Macrocyclic Antibiotics as Effective Chiral Selectors in Liquid Chromatography for Enantiomeric Separation of Pharmaceutical Compounds: A Review

Chiral separation techniques play a crucial role in the pharmaceutical industry, where the enantiomeric purity of drugs can have a significant impact on their efficacy and safety. Macrocyclic antibiotics are highly effective chiral selectors used in various chiral separation techniques, including LC, HPLC, SMB, and TLC, offering reproducible results and a wide range of applications. However, developing robust and efficient immobilization mechanisms for these chiral selectors remains a challenge. This review article focuses on various immobilization approaches, such as immobilization, coating, encapsulation, and photosynthesis, that have been applied to immobilize macrocyclic antibiotics on their support. Commercially available macrocyclic antibiotics for conventional liquid chromatography include Vancomycin, Norvancomycin, Eremomycin, Teicoplanin, Ristocetin A, Rifamycin, Avoparcin, Bacitracin, and others. In addition, capillary (nano) liquid chromatography has also been used in chiral separation utilizing Vancomycin, Polymyxin B, Daptomycin, and Colistin Sulfate. Macrocyclic antibiotic-based CSPs have been extensively applied due to their reproducible results, ease of use, and broad range of applications, capable of separating a large number of racemates.

Enantioseparation of β-Blockers Using Silica-Immobilised Eremomycin Derivatives as Chiral Stationary Phases in HPLC

The regularities of chromatographic retention and separation enantioselectivity of the selected β-blockers (propranolol, pindolol, alprenolol, atenolol, oxprenalol, metoprolol, clenbuterol, sotalol, pronethalol, salbutamol, and labetalol) were studied with eight chiral stationary phases (CSPs) in polar ionic mode (PIM) elution system. A range of novel CSPs was prepared by immobilisation of macrocyclic glycopeptide antibiotic eremomycin (E-CSP); structurally related antibiotics chloreremomycin (Chloro-E-CSP) and semi-synthetic oritavancin (O-CSP); and five eremomycin derivatives including amide- (Amide-E-CSP), adamantyl-2-amide- (Adamantylamide-E-CSP), eremomycin aglycon (EAg-CSP), eremosaminyl eremomycin aglycon (EEA-CSP), and des-eremosamynyl eremomycin (DEE-CSP) onto microspherical silica (Kromasil, particle size 5 micron, pore size 11 nm). The effect of different functional groups in eremomycin structure on chiral recognition of β-blockers was studied. The original E-CSP revealed moderate enantioseparation for all studied β-blockers. The presence of a free carboxylic group in a chiral selector molecule is found to be critical for the general retention of enantiomers as no separation enantioselectivity was recorded for Amide-E-CSP and Adamantyl-E-CSP. Modification of the aromatic system of eremomycin by the introduction of a chloro- substituent in the aromatic ring (Chloro-E-CSP) or a hydrophobic 4’-chlorobiphenylmethyl substituent to the disaccharide sugar residue (O-CSP) resulted in decreased enantioselectivity. The best enantioseparation of β-blockers was obtained for CSPs with eremosaminyl eremomycin aglycon and des-eremosamynyl eremomycin as chiral selectors.

Polysaccharide-Based Chiral Stationary Phases on Gold Nanoparticles Modified Silica Beads for Liquid-Phase Separation of Enantiomers

Au nanoparticles (AuNPs) (10−15 nm in size) were prepared and deposited on the surfaces of silica particles functionalized using 3-aminopropyltriethoxysilane as the seeds under mild conditions. Then, Au seeds grew further and formed nanosheets by the method of gold chloride hydrate reduction. 3, 5-dimethylphenyl isocyanate derivative of cellulose as chiral selector was coated on the surfaces of SiO2/Au. The obtained spheres possessed a sandwich structure in which silica bead, the packed Au NPs monolayer and cellulose derivative were the core, the interlayer and the shell, respectively. The resultant packing material was evaluated by high-performance liquid chromatography (HPLC) as chiral stationary phase (CSP). The separations of nine pairs of enantiomers were achieved in the normal-phase liquid chromatography mode. The results showed that the new CSP has sufficient interaction with the analytes due to the existence of AuNPs on silica surfaces compared with coated cellulose-silica column.