Influence of organic solvent on the behaviour of camphor and alpha-pinene enantiomers in reversed-phase liquid chromatography systems with alpha-cyclodextrin as chiral additive.

Reversed-phase liquid chromatography has been applied in order to gain insight into the alpha-cyclodextrin (alpha-CD)-solute complexation process, which occurs in the aqueous mobile phases containing a secondary achiral modifier. The model compounds tested were (+/-)-camphor and (+/-)-alpha-pinene. Methanol, ethanol, and 1 or 2-propanol were used as secondary modifiers. Retention factors and enantioseparation factors have been determined on a RP 18 stationary phase as a function of the alpha-CD concentration, secondary modifier content, and temperature changes. The shortest retention and the best separation of studied compounds were achieved for aqueous-methanol eluents. Apparent stability constants in various binary aqueous-organic solvent mixtures have been evaluated for alpha-CD complexes of camphor enantiomers. Using the competition concept, values for the stability constants in pure water have been calculated. It has been found that: (1) the quotient of the stability constants for both enantiomers, denoted as absolute enantioselectivity E, always remains constant at a fixed value (E approximately 1.9), which may indicate that the complex composition does not change, (2) only the first step in the complexation process is altered by changing the solvent, which does not seem to affect the separation of the enantiomers, (3) the remarkable enantioselectivity that is observed results from the second step in the complexation process, (4) enthalpy changes are much more favourable for camphor-alpha-cyclodextrin complex formation than for the transfer of camphor to the stationary phase, which means that complexation dominates over adsorption and retention is shorter at lower temperatures, (5) the difference in free energy changes of complexation (AAG) between the enantiomers of camphor is about 1.5 kJ/mol at 20 degrees C.

Comparative study on camphor enantiomers behavior under the conditions of gas-liquid chromatography and reversed-phase high-performance liquid chromatography systems modified with alpha- and beta-cyclodextrins.

The dependence of retention and selectivity parameters of camphor enantiomers on the concentration of alpha- and beta-cyclodextrins were studied under conditions of GLC (matrix solvent: Glycerol, 95 degrees C) and RP-HPLC (matrix solvent: Aqueous methanolic, 20 degrees C). It has been found that beta-cyclodextrin forms complex of 1:1 stoichiometry and does not recognize enantiomers of camphor. In contrast alpha-CD forming complexes of 1:2 stoichiometry appeared to be very efficient chiral selector of (+) and (-)-camphor. Relatively considerable differences have been observed between stability constants determined by GLC and RP-HPLC, what may be explained by the various natures of the matrix solvents and the various temperatures of the measurements. On the contrary, the enantioseparation factor alpha observed at higher concentrations of alpha-cyclodextrin stabilizes on the very similar value alpha+/-(GLC) approximately = alpha-/+(HPLC) approximately = 1.6. Simple theoretical considerations focusing on the differences in the mechanisms of the studied processes have been performed. According to them the enantiomer forming the more stable complex with the cyclodextrin should be eluted from the RP-HPLC column first and GLC column last. This fact has been confirmed experimentally.

Separation ability and stoichiometry of cyclodextrin complexes.

Gas-liquid chromatography has been applied to search relations between selectivity towards isomers and stoichiometry of cyclodextrin complexes. The model tested compounds were: dimethylnaphthalenes and alpha- and beta-pinenes as constitutional isomers; cis/trans decalins, anetholes and isosafroles as diastereomers and as enantiomers (+/-)-alpha-pinenes and (+/-)-camphenes. Experimental retention data are used to confirm a simple theoretical model that allows distinguishing formation of G x CD complexes (1:1) and G x CD2 complexes (1:2). Based on the experimental data, stability constants K were evaluated. It has been found that remarkable selectivity factor alpha may appear both within the range of 1:1 stoichiometry (beta-CD complexes of decalins and of alpha- and beta-pinenes) and 1:2 stoichiometry (alpha-CD complexes with (+/-)-alpha-pinenes and (+/-)-camphenes). Occasionally selectivity arises from a different composition, when one isomer forms a 1:1 stoichiometry complex while another forms a 1:2 complex (dimethylnaphthalenes, cis/trans-anetholes and cis/trans-isosafroles).

Chromatographic studies of the enantiomeric composition of some therapeutic compositions applied in the treatment of liver and kidney diseases.

A gas-liquid chromatographic system with alpha-cyclodextrin in formamide medium (coated on Chromosorb) was used for the separation of enantiomers of alpha-pinene, beta-pinene, limonene and camphene in medicines applied in the therapy of liver and kidney diseases. The drugs under investigation were produced in Poland (Terpichol and Terpinex), in Germany (Rowachol and Rowatinex) and in Slovenia (Uroterp). It was found that, depending on the manufacturer, medicines possessing similar chemical compositions differ considerably from one another regarding the content of enantiomers, mainly those of alpha-pinene.

Resolution of chiral barbiturates into enantiomers by reversed-phase high-performance liquid chromatography using methylated beta-cyclodextrins.

The correlation between the capacity factors of enantiomers of chiral barbiturates and the concentrations of beta-cyclodextrin, heptakis(2,6-di-O-methyl)-beta-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin dissolved in the mobile phase was studied using LiChrosorb RP-18 as the stationary phase. Owing to the very strong adsorption of permethylated beta-cyclodextrin on the ODS surface a chiral stationary phase is generated dynamically and forms complexes with the solutes; this mechanism has been found to be the only factor responsible for the chiral recognition of the investigated compounds at all applied concentrations. The inclusion of barbiturates in the cavities of permethylated beta-cyclodextrin involves a distinct and entirely new kind of enantioselectivity compared with that observed for beta-cyclodextrin and its dimethyl derivative. Using permethylated beta-cyclodextrin baseline resolutions have been obtained with barbiturates containing a chiral centre in the heterocyclic ring or in the aliphatic side-chain.