Page 50 - Chiral Separation Techniques

26 2 Method Development and Optimization of Enantiomeric Separations Using …

2.2 Characteristics of Macrocyclic Glycopeptide CSPs

2.2.1 Chiral Recognition Mechanisms

Vancomycin, ristocetin A and teicoplanin are produced as fermentation products of
Streptomyces orientalis, Nocardia lurida and Actinoplanes teichomyceticus, respec-
tively. All three of these related compounds consist of an aglycone “basket” made up
of fused macrocyclic rings and pendant carbohydrate moieties (Fig. 2-1). The
macrocycles contain both ether and peptide linkages. The aglycones of vancomycin
and teicoplanin contain two chloro-substituted aromatic rings, while the analogous
portion of ristocetin A contains no chloro substituents.
Vancomycin is the smallest of the three molecules, consisting of three macro-
cyclic rings and an attached disaccharide comprising D-glucose and vancosamine.
The other two glycopeptides are somewhat larger and have four fused macrocyclic
rings and different types of pedant sugar moieties. Teicoplanin has three attached
monosaccharides, two of which are D-glucosamine and one of which is D-mannose.
Ristocetin A has a pendant tetrasaccharide and two monosaccharide moieties. These
saccharides include D-arabinose, D-mannose, D-glucose, and D-rhamnose.
Teicoplanin has one unique characteristic: namely, it has a hydrophobic acyl side
chain (“hydrophobic tail”) attached to a 2-amino-2-deoxy-β-D-glucopyranosyl moi-
ety. The structural characteristics of the three macrocycles are outlined in Table 2-1.

Table 2-1. Structural characteristics of macrocyclic glycopeptides.

Vancomycin Teicoplanin Ristocetin A
Molecular weight 1,449 1,877 2,066
Stereogenic centers 18 23 38
Macrocycles 3 4 4
Sugar moieties 2 3 6
Hydroxyl groups 9 15 21
Amine groups 2 1 2
Carboxyl groups 1 1 0
Amido groups 7 7 6
Aromatic groups 5 7 7
Methyl esters 0 0 1
Hydrophobic tail 0 1 0
pI values 7.2 4–6.5 7.5

All three glycopeptides have analogous ionizable groups which control their
charge and are thought to play a major role in their association with analytes and chi-
ral recognition. For example, there is an amine on the aglycone portion of each com-
pound. Vancomycin has a secondary amine, while the other two macrocycles have
primary amine groups. All three compounds also have amino saccharide moieties.
However, teicoplanin is unique in that it has two amino saccharides, both of which
are N-acylated. There is a carboxylic acid moiety on the aglycone of both van-
comycin and teicoplanin, while the equivalent group on ristocetin A is esterified. The