Mechanisms of Chiral Discrimination in LEC

Of many enantioselective chromatographic systems developed thus far, ligand exchanging systems are the most examined. Understanding of the mechanism of chiral discrimination is greatly facilitated here by the fact that, of the three interactions needed for the chiral selector to recognize the solute enantiomers, two are well defined as the trans-coordination bonds of pairs of carboxylic and amino groups in the main coordination square of the Cu(II) ion. The third interaction should be looked for in the out-of-plane steri-cal interactions or coordination in remote axial positions of the coordination sphere. Thus with l-proline or l-hydroxyproline incorporating polystyrene-type resins, retention of l-isomers of amino acids is diminished by sterical interactions with the water molecule coordinated in one of the axial positions. On the other hand, the binding of d-isomers is facilitated by favourable hydrophobic interaction between the solute a-substituent and the polystyrene matrix, as shown below:

It is easy to imagine that, by using a mobile phase Tridentate amino acids, l-His, l-Asp, l-Glu, l-with a chiral selector of the opposite configuration, aHyp, manage to replace the water molecule from the we reverse the elution order of the resolved enantio- axial position and form one additional coordination bond, which logically explains the inverted elution shown above for the first resin. Conversely, with order of these solutes: an l-phenylalanine-type chiral selector in a poly-

With tridentate l-allo-hydroxyproline as the chiral selector, both bifunctional and trifunctional solutes can form two coordinate bonds only and elute in the order l before d without exception:

acrylamide matrix, the balance of steric interactions results in a reduction in the retention of l-amino acids (see above).

The decisive role of interactions with the achiral sorbent matrix in the enantiomer recognition was discovered by Davankov in chiral-coated and chiral mobile phase reversed phase systems. Here, only one of two solute enantiomers can realize favourable hydrophobic interactions with the silica immobilized alkyl groups, thus forming the stronger retained diastereomeric sorption complex.

With the l-Pro type selectors immobilized on polyglycidylmethacrylate, polyacrylamide, and poly-vinylpyridine matrixes, which occupy three coordination positions of Cu(II), the steric repulsion interactions diminish retention of the d-amino acid isomers as compared to l-isomers, as this was

The mechanism of chiral discrimination is less clear with selectors and/or analytes other than a-amino or a-hydroxy acids, and metal ions other than Cu(II), since the coordination mode of ligands in these complexes is less well known.

Figure 4 Enantiomeric separation of R,S-hydroxy acids with a chiral mobile phase. Conditions: 8mM L-PheNH2, 4mM Cu(CH3COO)2, pH 6.0, ambient temperature, column Spherisorb 3 ODS-2 (3 ^m, 150 x4.6 mm), flow rate 1 mL min~1, postcolumn derivatization with Fe(CIO4)3, UV 436 nm. (Reprinted with permission from Galaverna G, Panto F, Dossena A, Marchelli R and Bigi F (1995) Chirality7: 331-336. Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.)

Figure 4 Enantiomeric separation of R,S-hydroxy acids with a chiral mobile phase. Conditions: 8mM L-PheNH2, 4mM Cu(CH3COO)2, pH 6.0, ambient temperature, column Spherisorb 3 ODS-2 (3 ^m, 150 x4.6 mm), flow rate 1 mL min~1, postcolumn derivatization with Fe(CIO4)3, UV 436 nm. (Reprinted with permission from Galaverna G, Panto F, Dossena A, Marchelli R and Bigi F (1995) Chirality7: 331-336. Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.)

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