Liquid Chromatography - Cyclodextrin 1
There are two forms of Avoparcin the unsubstituted a-Avoparcin structure and the chlorinated structure b-Avoparcin, the molecular weights being 1909 and 1944 respectively. The ratio of a-Avoparcin to b-Avoparcins is about 1:4. The aglycon portion of Avoparcin contains three connected semie rigid macrocyclic rings (one 12-membered, and two 16-membered) which form a pocket providing possible solute inclusion. The glycopeptide contains seven aromatic rings with four phenol moieties, four carbohydrate chains, 16 hydroxyl groups, one carboxylic acid, two primary amines, one secondary amine, six amide linkages, two chlorine atoms for b-Avoparcin (only one for a-Avoparcin) and 32 stereogenic centers.
It is clear that, there is a wide diversity of interactive possibilities ranging from weak and strong dispersive interactions, to polar interactions that span from induced dipole interaction, through dipole–dipole interaction, to strong hydrogen bonding. In addition, at the right pK, basic and acidic ionic interactions can also be invoked. More importantly, with 32 stereogenic centers the probability of interaction between chiral centers of solute and stationary phase is relatively high.
The cyclodextrin based chiral stationary phases are some of the more popular materials used for contemporary chiral separations. One of their advantages lies in their use with all types of solvent. They can be used very effectively in the reversed phase mode and, as well as being usable as a normal phase. The cyclodextrins and their derivatives have been widely used for all types of chiral separations and can often be used for preparative separations. Cyclodextrin-based phases are readily available, covalently bonded to spherical silica gel particles 5 mm in diameter. The cyclodextrins are produced by the partial degradation of starch followed by the enzymatic coupling of the glucose units into crystalline, homogeneous toroidal structures of different molecular size. The molecular structure of a, b, and g cyclodextrins are shown in figure 54. The alpha-, beta- and gamma-cyclodextrins and have been shown to contain 6 (cyclohexamylose), 7 (cycloheptamylose) and 8 (cyclooctamylose) glucose units, respectively. These cyclic, chiral, torus shaped macromolecules contain the D(+)-glucose residues bonded through a-(1-4)glycosidic linkages.
Courtesy of ASTEC Inc.
Figure 54 The Molecular Structure of a, b, and g Cyclodextrins