Chiral separation by chromatographic and electromigration techniques. A review

Separation of neutral dihydropyridines and their enantiomers using electrokinetic chromatography

The separation and simultaneous enantiomeric separation of three neutral 1,4-dihydropyridine (DHP) derivatives (nimodipine, nisoldipine and nitrendipine) was studied using electrokinetic chromatography. Bile salts allowed the non-chiral separation of these DHP derivatives. With the taurine-conjugated bile salts a beginning of enantiomeric separation was observed for nimodipine and nisoldipine. Achiral micelles of sodium dodecyl sulphate mixed with neutral cyclodextrins did not allow enantioseparation. Baseline chiral separation of nisoldipine and nimodipine was obtained with carboxymethyl-beta-cyclodextrin at pH 5.0. The buffer type affected the chiral separation, especially in the case of nisoldipine. The addition of organic solvent decreased the enantioresolution of nimodipine. However, the resolution between the nisoldipine enantiomers was increased when methanol or ethanol were added to the background electrolyte. Varying the temperature had almost no effect on the enantioresolution of nisoldipine, whereas with nimodipine a clear improvement at lower temperatures was observed. Using the optimised method, the selectivity of this method was investigated for three possible impurities of nisoldipine.

Chiral separation of natural and unnatural amino acid derivatives by micro-HPLC on a Ristocetin A stationary phase

This paper deals with the chiral separation of Fmoc- and Z-derivatives of natural and unnatural sulfur containing amino acids by micro-HPLC. The separations were carried out in microbore columns packed with a new material based on Ristocetin A bonded to 3.5 microm silica gel. The columns were run in the normal phase, polar organic mode as well as in the reversed phase mode, whereby best results were obtained with the reversed-phase mode using mixtures of triethylamine acetate (TEAA) buffer and methanol as mobile phases.

Comparison study of Chiralpak AD-H with AD columns in chromatographic enantioseparation of dihydropyrimidinone acid and its methyl ester

This paper reports a comparison study of the difference between Chiralpak AD-H and AD columns in enantioseparation of dihydropyrimidinone (DHP) acid and its methyl ester under normal phase LC conditions. Unlike those of the AD phase, the van't Hoff plots of retention factors for DHP acid on the AD-H phase were linear. The cyclic van't Hoff plots of selectivity factors for DHP acid on the AD-H phase were non-linear and slightly non-superimposable. No conformational transition was observed on the AD-H phase in the whole temperature range. A single-step temperature program on the AD-H phase showed that the selectivity factors of DHP acid only increased approximately 1.7% in 24 h (versus approximately 50% on the AD phase). For DHP ester, the single-step temperature program showed that the selectivity factors on the AD-H phase remained the same in 24 h while those on the AD phase increased around 3.1%. The enantioselectivity of DHP acid on the AD-H phase was lower than that on the AD phase while the enantioselectivity of DHP ester on the AD-H phase was higher than that on the AD phase. The resolution of DHP acid on the AD-H phase was about the same as that on the AD phase while the resolution of DHP ester on the AD-H phase was much higher than that on the AD phase. The results of DHP acid are opposite of what the vendor suggested while the results of DHP ester are the same as the vendor's application notes. This indicates that the differences between Chiralpak AD-H and AD columns are not only in their particle size, but also in the solvated conformations.

Properties of poly-aminophenylboronate coatings in capillary electrophoresis for the selective separation of diastereoisomers and glycoproteins

The polymerisation of 3-aminophenylboronic acid (APBA) in aqueous environment has been used for the open tubular modification of capillary electrophoresis (CE) capillaries. Being poly-APBA endowed with boronic acid, aromatic rings and secondary amines groups, it posses a variety of functional groups affecting selectivity. Diastereoisomers (e.g. ascorbic and isoascorbic acid) and proteins (e.g. haemoglobins) were successfully separated onto poly-APBA column, by means of a combination of electrophoresis and open tubular electrochromatography. The mechanism of selection was investigated: results indicate an interplay between enhancing or silencing the contribution of the protonable functionalities (amino groups, boronic acid). The properties of APBA polymer coating make it attractive for CE separation and for further application in affinity separations and chip technologies.

Comparative performances of selected chiral HPLC, SFC, and CE systems with a chemically diverse sample set

Pharmaceutical companies have a continuous need to resolve new racemates. Analysis may be required in aqueous and nonaqueous media, or in the presence of several different sets of potentially interfering compounds. There is often a preparative requirement. For these reasons analysts may require a number of different separation systems capable of resolving a given pair of enantiomers. We wished to improve upon existing approaches that address this situation and undertook a program of work to screen over 100 racemates, selected for their chemical diversity, on over 100 different chiral HPLC, SFC, and CE systems. Here we report results of this comparison and illustrate the use of rapid gradient screening as a valuable tool for chiral method development.

Enantioselective cyclization of racemic supramolecular polymers

Homochiral hydrogen-bonded cyclic assemblies are formed in dilute solutions of racemic supramolecular polymers based on the quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidinone unit, as observed by 1H NMR and SEC experiments. Preorganization of the monomers and the combined binding strength of the eight hydrogen bonds result in a very high stability of the cyclic aggregates with pronounced selectivity between homochiral and heterochiral cyclic species, usually only observed in crystalline or liquid crystalline phases.

Comparing cyclodextrin derivatives as chiral selectors for enantiomeric separation in capillary electrophoresis

A total of 26 different cyclodextrin (CD) derivatives with different functional groups and degrees of substitution were tested against 35 basic pharmaceutical compounds in an effort to investigate their effectiveness as chiral selectors for enantiomeric separation in capillary electrophoresis (CE). Testing was performed under the same conditions using a low pH buffer (25 mM phosphate buffer at pH approximately 2.5). Five CD derivatives, namely, highly sulfated-beta-CD, highly sulfated-beta-CD, hydroxypropyl-beta-CD (degree of substitution approximately 1), heptakis-(2,6-O-dimethyl)-beta-CD, and heptakis(2,3,6-O-trimethyl)-beta-CD were identified to be most effective for enantiomeric separations and have a wide range of enantiomeric selectivity towards the model compounds. Over 90% of the model compounds were enantiomerically resolved with the five identified CD derivatives, at a minimum resolution of 0.5. An additional 20 compounds were also tested to demonstrate the validity of the identified CD derivatives. The five CD derivatives were recommended as the starting chiral selectors in developing enantiomeric separation methods by CE.