In this case, the mobile phase consisted of a mixture of acetonitrile/water : 8/2 v/v. It is seen that the cyclodextrin based stationary phase elegantly resolves both pairs of enantiomers. The separation of the diastereomers formed by reacting the reagents with some chiral drugs is shown in figure 51. The column was the Intersil ODS 80A, 15 cm long, 4.6 mm I.D., packed with 5 mm particles. The largest peak in the chromatogram was the reagent itself. Peak 1 was (R)-(+)-Propanolol, peak 2 was (S)-(-)-Propanolol, peak 3 (R)-(+)-Alprenolol and peak 4 (S)-(–)-Alprenolol. Each peak, except that of the reagent, represents 25 ng of the original drug.

Courtesy of the Royal Society of Chemistry, [Ref. 15]

Figure 51 The Separation of the Diastereomers of Propanolol and Alprenolol on a Reversed Phase Column

Isomers of Terbutaline Using Macrocyclic Glycopeptides

Another group of chiral stationary phase are the macrocyclic glycopeptide phases. The macrocyclic glycopeptides were first introduced by Armstrong [16]. One way of preparing the glycopeptide phases is to covalently bond Vancomycin to the surface of silica gel particles. Vancomycin contains 18 chiral centers surrounding three 'pockets' or 'cavities' that are bridged by five aromatic rings. Strong polar groups are proximate to the ring structures to offer strong polar interactions with the solutes. This type of stationary phase is stable in mobile phases containing 0–100% organic solvent. The proposed structure of Vancomycin is shown in figure 52.

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 52 The Proposed Structure of Vancomycin