Electrophoresis zyxwvutsrqponm 1997, zyxwvutsrqponm 18, 897-904 zyxwvutsrqponml Cyclodextrin electrokinetic chromatography 897 z Hansjorg Jakubetz Markus Juza Volker Schnrig Electrokinetic chromatography employing an anionic and a cationic P-cyclodextrin derivative Institut f~r Organische Chemie, Universitiit Tiibingen, Tiibingen, Germany f3-Cyclodextrin, 1, can act as a chiral buffer additive in capillary zone electro- phoresis (CZE) owing to its ability to form diastereomeric complexes with the enantiomers of ionic compounds. Reaction of 1 with 2,3-epoxy-propyltri- methylammonium chloride gave preponderantly zyx 6-0-(2-hydroxy-3-trimethyl- ammoniopropyl) derivatives (2) of varying degrees of substitution (d.s.). Anal- ogous reaction of 1 with 1,3-propanesultone yielded 6- 0-(sulfo-n-propyl) deriv- atives (SPE-0-CD, 3) of varying d.s. The purity and the d.s. of 2 and 3 were determined by ‘H NMR and electrospray ionization mass spectrometry (ESI- MS). These charged B-cyclodextrin derivatives were used for the chromato- graphic enantiomer separation of a series of neutral barbiturates, of chlorthali- done, terbutaline, warfarin, salbutamol and brompheniramine by cyclodextrin electrokinetic chromatography (CD-EKC). During the separation of chlorthali- done with 3 as chiral additive, a reversible interconversion of the enantiomers (enantiomerization) 1 Introduction Although capillary zone electrophoresis (CZE) has a high resolving power, it can separate only ionic or charged compounds because its separation principle is based on their difference in the electrophoretic mobili- ties, kEP, in a background electrolyte or gel [l]. To over- come this problem, Terabe and co-workers [2-51 intro- duced “electrokinetic chromatography” with cyclodex- trins (CD-EKC). In CD-EKC, neutral as well as ionic compounds can be separated into enantiomers. Even the migration order of the enantiomers can be manipulated by selecting neutral or charged CDs; this offers great advantages for enantiomeric purity testing 161. Most of the charged CDs show a considerably higher water solu- bility (up to 40 mM/L) [7, 81 compared to underivatized B-cyclodextrin 1 (maximally 16 mM/L water) [9, 101. This enhanced solubility is sometimes very favorable for elec- trokinetic separations which require relatively high con- centrations of CDs to move complexation towards its maximum [ll]. However, the higher the concentration of charged cyclodextrins in the buffer becomes, the higher the electric current will be due to the increased ionic strength. This drawback prompted us to synthesize and characterize other charged cyclodextrins [12] with an expected wide range of applicability in CD-EKC and a relatively low degree of substitution (d.s.), i.e., a small number of charged substituents, but nevertheless endowed with improved solubility and sufficient UV transparency. Terabe and co-workers prepared two charged p-CD deriv- atives for use in CD-EKC. Enantiomer separation of Correspondence: Professor Dr. V. Schurig, Institut fur Organische Chemie, Auf der Morgenstelle 18, D-72076 Tubingen, Germany (Tel: +49-7071-2976257; Fax: +49-7071-296257; E-mail: volker.schurig@uni- tuebingen.de) Nonstandard abbreviations: BGE, background electrolyte; CD, cyclo- dextrin; CD-EKC, cyclodextrin electrokinetic chromatography; CZE, capillary zone electrophoresis; d.s., degree of substitution; SPE-B-CD, 6- 0-(sulfo-n-propyl-B-cyclodextrin Keywords: Charged cyclodextrins zyxwvutsrqp / Cyclodextrins / Electrokinetic chro- matography / Enantiomerization was observed at temperatures below 20°C. six dansyl amino acids was successfully achieved by the positively charged mono-(6-@-aminoethylamino-6- desoxy)-P-CD [4, 131; however, hydroxypropylcellulose had to be added to suppress the EOF. The second selector introduced by Terabe was negatively charged 2- 0-carboxymethyl-B-CD [5], a compound later used by Schmitt and Engelhardt [14, 151 as randomly carbo- xymethylated p-CD (d.s. 0.4-0.8). Nardi et al. [16] employed the positively charged 6*-methylamino and 6A,6D-dimethylamino-B-CDs for the enantiomer separa- tion of several 2-hydroxy acids, including mandelic acid. Recently Fanali and Camera presented new applications of 6*-methylamino P-CD and the per 6-methylamino-@- CD [ 171. Dette et al. [18] used tetra-(6-0-(sulfo-n-buty1))- B-CD (Na’ salt) (SBE-B-CD), a sulfonated B-CD intro- duced by Tait et zyxwv al. [19], as the anionic carrier in CD-EKC for the separation of four ephedrine alkaloids. SBE-B-CD has been intensively studied by Blaschke and co-workers [20] for the separation of nine basic drugs. The homologous tri-(6-O-(sulfo-n-propyl))-fi-CD (Na’ salt) (SPE-B-CD, 3) was used by Mayer and Schurig et al. in 1993 [21-241 for the separation of 1,l’- binaphthyl-2,2’-diylhydrogenphosphate, 1,l‘-binaphthyl- 2,2’-diol, dansyl-phenylalanine, hexobarbital and 4,s-dihydrodiazepam. To our knowledge, SPE-fi-CD rep- resents the first use of a sulfoalkylether of a CD for enantiomer separation in CD-EKC [21, 221. The above- mentioned studies already recognized that these anionic CD alkyl sulfates exhibit a chiral recognition ability at a very low concentration in the background electrolyte (BGE) [23]. SPE-0-CD has also been used for a dual chiral recognition system introduced by Mayer und Schurig [22]. This system exploits simultaneously the combination of a column coated with an immobilized B-cyclodextrin and the SPE-B-CD in the buffer and allows a reversal of the elution order of enantiomers by changing the concentration of CD in the mobile phase. Some charged cyclodextrins have also been investigated as models for enzyme action [25-281. These mono- or disubstituted ammonio derivatives were obtained by nucleophilic displacement of mono- or disulfonated derivatives of B-CD, but their purification was tedious and the overall yields were low. Therefore, we employed @ VCH Verlagsgesellschaft mbH, 69451 Weinheim, 1997 0173-0835/97/0606-0897 $17.50+.50/0