Dual chiral recognition system involving cyclodextrin derivatives in capillary electrophoresis II. Enhancement of enantioselectivity

Abstract

The enantiomer separation of hexobarbital was investigated by open tubular electrochromatography (OTEC) using the chiral stationary phase (CSP) CHIRASIL-DEX (a permethylated beta-cyclodextrin covalently linked to a dimethylpolysiloxane) and by cyclodextrin-electrokinetic chromatogaphy (CD-EKC) using anionic beta-cyclodextrin-sulfo-n-propyl ether (SPE-beta-CD) and cationic beta-cyclodextrin-2-hydroxy-3-trimethylammoniumpropyl ether chloride (HTAP-beta-CD) added to the running buffer. By employing two chiral selectors, the enantiomer separation of hexobarbital was then studied simultaneously by OTEC with CHIRASIL-DEX and by CD-EKC with either SPE-beta-CD or HTAP-beta-CD in the dual chiral recognition mode. In conjunction with CHIRASIL-DEX, anionic SPE-beta-CD decreased the chiral separation factor alpha due to compensation of enantioselectivity whereas the cationic additive HTAP-beta-CD increased the chiral separation factor alpha due to enhancement of enantioselectivity. It is concluded that CHIRASIL-DEX imparts an opposite enantioselectivity to the enantiomers of hexobarbital as compared to the charged CDs SPE-beta-CD and HTAP-beta-CD. Unusual peak broadening phenomena are observed in the dual chiral recognition system comprised of CHIRASIL-DEX and HTAP-beta-CD. The possible consequences of accidental dual chiral recognition systems caused by wall stacking effects of the mobile phase additives onto the inner surface of the capillary column are discussed.

Determination of enantiomerization barriers by dynamic and stopped‐flow chromatographic methods

“…Dual recognition systems have proven to solve chiral separation problems that could hardly be handled by HPLC before. [91][92][93][94][95] So far, few examples of the determination of interconversion barriers by capillary electrophoretic techniques have been described. Possible reasons for this lie in the difficulty of simulating chromatograms with very high plate numbers.…”

Investigation of the stereodynamics of molecules and catalyzed reactions by CE

“…dual recognition systems have proven to solve separation problems that could hardly be solved by HPLC [69][70][71][72]. Furthermore, initial experimental problems with thermostatting of capillaries, which is important to perform kinetic studies, have been resolved and theoretical insights into the understanding of Joule heating [73,74] lead to experimental improvements.…”

Recent innovations in enantiomer separation by electrochromatography utilizing modified cyclodextrins as stationary phases

“…Indeed, according to Fig. 6, the expected reversal of the elution order was observed [40]. Moreover, the result implies that the R-enantiomer undergoes a stronger interaction with the positively and negatively charged selectors as compared to the S-enantiomer (cf.…”

“…The discussion presented herein has some important practical implication since accidental compensation of enantioselectivity may arise if the chiral mobile phase additive is absorbed on the column wall generating a chiral stationary phase via stacking and giving rise to a real dual chiral recognition system [40]. Since, in this particular case, the identity of the chiral selector is exactly the same, the enantioselectivity imparted on any selectand should also be identical.…”