Direct thin layer chromatography enantioresolution of some basic DL-amino acids using a pharmaceutical industry waste as chiral impregnating reagent

Sustainable solutions for direct TLC enantioseparation with in-home thought-out, prepared/modified chiral stationary phases

TLC is used globally, yet less attention has been paid to TLC (in enantioseparation) despite its advantages. The present paper describes/reviews successfully practiced direct approaches of 'chiral additive in achiral stationary phase' (as an application of in-home thought out, prepared, tested, and modified chiral stationary phase), 'pre-mixing of chiral reagent with the enantiomeric mixture' (an approach using both achiral phases during chromatographic separation) and 'chiral additive in mobile phase', and chiral ligand exchange for enantioseparation of DL-amino acids, their derivatives, and some active pharmaceutical ingredients. It provided efficient enantioseparation, quantitative determination, and isolation of native forms via in-situ formation of non-covalent diastereomeric pair. The mechanism of enantioseparation in these approaches has been discussed along with the isolation and establishment of the structure of diastereomers. This may help chemists gain useful insights into fields outside their specialization and the experts get brief accounts of recent key developments, providing solutions for sustainable development of less expensive methods for control of enantiomeric purity and isolation of native enantiomers.

Silica-coated gold nanorod arrays for nanoplasmonics devices

A facile method for growing silica layer on lithographically designed gold nanorod arrays (GNRAs) using a convenient sol-gel method is presented herein. The silica layer thickness was controlled on GNRAs with the reaction time. The localized surface plasmon resonance (LSPR) spectra of these hybrid metal/dielectric nanoparticles were recorded before and after the coating and the effect of different solvents on the LSPR were also assessed. The change in the fluorescence and SERS intensities of a probe molecule (Rh6G) deposited on GNRAs and silica-coated GNRAs revealed that the as-fabricated silica layer does inhibit the quenching of molecular excited states and enhances photophysical/photochemical processes. This kind of hybrid metal/dielectric nanoparticle arrays hence turn out to be real good candidates to design new "plasmonic-active" devices.

Synergistic Effect of Cationic-Nonionic Surfactants on Simultaneous Separation of Phenylalanine and Tyrosine

A new thin layer chromatographic system comprising of silica gel impregnated with 0.1% aqueous solution (below CMC) of nonionic surfactant Brij-35 as stationary phase and 0.1% aqueous solution (below CMC) of cationic surfactant (cetrimide) as eluent has been found most suitable for the separation of closely related amino acids (phenylalanine and tyrosine) from their mixture. Effect of substitution of Brij-35 by cetrimide and sodium cholate has been examined to assess the impact of charge of impregnant on the chromatographic behavior of analytes. The changes brought about by the impregnation in the structure and homogeneity of silica gel have been studied by scanning electron microscopy (SEM) and fourier transform infra-red spectroscopy (FTIR) techniques. Optimizations of concentration of Brij-35 and cetrimide in the stationary and mobile phases respectively and the effect of bed height on the separation have also been examined. Chromatographic parameters like ΔRF, separation factor (α) and resolution (RS) were calculated for the separation of tyrosine from phenylalanine and separation mechanism have been proposed. The limits of detection for phenylalanine and tyrosine were found to be 0.25 and 0.23 μg respectively. The proposed method studied is simple, rapid and free from the use of volatile organic solvents.

Enantioseparations by thin-layer chromatography

An up-to-date overview of thin-layer chromatography (TLC) techniques for chiral separations of various significant and/or recent examples of enantioresolutions is reported. Furthermore, examples for chiral separations obtained on achiral commercially available C18 TLC plates are described in detail. These include the enantioseparation of methylthiohydantoin-phenylalanine and methylthiohydantoin-tyrosine using hydroxyethyl-β-cyclodextrin as mobile phase additive and the separation of the enantiomers of warfarin and p-chlorowarfarin using bovine serum albumin as mobile phase additive.

Indirect resolution of enantiomers of penicillamine by TLC and HPLC using Marfey's reagent and its variants

TLC and HPLC methods were developed for indirect chiral separation of penicillamine (3,3-dimethylcysteine) enantiomers after derivatization with Marfey's reagent (FDNP-Ala-NH(2)) and two of its structural variants, FDNP-Phe-NH(2) and FDNP-Val-NH(2). The binary mobile phase of phenol-water (3:1 v/v) and solvent combinations of acetonitrile and triethylamine phosphate buffer were found to give the best separation in normal and reversed-phase TLC, respectively. The diastereomers were also resolved on a reversed-phase C18 HPLC column with gradient elution of acetonitrile and 0.01 m trifluoroacetic acid. The results due to these three reagents were compared. The method was successful for checking the enantiomeric impurity of l-penicillamine in d-penicillamine and to check the enantiomeric purity of pharmaceutical formulations of d-penicillamine. The method was validated for linearity, repeatability, limit of detection and limit of quantification.

Thin-layer chromatography separation of enantiomers of verapamil using macrocyclic antibiotic as a chiral selector

Silica gel thin-layer chromatography plates impregnated with macrocyclic antibiotic, vancomycin, as chiral selector were prepared and used for the resolution of (+/-)-verapamil. A mobile phase system of acetonitrile-methanol-water (15:2.5:2.5, v/v) was worked out systematically. The effects of chiral selector, temperature and pH on resolution were also studied. The spots were detected with iodine vapors and the detection limit was found to be 0.074 microg of each enantiomers.

Direct enantiomeric resolution of (+/-)-atenolol, (+/-)-metoprolol, and (+/-)-propranolol by impregnated TLC using L-aspartic acid as chiral selector

Resolution of three commonly used beta-blockers, (+/-)-atenolol, (+/-)-metoprolol and (+/-)-propranolol, into their enantiomers has been achieved using normal-phase TLC on silica gel plates impregnated with L-aspartic acid as the chiral selector. Different combinations of acetonitrile-methanol-water as mobile phase were found to be successful in resolving the enantiomers. The spots were detected with iodine and the detection limits were found to be 0.26 microg for atenolol and 0.23 microg for each of metoprolol and propranolol as racemate.

Use of surface plasmon resonance biosensor technology as a possible alternative to detect differences in binding of enantiomeric drug compounds to immobilized albumins

The use of biosensors for monitoring real time interactions between biomolecules and drug compounds has a lot of advantages over presently existing detection methods, the major ones being the elimination of radio labels and rapid screening. We can also obtain information about the kinetic parameters and these values may serve as useful indicators towards subtle differences in the binding strength and characteristics of closely related drug compounds and enantiomers. The Biacore 3000 biosensor based on the Surface Plasmon Resonance (SPR) technology was used to assess the albumin protein binding differences between two enantiomers of a drug compound. Normalized responses (NRU) and affinity constants (K(D)) were readily calculated. Statistical parameters like mean normalized responses, %CV values were determined to make the technique robust. The %CV values obtained were within the preset limits of < or = 25% (FDA limits for drug development and method validation protocols) for the binding interactions for majority of the concentrations studied. For example, the %CV values for the normalized responses for the binding of the control drug warfarin to human albumin ranged from 7.9 to 24.3%. The method gave reproducible results, and the results indicated slight differences in binding patterns of the enantiomers to human and rat albumin.

Resolution of enantiomers of DL-amino acids on silica gel plates impregnated with optically pure (-)-quinine

TLC resolution of enantiomers from racemic amino acids was achieved on silica gel plates impregnated with optically pure (-)-quinine. The successful solvent systems were butanol-chloroform-acetic acid (3:7:5, v/v) for DL-methionine; 6:8:4, v/v for alanine; 10:1:4; v/v for threonine; and ethyl acetate-carbon tetrachloride-propionic acid (10.5:6.5:3.5, v/v) for valine. Minimum detection limits were found to be different for each of the amino acid, ranging between 0.9 and 3.7 microg. The effects of concentration of impregnating reagent, temperature and pH on resolution of enantiomers have been studied in details.

Separation of amino acids, their derivatives and enantiomers by impregnated TLC

The present state of TLC with respect to separation of amino acids, their different derivatives and their enantiomers by the technique of impregnation is discussed. The main approaches to impregnation viz. mixing of a suitable reagent with the adsorbent prior to plate-making, immersion of the untreated plate in the solution of impregnating reagent prior to development, and modification of the adsorbent, have been identified and discussed for each class of these compounds. The role of impregnation in resolving enantiomers or in improving the separation of mixtures of amino acids or their derivatives in terms of ion pairing, complex formation, ligand exchange or other steric interactions has been elaborated in each category.

Direct resolution of (+/-)-ephedrine and atropine into their enantiomers by impregnated TLC

Direct resolution of (+/-)-ephedrine and atropine into their enantiomers was achieved by normal-phase thin layer chromatography on silica gel plates impregnated with optically pure L-tartaric acid and L-histidine, respectively, as chiral selectors. The mobile phases enabling successful resolution were different combinations of acetonitrile-methanol-water. The spots were detected with iodine vapours and the detection limits were 2 and 6 microg, respectively, in terms of the racemate. The effects of concentration of the impregnating reagent, temperature and pH on resolution have been studied.