Polar organic modes, glycopeptide

The macrocyclic glycopeptides CSPs arc capable of operating in three different mobile phase systems reversed phase, normal phase, and the new polar organic mode. The new polar organic mode refers to the approach when methanol is used as the mobile phase with small amounts of acid and/or base as the modifier to control... 

Statistically, of the compounds enantioresolved by macrocyclic glycopeptide CSPs, new polar organic mode accounts for more than 40 %, balanced by reversed-phase mode, while typical normal-phase operation resulted in approximately 5 % of separations. Some categories of racemic compounds that are resolved on the glycopeptide CSPs at different operating modes are listed in Table 2-4. 

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... 

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. 

The specificity of the amino acid charge-charge docking interaction must be pointed out. Non-amino acid enantiomers could still be separated by vancomycin and teicoplanin column in presence of copper ions clearly showing the multiple possible mechanisms of the macrocyclic glycopeptide selectors. The two enantiomers of tryptophan ethyl ester, a carboxylic acid blocked amino acid, could be separated on a ristocetin A chiral column in polar organic mode [16]. The separation did not involve the secondary amine on the macrocycle but other possible multiple interactions with the ristocetin A selector. 

Analytical Properties Substrate has 38 chiral centers and 7 aromatic rings surrounding 4 cavities (A, B, C, D), making this the most structurally complex of the macrocyclic glycopeptides substrate has a relative molecular mass of 2066 this phase can be used in normal, reverse, and polar organic phase separations selective for anionic chiral species with polar organic mobile phases, it can be used for a-hydroxy acids, profens, and N-blocked amino acids in normal phase mode, it can be used for imides, hydantoins, and N-blocked amino acids in reverse phase, it can be used for a-hydroxy and halogenated acids, substituted aliphatic acids, profens, N-blocked amino acids, hydantoins, and peptides Reference 47, 48... 

There are many classes of CSPs applicable in different mobile-phase modes. In particular, CSPs based on derivatized polysaccharides, native and derivatized cyclodextrins, macrocyclic glycopeptides, and Pirkle-type chiral selectors operate quite well in four separation modes, i.e RP, polar organic phase, NP, and super- or subcritical fluid chromatography (SFC) conditions. It is common that a chiral compound can be separated on the same CSP in more than one separation mode [58, 160, 166, 170-176]. For example, Nutlin-3, a small molecule antagonist of MDM2, has been baseline resolved from its enantiomer in all four mobile-phase conditions (Fig. 16) [170]. Multimodal enantioseparation on the same CSP would be greatly beneflcial for chiral method development in pharmaceutical industry. 

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