Chiral glycopeptides

Method Development and Optimization of Enantiomeric Separations Using Macrocyclic Glycopeptide Chiral Stationary Phases... 

Table 2-2. The relative strength of potential interactions between glycopeptide CSPs and chiral analytes.

The enantioselectivity of the macrocyclic CSPs are different in each of the operating modes, probably because of different separation mechanisms functioning in the different solvent modes. The possible chiral recognition mechanisms for three mobile phase compositions on glycopeptide phases are listed in Table 2-3 in descending order of strength. 

Each glycopeptide CSP has unique selectivity as well as complementary characteristics, and a considerable number of racemates have been resolved on all three of them. Interestingly, most of the resolved enantiomers have the same retention order on these macrocyclic CSPs. When they are mixed or coupled with each other, the selectivity on one CSP will not be canceled by another. Even if some compounds may not have the same retention order, the complementary effects will result in an identifiable selectivity. Therefore, the coupled chiral columns can be used as a screening tool and save chromatographers substantial time in method development. 

This relatively new class of CSPs incorporates glycopeptides attached covalently to silica gel. These CSPs can be used in the normal phase, reversed phase, and polar organic modes in LC [62]. Various functional groups on the macrocyclic antibiotic molecule provide opportunities for tt-tt complexation, hydrogen bonding, and steric interactions between the analyte and the chiral selector. Association of the analyte... 

Chirobiotic Handbook, Guide to using macrocyclic glycopeptide bonded phases for chiral LC separations, Advanced Separation Technologies Inc. (ASTEC), 2nd Ed. Whippany, New York... 

Because plasma and urine are both aqueous matrixes, reverse-phase or polar organic mode enantiomeric separations are usually preferred as these approaches usually requires less elaborate sample preparation. Protein-, cyclodextrin-, and macrocyclic glycopeptide-based chiral stationary phases are the most commonly employed CSPs in the reverse phase mode. Also reverse phase and polar organic mode are more compatible mobile phases for mass spectrometers using electrospray ionization. Normal phase enantiomeric separations require more sample preparation (usually with at least one evaporation-to-dryness step). Therefore, normal phase CSPs are only used when a satisfactory enantiomeric separation cannot be obtained in reverse phase or polar organic mode. 

All these chemically modified glycopeptides have been helpful in understanding the mechanism of the chiral recognition process mediated by these macrocycles. 

In an attempt to change and broaden the capabilities of the vancomycin CSP, the glycopeptide was derivatized with (R)- and (S )-(l-naphthylethyl) isocyanate (NEIC) and then bonded to a silica-gel support [48]. A variety of chiral compounds was tested on the two composite stationary phases and the results were compared with the ones obtained using the underivatized vancomycin CSP. The advantages of the NEIC derivatization were not as obvious or substantial as they were in the case of cyclodextrin phases [49]. Moreover, the exact chemical structures of the synthesized NEIC derivatives of vancomycin were not reported. 

Glycopeptide antibiotics have successfully been used as chiral selectors to resolve the enantiomers of a variety of chiral compounds by means of both chromatographic and electrophoretic techniques. The idea of testing glycopeptide antibiotics as chiral selectors was first introduced by Armstrong and coworkers, at the Pittsburgh Conference in 1994. 

A comprehensive study on the temperature effect was done in 2004 for 71 chiral compounds on four glycopeptide CSPs TE, TAG, ristocetin A, and vancomycin phases, using the three RP, POM, and NP elution systems [95]. The separations were studied in the 5 5°C temperature range. Peak efficiencies always increased with temperature, but in only 17% of the separations studied, a small increase of the resolution was observed. In the rest of the cases, the resolution decreased or even vanished when temperature increased. All van t Hoff plots were linear, showing that... 

The majority of the HPLC chiral separations obtained with glycopeptides-containing CSPs are anyway achieved by UV/visible (see Section 2.3.2) or fluorescence [102] detection. 

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