Macrocyclic glycopeptides Vancomycin

The column was 25 cm long, 4.6 mm I.D. and packed with Partisil 10. It is seen that linear curves were obtained for three different solutes and two different moderators in n-heptane. Scott and Beesley [14] obtained retention data for the two enantiomers, (S) and (R) 4-benzyl-2-oxazolidinone. The column chosen was 25 cm long, 4.6 mm I.D. packed with 5 mm silica particles bonded with the stationary phase Vancomycin (Chirobiotic V provided by Advanced Separations Technology Inc., Whippany, New Jersey). This stationary phase is a macrocyclic glycopeptide Vancomycin that has a molecular weight of 1449.22, and an elemental composition of 54.69% carbon. 

Macrocyclic glycopeptides. The first of these CSPs - based on the cavity of the antibiotic vancomycin bound to silica - was introduced by Armstrong [25]. Two more polycyclic antibiotics teicoplanin and ristocetin A, were also demonstrated later. These selectors are quite rugged and operate adequately in both normal-phase and reversed-phase chromatographic modes. However, only a limited number of such selectors is available, and their cost is rather high. 

Scheme 2.1 General synthetic schemes for the immobilization of macrocyclic antibiotics. (a) (i) 1,6-Diisocyanatohexane (ii) glycopeptide (TE, TAG, and A-40,926). (b) (i) Glycopeptide (eremomycin, ristocetin A, vancomycin), (c) (i) NaI04 (ii) glycopeptide (vancomycin, ristocetin A, MDL 63,246). (d) (i) 3-Isocyanatopropyl-silyl derivative of macrocyclic antibiotic (TE, rifamycin B, vancomycin, DMP-vancomycin, NVC).

Although the macrocyclic glycopeptide antibiotic CSPs are very effective for the chiral resolution of many racemic compounds, their use as chiral mobile phase additives is very limited. Only a few reports are available on this mode of chiral resolution. It is interesting to note that these antibiotics absorb UV radiation therefore, the use of these antibiotics as the CMPAs is restricted. However, Armstrong et al. used vancomycin as the CMPA for the chiral resolution of amino acids by thin-layer chromatography, which will be discussed in Section 10.7. 

Two EPMEs based on macrocyclic glycopeptide antibiotics—vancomycin and teicoplanin—were designed for the assay of acetyl-L-carnitine [44]. The linear concentration ranges for the proposed electrodes were 10 5-10-2 mol/L for the vancomycin-based electrode and 10 4-10-2 mol/L for the teicoplanin-based electrode, with slopes of 58.1 and 55.0mV/p(acetyl-L-camitine), respectively. The enantioselectivity was determined over D-carnitine. 

Three EPMEs based on macrocyclic glycopeptide antibiotics— vancomycin and teicoplanin (modified or not with acetonitrile)—were proposed for the determination of l- and D-enantiomers of methotrexate (Mtx) [48]. The linear concentration ranges for the proposed enantioselective membrane electrodes were between 10 6 and 10-3 mol/L for l- and D-Mtx. The slopes of the electrodes were 58.00 mV/pL-Mtx for vancomycin-based electrode, 57.60 mV/pD-Mtx for teicoplanin-based electrode and 55.40 mV/pD-Mtx for teicoplanin modified with acetonitrile-based electrode. The detection limits of the proposed electrodes were of 10 8 mol/L magnitude order. All proposed electrodes proved to be successful for the determination of the enantiopurity of Mtx as raw material and of its pharmaceutical formulations (tablets and injections). 

The group of Bakhtiar [57-59] described the chiral bioanalysis of MPH in various matrices, utilizing a number of sample pretreatment strategies, separation on a Chirobiotic V columns, and the use of positive-ion APCI-MS in SRM mode. The chiral selectivity of the Chirobiotic V column is based on the use of the macrocyclic glycopeptide antibiotic vancomycin. The column can be used in both aqueous and organic mobile phase. 

Tesarova and Bosakova [58] proposed an HPLC method for the enantio-selective separation of some phenothiazine and benzodiazepine derivatives on six different chiral stationary phases (CSPs). These selected CSPs, with respect to the structure of the separated compounds, were either based on b-CD chiral selectors (underivatized (J>-CD and hydroxypropyl ether (3-CD) or on macrocyclic antibiotics (vancomycin, teicoplanin, teicoplanin aglycon and ristocetin A). Measurements were carried out in a reversed-phase separation mode. The influence of mobile phase composition on retention and enantio-selective separation was studied. Enantioselective separation of phenothiazine derivatives, including levopromazine (LPZ), promethazine and thioridazine, was relatively difficult to achieve, but it was at least partly successful with both types of CSPs used in this work (CD-based and glycopeptide-based CSP), except for levomepromazine for which only the [CCD-based CSP was suitable. 

Figure 10.6. Structures of the macrocyclic glycopeptide chiral selectors vancomycin (A) and teicoplanin (B).

The chromatographic system chosen to examine was the chiral stationary phase Chirobiotic V (as already described, this is a macrocyclic glycopeptide called Vancomycin bonded to silica gel particles 5 pm in diameter) which was used in the normal phase mode (solute retention controlled largely by polar interactions), employing mixtures of hexane and ethanol as the mobile phase. 

The antibiotics vancomycin (shown in Table 1), teicoplanin, and ristocetin A can be bonded to silica, giving a unique class of macrocyclic glycopeptide CSPs. They can be used in the normal-phase mode with a nonpolar eluent as well as in the reversed-phase mode with an aqueous eluent. They show unique selectivity for a large number of analytes. Figure 3 gives an example with the separation of the antidepressant citalopram and its metabolites on a vancomycin CSP. 

Figure 4.16 Chemical structures of the macrocyclic glycopeptide antibiotics (a) vancomycin, (b) teicoplanin, (c) avoparcin, (d) ristocetin A, that have been used as chiral selectors in CSPs for HPLC. Reproduced from Ward and Farris, J. Chromatogr. A 906 (2001), copyright (2001), with permission from Elsevier.

We performed this difficult study using the Supelco Chirobiotic0 CSPs based on macrocyclic glycopeptide selectors. The system parameters, e, s, a, b, and v, of the four different columns, namely ristocetin, vancomycin, teicoplanin, and teicoplanin aglycon, were obtained for six different normal heptane-ethanol mobile phases and ten different acetonitrile or methanol-water reversed mobile phases [47, 48]. The 44 achiral probe solutes contained aldehydes, ketones, amides, halogenated phenols, nitro-substituted benzenes, and nitro-alkanes. About 2,400 chromatograms were needed to obtain the 5 fl-v system parameters for the 4 CSP columns and the 16 mobile phases [47]. 

This chapter will focus on chiral recognition mechanisms and molecular interactions. The commercialized macrocyclic glycopeptides ristocetin, R, vancomycin, V, teicoplanin, T and its aglycon form, teicoplanin aglycon, TAG will be the main subject of the study since a large database is available in the literature [3-7]. However, non-commercialized avoparcin, balhimycin, and the recently introduced eremomycin macrocyclic glycopeptides [8] are also included in the study for more information on possible mechanisms. 

Table 2 lists a selected set of the considerable data published on the separation of native or derivatized amino acids on macrocyclic glycopeptide CSPs [1, 2, 11-14], The vancomycin chiral selector was found to have a lower efficacy in the separation of amino acids compared to that of the three other commercialized CSP (Chirobiotic T, TAG, and R) [14]. 

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. 

The macrocyclic glycopeptide chiral selectors are now a very important class of CSPs that must be part of the column set of any laboratory involved in enantiomeric separations. The variety of functionalities found in these relatively small molecules allow for many different interactions leading to successful enantioseparations [29]. The similarities between members of this class of chiral selectors produced the complementary separation property [14, 30, 31]. If a partial separation of an enantiomeric pair is observed on a macrocyclic selector, say vancomycin, a baseline separation may very likely be observed on a different selector, say teicoplanin. This interesting property in method development illustrates the large number of selector-selectand possible interactions. Such complementarities are due to the... 

Abstract Medical studies established that vancomycin and other related macro-cyclic antibiotics have an enhanced antimicrobial activity when they are associated as dimers. The carbohydrate units attached to the vancomycin basket have an essential role in the dimerization reaction. Covalently synthesized dimers were found active against vancomycin-resistant bacterial strains. A great similarity between antibiotic potential and enantioselectivity was established. A covalent vancomycin dimer was studied in capillary electrophoresis producing excellent chiral separation of dansyl amino acids. Balhimycin is a macrocyclic glycopeptide stmcturally similar to vancomycin. The small differences are, however, responsible for drastic differences in enantioselectivity in the same experimental conditions. Contributions from studies examining vancomycin s mechanism for antimicrobial activity have substantially aided our understanding of its mechanism in chiral recognition. 

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