The Mechanism of Chromatographic Retention - Chiral Chromatography > Chiral Polysiloxane Stationary Phases > Page 71
The second type of chiral stationary phase consisted of relatively small molecular weight chiral substances bonded to silica and were pioneered by Pirkle (20). Although each bonded group has a limited number of chiral centers available, due to their small size, there are a large number of them on the silica (as opposed to much larger complex chiral moieties), so, a relatively high interaction probability with a chiral center is maintained. The advantage of the Pirkle chiral phase is that the overall interacting molecule is small, and. so, the extra interactive contributions to retention are also small. It follows, that the chiral selectivity becomes the dominant factor controlling retention. The third type are the polymers of cellulose and amylose developed by Okamato (21) The polymers are derivatized to link appropriate interactive groups to the cellulose polymer which is then physically coated onto a silica support. The fourth type is based on the macrocyclic glycopeptides introduced by Armstrong (22). These materials contain a large number of chiral centers, together with molecular cavities in which solute molecules can enter and interact with neighboring groups. The spatial character of the solute will determine the degree of entry and consequently the proximity of interaction which, in turn, will determine the energy of interaction and the magnitude of the retention. The macrocyclic glyco peptides are one of the more popular chiral phases and the structure of Vancomycin is shown in figure 35.
A, B and C are inclusion cavities. Molecular weight 1449. Chiral centers 18. pK's 2.9, 7.2, 8.6, 9.6, 10.4, 11.7. Isoelectric point 7.2
Courtesy of ASTEC Inc.
Figure 35 The Proposed Structure of Vancomycin