Amino Acid Enantioseparation via Macrocyclic Glycopeptide Stationary Phases

There are three commonly used macrocyclic glycopeptides and they are the antibiotics vanco-mycin, teicoplanin an avoparcin all of which were introduced as chiral stationary phases by Armstrong. They contain a large number of chiral centres, together with molecular cavities in which solute molecules can enter and interact with neighbouring groups. Vancomycin, for example, contains 18 chiral centres surrounding three 'pockets' or 'cavities' which are bridged by five aromatic rings. Strong polar groups are proximate to the ring structures that can offer strong polar interactions with the solutes. This type of stationary phase is stable in mobile phases containing 100% organic solvent.

The macrocyclic glycopeptides have a higher loading capacity than the traditional protein phases and are more stable. They can also tolerate a much wider range of solvents than the cellulose and amylose phases.

The macrocyclic glycopeptide stationary phases can also be used very effectively for the separation of amino acids and their derivatives. The separation of the isomeric bromophenylalanines as their FMOC derivatives formed by reacting them with 9-fluorinyl-methylchloroformate is shown in Figure 3. The two enantiomers are very well separated indicating that the chiral selectivity of the telcoplanin stationary phase was extremely high. It should be noted, that the 'pure' (S) enantiomer actually contained a significant amount of the (R) enantiomer. The macrocyclic glycopeptide stationary phases often exhibit high selectivity for chiral substances of biological origin, perhaps due to their being biological products themselves.

0 10 20 Retention time (minutes)

Figure 2 The separation of N-benzyloxycarbonyl alanine ethyl ester on cellulose tris(3,5-dimethylphenylcarbamate).

0 10 20 Retention time (minutes)

Figure 2 The separation of N-benzyloxycarbonyl alanine ethyl ester on cellulose tris(3,5-dimethylphenylcarbamate).

Figure 3 The separation of the enantiomers of 2-bro-mophenylalanine and 3-bromophenylalanine. (A) A 'pure' sample of the S enantiomer of FMOC 2-bromophenyl alanine. (B) A ra-cemic mixture of FMOC 3-bromophenylalanine. The separation was carried out on a CHIROBOTIC T(teicoplanin) column, 25-cm long, 4.6-mm i.d., packed with 5 ^m particles. The mobile phase was programmed from methanol-1 % triethylamine acetate (pH 4.5) (40 : 60 v/v) to methanol-1 % triethylamine acetate (pH 4.5) (60 : 40 v/v) over 20 min. The flow rate was 1.0 mL min~1 and the sample was injected as a solution in acetone. (Courtesy of Chirotech Technology Ltd.)

Figure 3 The separation of the enantiomers of 2-bro-mophenylalanine and 3-bromophenylalanine. (A) A 'pure' sample of the S enantiomer of FMOC 2-bromophenyl alanine. (B) A ra-cemic mixture of FMOC 3-bromophenylalanine. The separation was carried out on a CHIROBOTIC T(teicoplanin) column, 25-cm long, 4.6-mm i.d., packed with 5 ^m particles. The mobile phase was programmed from methanol-1 % triethylamine acetate (pH 4.5) (40 : 60 v/v) to methanol-1 % triethylamine acetate (pH 4.5) (60 : 40 v/v) over 20 min. The flow rate was 1.0 mL min~1 and the sample was injected as a solution in acetone. (Courtesy of Chirotech Technology Ltd.)

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