1,720,966 research outputs found
Study of retention, efficiency and selectivity in Chiral Ligand-Exchange Chromatography with a dynamically coated stationary phase
No full textA Bakerbond ODS column was dynamically coated with the chiral selector N-decyl-L-histidine, and then loaded with copper(II) ions. A number of racemic mixtures of underivatized amino acids were resolved on such a column via Chiral Ligand-Exchange Chromatography. The most important experimental parameters influencing column efficiency, retention and selectivity (eluent flow rate, analyte concentration, temperature and mobile phase composition) were exten-sively investigated. Results are discussed in light of CLEC theory and thermodynamic data on mod-el systems in aqueous solution. The most likely structures for the stationary ternary complex are suggested
Binary and ternary Cu(II) complexes of Ntau- and Npi-methyl-L-histidine in aqueous solution
No full textProtonation, binary and ternary Cu(II) complex formation constants of N-methyl-L-histidine (N-MH) and N-a-methyl-L-histidine (N-a-MH) have been measured through potentiometric titrations. UV/VIS spectra have been recorded for both the binary systems; the Cu(II)/N-a-MH complexes have been investigated by means of Fast Atom Bombardment mass spectroscopy as well.
The data on protonation equilibria of N-MH and N-a-MH show the existence of a difference in basicity between the respective nitrogen atoms, thus giving information on the tautomerism involving the imidazole residue of L-histidine (L-His) in the physiological pH range.
The complexation behaviour of N-MH is quite similar to that of underivatized L-His: the same species are formed and their log beta values are very close. On the contrary, the presence of a methyl substituent on the N alpha atom deeply affects the complexing properties of the ligand. N-a-MH cannot act as a tridentate ligand for steric reasons and tends to form polynuclear complexes
Host-guest complexation in chiral chromatography: study of retention, selectivity and efficiency of a HPLC column containing a chiral crown ether
No full text18-crown-6 ethers are able to form inclusion complexes with metal and ammonium cations as well as with protonated primary amines. If the latter contain a chiral centre close to the amino group and the crown ether bears some chiral barrier, the stability of diastereomeric complexes can be very different. This principle has been used to develop new chiral discrimination methods, particularly useful for amino acid enantiomer separation.
In the present work, the resolution of racemic mixtures of underivatized amino acids and oligopeptides on a Daicel Crownpak CR (+) column was extensively studied. Sample retention and selectivity were optimised varying eluent composition and column temperature. The effect of a number of experimental parameters on column efficiency was deeply investigated
Dynamic column coating procedure for chiral ligand-exchange chromatography
No full textThe preparation of HPLC columns for chiral ligand-exchange chromatography by dynamic adsorption of a histidine-based chiral selector (N(tau)-n-decyl-L-histidine, LNDH) on commercial reversed-phase, C18 columns was investigated. The most suitable solvent for column preparation was sought through a study of the behaviour of LNDH on the reversed-phase column eluting with different methanol-water ratios at varying pH. It is shown that the use of an aqueous solvent instead of pure methanol is advantageous, despite the decrease in LNDH solubility, and that the solvent pH must be controlled. Open and closed-cycle procedures for flowing the selector solution through the column are compared through a study of chromatographic retention, selectivity and efficiency parameters of selected enantiomers. Even though no significant difference in column loading was found between the two procedures, the open cycle method gave rise to more efficient and selective chiral columns
Synthesis of glycil-L-spinacine and study of its protonation and Cu(II) complex-formation equilibria, in aqueous solution
No full textA new dipeptide, glycil-L-spinacine, has been synthesised and fully characterised. Protonation constants have been determined and binary Cu(II) complex formation equilibria investigated in an aqueous solution (25 °C, I = 0.1 mol dm-3, KNO3) using the potentiometric and spectrophotometric techniques. Mononuclear and binuclear complex species have been found to form. Binding sites and structure hypotheses are discussed on the basis of available experimental and literature data
Copper and nickel complex-formation equilibria with Lys-Gly-His-Lys, a fragment of the matricellular protein SPARC
No full textComplex-formation equilibria of the tetrapeptide Lys-Gly-His-Lys with the Cu(II) and the Ni(II) ions have been studied in aqueous solution, at I=0.1 mol dm-3 (KNO3) and T=25 C. Protonation and complex-formation constants have been
potentiometrically determined. The structure of the main complex species is discussed on the basis of thermodynamic data obtained
by direct calorimetry as well as the CD, ESR and electronic spectra. The participation of two amide nitrogens in complex-formation
is suggested for both the metal ions, while no evidence supports the participation of the NH2 side groups of Lys residues in
coordination
Synthesis of spinacine and spinacine derivatives: crystal and molecular structures of N-pi-hydroxymethyl spinacine and N-alpha-methyl spinaceamine
No full textThe natural amino acid L-Spinacine (4,5,6,7-tetrahydro-IH-imidazo[4,5-c]pyridine-6-carboxylic acid) has been synthesized following a new pathway which gives a chemically and optically pure product with an excellent yield. The crystal structures of a synthetic intermediate, N-hydroxymethyl-spinacine, and a spinacine derivative, N-methyl-spinaceamine, have been investigated through X-ray diffraction.
Spi(MeOH) crystallizes with a water molecule and displays a zwitterionic character. The carboxylate group is in equatorial position and forms a short electrostatic interaction of 2.618(2)
A, between one of its oxygens and the protonated nitrogen of the tetrahydropyridine ring. The crystal packing is assured by strong O-H- - -O, O-H-- -N, N-H- - -N intermolecular hydrogen bonds and C-H---O close contacts. The biprotonated compounds Spm(alpha-Me) crystallizes with two Cl- anions and a water molecule. The positive charge on the imidazole ring is delocalized on the conjugated moiety N=C-N. The crystal is built up by clusters formed by two biprotonated Spm(aMe) molecules, tour Cl- anions and two water molecules linked together by hydrogen bonds
Nickel complexes of dipeptides: a thermodynamic and spectroscopic study
No full textComplex formation equilibria of some dipeptides (glycyl-alanine, glycyl-lysine, alanyl-alanine, alanyl-lysine, histidyl-alanine
and histidyl-lysine) with the Ni(II) ion have been studied in aqueous solution at I=0.1 mol dm−3 (KNO3) and T=25°C.
Protonation and complex formation constants have been potentiometrically determined. The structure of the main complex
species is discussed on the basis of thermodynamic data obtained by direct calorimetry, CD and absorption spectra. The
participation of the amide nitrogen in complex formation is suggested, while no evidence supports the participation of the NH2 side group of lysine in coordination
Non-covalent interactions in thermodynamic stereoselectivity of mixed-ligand copper(II)-D- or L-histidine complexes with L-amino acids. A possible model of metal ion-assisted molecular recognition
No full textFormation constants of ternary complexes of copper(II), L/D-histidine and, in turn, glycine, L-alanine, L-valine, L-leucine, L-tryptophan, or L-phenylalanine have been determined potentio-metrically at 25 °C and I= 0.1 mol dm–3(KNO3). In the case of amino acids with aromatic side chains the ternary complexes containing ligands of opposite chirality are more stable than those having ligands of the same chirality; the opposite is true for amino acids with aliphatic residues. Calorimetric measurements have been carried out to obtain the enthalpy and entropy values associated with complex formation. Copper(II)–histamine ternary systems with L-alanine or L-phenylalanine have also been investigated. Comparison of the thermodynamic parameters pertinent to formation of the histamine complexes with those of the analogous histidine complexes allows one to ascertain the number of donor atoms involved in the co-ordination to copper(II), in the histidine systems. The determination of H and S values renders easier understanding of the factors determining Stereoselectivity in the above systems. The Stereoselectivity may be explained in terms of non-covalent interactions between side-chain residues. The role played by the histidine carboxylate in the molecular recognition of amino acids is also discussed
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