CHIRAL RECOGNITION IN LIQUID-CHROMATOGRAPHY UTILIZING CHARGEABLE CYCLODEXTRINS FOR RESOLUTION OF DOXAZOSIN ENANTIOMERS

The chromatographic resolution of rac-doxazosin using reversed-phase h igh performance liquid chromatography (HPLC) with the chargeable chira l mobile phase additive, carboxymethyl-beta-cyclodextrin (CM-beta-CD), is described. The effects of different modifiers (acetonitrile, metha nol and tetrahydrofuran), pH, temperature, and cyclodextrin concentrat ion were investigated to a) assess the key chromatographic parameters for subsequent chemometric optimisation, and b) explore the enantiosel ective mechanism. Assuming a 1:1 complex between each doxazosin enanti omer and CM-beta-CD, studies of the relationship between the capacity factors (k') and functions of CM-beta-CD concentration indicate that t he mechanisms for retention and chiral selectivity are comparable with those proposed earlier by Sybilska et al.(1) Stability constants (K-G ) calculated for rac-doxazosin complexed with CM-beta-CD (647 +/- 55 a nd 594 +/- 45 M-1 for each enantiomer respectively) are significantly larger than those calculated for the barbiturates complexed with beta- CD (ca. 101-108 M-1).(1) Investigations on pH indicate an ionic or ion -pair interaction between the anionic CM-beta-CD and the cationic doxa zosin enantiomers. A central composite design was used to optimise the key chromatographic parameters: pH, methanol (v/v) and CM-beta-CD con centration. The Kaiser peak separation index, P-i, was used for the re sponse function. The predicted response for this chiral separation has been compared with that observed experimentally and samples of the fo ur-dimensional response surface have been assessed for their value in showing robustness. (C) 1997 Wiley-Liss, Inc.