Amino acids ligand exchange

Amino acids bonded to silica and loaded with Cu ions can interact in stereoselective manner with amino acids in aqueous solution. The copper ion forms a complex with both the bound and the sample amino acids. Ligand exchange phases are suited for the separation of amino acids as well as of some -amino alcohols and similar molecules because these compounds bear two polar functional groups with appropriate spacing. This approach has found limited interest because the column efficiencies are rather low, the detectability of the nonderivatized sample... 

Achiral Columns Together with Chiral Mobile Phases. Ligand-exchange chromatography for chiral separation has been introduced (59), and has been appHed to the resolution of several a-amino acids. Prior derivatization is sometimes necessary. Preparative resolutions are possible, but the method is sensitive to small variations in the mobile phase and sometimes gives poor reproducibiUty. 

Some ligand-exchange CSPs have been used at preparative level [31, 32]. In this case it must be taken into account that an extraction process, to remove the copper salts added to the mobile phase, must be performed following the chromatographic process [33]. Teicoplanin, in contrast, resolves all ordinary a and (3-amino acids with mobile phases consisting of alcohol/water mixtures. No buffer is needed in the... 

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

Ligand exchange has been a favoured method for the concentration of amino acids from solution because of its selectivity [268]. Ion exchange has often then been used for the final separation of the acids [269-272],... 

Exchange of complex cations. Complexation of transition metal cations with uncharged ligands such as with amines and with amino acids results in a selectivity enhancement compared to the selectivity of the aqueous metal cation (27, 65-72). Fig. 3 shows an example for the Cu(ethylenediamine) adsorption in montmorillonites of different charge density. Standard thermodynamic data for other cases are given in table IV. In all cases the free ligand concentration in equilibrium solution was... 

Galli, B. et al., Enantiomeric separation of DNS-amino acids and DBS-amino acids by ligand exchange chromatography with (5) - and (R)-phenylalaninamide modified silica gel, J. Chromatogr. A, 666,11, 1994. 

The next question is, where the protons go to in the active site during the catalytic cycle. For the base, there are too many possibilities to be certain. Groups near to the dinuclear cluster that can accept or exchange protons include the side chains of the amino acids arginine and histidine, thiolate ligands to the cluster, and peptide NH groups. 

Chiral ligand-exchange chromatography (CLEC) ° separates enantiomers by the formation of diastereomeric metal complexes. In a first instance the technique was mainly used for the separation of amino acids. Impressive results of the first separations gave rise to intensive investigation in the field and numerous publications appeared in the literature, which have been reviewed. 

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