyl 2,4-dioxopentanoate with 1H-tetrazol-5-amine and 2-fluorobenzaldehyde in boiling acetic acid gave methyl 6-acetyl-5-hydroxy-7-(2-fluorophenyl)-4,5,6,7-tetrahydrotetrazolo[1,5-a]pyrimidine-5-carboxylate.     

The thermodynamic characteristics of solution of L-α-histidine and L-α-phenylalanine in water at 273–373 K

The solubility of L-phenylalanine and L-histidine in water at 298.15 and 318.15 K and the heat effects of solution of the amino acids at 328.15 K were determined. These results and the data obtained earlier were used to calculate all the standard thermodynamic functions of solution of the amino acids and the solubilities of L-phenylalanine and L-histidine over the temperature range 273–373 K. The selection of the form of the Δ_sol H ^o = f(T) dependence had a negligible effect on the free energies of solution and solubilities of the amino acids. This selection primarily influenced the entropy and heat capacity characteristics of the process.     

Thermochemical study of the complex formation of copper(II) and nickel(II) iminodiacetates with amino acids in aqueous solutions

The formation of mixed-ligand complexes in the M(II)–Ida–L systems (M = Cu, Ni, L = His, Orn, Lys), where Ida is the iminodiacetic acid residue, was studied by pH-metry, calorimetry, and spectrophotometry. The thermodynamic parameters (logK, Δ_rG⁰, Δ_rH, Δ_rS) of formation of the complexes were determined at 298.15 K and the ionic strength I = 0.5 (KNO₃). The most probable mode of coordination of the chelating agent and the amino acid in the mixed-ligand complexes was elucidated.     

Separation of β-blocker and amino acid enantiomers on a mixed chiral sorbent modified with macrocyclic antibiotics eremomycin and vancomycin

A mixed chiral sorbent based on silica with immobilized macrocyclic antibiotics eremomycin and vancomycin was synthesized. A possibility of the separation of enantiomers of β-blockers (metoprolol, pindolol, alprenolol, oxprenolol, labetalol, and atenolol) and amino acids (tryptophan, phenylalanine, DOPA, methionine, and acetyl glutamic acid) on this chiral sorbent by HPLC was studied. The influence of the composition of the mobile phase (pH of buffer solution, its concentration, content of organic modifier, and its nature) on the retention times of β-blocker and amino acid enantiomers, selectivity, and resolution of peaks was studied. It was shown that the mixed chiral sorbent has enantioselectivity to both classes of compounds, while silica modified with vancomycin has no ability to the separation of enantiomers of non-derivatized amino acids, and silica modified with eremomycin has no ability to the separation of β-blocker enantiomers. High values of resolution for amino acids (max Rs > 4) and β-blockers (max Rs > 1) were obtained.     

Analysis of the Racemization of Tryptophan

Diastereoisomers of L- and D-tryptophan were formed with a chiral reagent, 1-thio-β-D-glucose tetraacetate and o-phthaldialdehyde, and were separated from the derivatives of the other amino acids that occur in food proteins on an achiral column by high performance liquid chromatography. Mercaptoethanesulfonic acid that is used as an agent for hydrolyzing proteins made the derivatization with o-phthaldialdehyde and 1-thio-β-D-glucose tetraacetate impossible. On the other hand the reaction was completed in the presence of p-toluenesulfonic acid, but the oxidative losses during hydrolysis were significant. During boiling, the racemization of tryptophan can be detected after 12 h above pH 9, but the rate of conversion was lower than expected (<1%). The decrease of L-tryptophan after 24 h was 2–5% depending on pH. Besides racemization, other reactions, e.g. oxidative deterioration may play a role in the loss of L-tryptophan.     

Catalytic hydrolysis of α-amino esters in the presence of chiral palladacycles

The rate of hydrolysis of esters derived from optically active α-amino acids, catalyzed by chiral cyclopalladated benzylamines, depends on the configuration of chiral centers in the substrate and catalyst. The catalytic hydrolysis of sulfur-containing amino esters follows an intramolecular mechanism, and the difference in the reaction rates for the stereoisomers increases in going from ortho-palladated primary benzylamines (k _S/k _R = 1.1) to tertiary amines (k _S/k _R = 1.5); the strongest catalytic effect is observed for an ester and a complex with the same absolute configuration of the chiral centers. The efficiency of intermolecular catalysis is greater for a complex and ester with opposite absolute configurations of the chiral centers, and the rate constants of catalytic hydrolysis for two pairs of stereoisomers coincide within experimental error. The maximal difference in the reaction rates is observed for cyclopalladated secondary benzylamines; it reaches 2.3 for the phenylalanine ester.     

Reaction of apple pectin modified by pharmacophores,with copper(II) cations

The reaction of apple pectin modified by organic pharmacophores (nicotinic, salicylic, 5-aminosalicylic, and anthranilic acids) with Cu(II) cations was studied by spectral methods. The compositions of the complexes were determined and their stability constants, as well as the thermodynamic parameters (ΔH ⁰, ΔG ⁰, ΔS ⁰) of the complex formation were calculated. The structure of the drug in the polymeric ligand was found to affect certain physicochemical properties of the metal complexes.     

Preparation of magnetite nanoparticles modified with poly(<Emphasis Type="Italic">о</Emphasis>-phenylenediamine) and their use as drug carriers

A procedure was suggested for preparing nanoparticles consisting of magnetite and poly(o-phenylenediamine) by combining the hydrolysis of a mixture of iron(II) and (III) sulfates with oxidative polymerization of o-phenylenediamine. These nanoparticles efficiently bind phenylalanine, thyrosine, tryptophan, and doxorubicin in aqueous solution.     

New method of preparation of carboxy-protected amino acid conjugates of glycyrrhizinic acid

A method for preparation of carboxy-protected amino acid conjugates of glycyrrhizinic acid with the use of N-hydroxybenzotriazole, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, and methyl esters of L-amino acids (phenylalanine, tyrosine, leucine, isoleucine, and methionine) in 85–90% yield was developed. The structure of the prepared compounds was confirmed by IR and ¹³C NMR spectra.     

Gas-phase synthesis of terbium and lutetium carboxylates

Gas-phase ligand exchange between volatile lanthanide dipivaloylmethanates (Ln(dpm)₃; Hdpm is dipivaloylmethane, Ln = Tb, Lu) and o-substituted aromatic carboxylic acids (HCarb = Hsal is o-hydroxybenzoic acid, Habz is o-aminobenzoic acid, Hpobz is o-phenoxybenzoic acid, Hpa is o-anilinobenzoic acid). The gas-phase reaction involves the formation of the mixed-ligand complex Ln (dpm)_3?n (Carb)n, which is subsequently converted into tris-carboxylate (Ln(Carb)₃) on heating of the product in vacuum.     

Study of caffeine binding to human serum albumin using optical spectroscopic methods

The binding of caffeine to human serum albumin (HSA) under physiological conditions has been studied by the methods of fluorescence, UV-vis absorbance and circular dichroism (CD) spectroscopy. The mechanism of quenching of HSA fluorescence by caffeine was shown to involve a dynamic quenching procedure. The number of binding sites n and apparent binding constant K _b were measured by the fluorescence quenching method and the thermodynamic parameters ΔH, ΔG, ΔS were calculated. The results indicate that the binding is mainly enthalpy-driven, with van der Waals interactions and hydrogen bonding playing major roles in the reaction. The distance r between donor (HSA) and acceptor (caffeine) was obtained according to the Förster theory of non-radiative energy transfer. Synchronous fluorescence, CD and three-dimensional fluorescence spectroscopy showed that the microenvironment and conformation of HSA were altered during the reaction.     

Synthesis,Spectroscopic and Computational Studies of CT Complexes of Amino Acids with Iodine as σ Acceptor

Understanding the charge transfer process between bioactive molecules and inorganic or organic molecules is significant as this interaction can be used to interpret bioactive molecule–receptor interactions. A comprehensive spectrophotometric study has been performed to explore the complexation chemistry of the amino acids, tyrosine, lysine and arginine, with iodine as σ acceptor. The molecular structure, spectroscopic characteristics and the interactive modes have been deduced from UV–Vis and IR spectra. The binding ratio of complexation has been determined to be 1:1 for iodine with the amino acids. The association constant (K), extinction coefficient (ε _max), ionization potential (IP), energy of the charge transfer complex (E _CT), resonance energy (R _N ), dissociation energy (W) and standard Gibbs energy (ΔG°) have been computed. An in silico study has been carried out using GAMESS computations to understand the structural features. Highest occupied molecular orbital and lowest unoccupied molecular orbital calculations helped us in characterizing the chemical reactivity and kinetic stability of the molecules. A good consistency between experimental and computational results has been found.     

β-Cyclodextrin derivative as chiral carrier in membrane transport of some aromatic amino acids

The chiral functionalized β-cyclodextrin (heptakis (2,3,6-tri-o-acetyl)-β-cyclodextrin) was studied as an extraction receptor in liquid–liquid extraction and carrier through bulk liquid membrane for highlighting its ability to enantiomeric discrimination of some native aromatic amino acids (tryptophane, phenylalanine, and tyrosine) and methylester derivatives (tryptophane methylester hydrochloride, phenylalanine methylester hydrochloride, and tyrosine methylester hydrochloride). The receptor exhibited carrier ability for aromatic amino acids transport through chloroform liquid membrane, especially for l-amino acids form aiming at their separation. Active transport of amino acids assisted by the pH gradient was carried out. Additional, the circular dichroism measurements were performed to point out the enantiomeric discrimination of aromatic amino acids by functionalized β-cyclodextrin. The results showed that the structural properties of amino acids are an important parameter in extraction and transport experiments.     

Measuring the enthalpies of interaction between glycine,L-cysteine,glycylglycine, and sodium dodecyl sulfate in aqueous solutions

Calorimetric measurements of enthalpies of solution Δ_sol H ^m for glycine, L-cysteine, and glycylglycine in aqueous solutions of sodium dodecyl sulfate (SDS) with concentrations of up to 0.05 mol kg^–1 are made. Standard enthalpy of solution Δ_sol H ⁰ and enthalpy of transfer Δ_tr H ⁰ of the dipeptide from water into mixed solvent are calculated. The calculated enthalpy coefficients of paired interactions of amino acids and dipeptide with SDS prove to be positive. Hydrophobic interactions between the biomolecules and SDS are found to have a major impact on the enthalpies of interaction in the three-component systems under study, within the indicated range of concentrations.