Intrinsic Selectivity in Capillary Electrophoresis for Chiral Separations with Dual Cyclodextrin Systems

Abstract

Defined as the ratio of the affinity factors of the analytes for a complexing agent, the intrinsic selectivity is representative of the very nature of the complexing agent. When more than one complexing agent are present in the background electrolyte, it is possible to define several intrinsic selectivities according to whether complexing agents are considered separately or all together. A theoretical model with respect to selectivity is presented for separations that involve two complexing agents, using the concept of apparent constant for complex formation. When only independent complexation occurs (absence of mixed complexes), then the intrinsic selectivity of a complexing agent X in the presence of a complexing agent Y can be easily related to the intrinsic selectivity of each complexing agent and to complex formation constants. Dual systems of cyclodextrins (CDs), implementing the cationic mono(6-amino-6-deoxy)-β-cyclodextrin (β-CD-NH(2)) and a neutral CD (trimethyl-β-CD (TM-β-CD) or dimethyl-β-CD (DM-β-CD)), were studied to illustrate this model and to offer an alternative to the separation of neutral enantiomers when β-CD-NH(2) shows no or insufficient stereoselectivity. With a dual β-CD-NH(2)/TM-β-CD system at pH 2.3, arylpropionic acid enantiomers were baseline resolved and benzoin derivatives were partially resolved. For the arylpropionic acids, β-CD-NH(2), which is not stereoselective, confers on them a nonzero mobility, while TM-β-CD allows the chiral recognition. A study of the respective influence of ΤM-β-CD and β-CD-NH(2) concentrations was performed to determine the optimal conditions with respect to resolution. This theoretical approach allowed characterization of the intrinsic selectivity of neutral CDs for pairs of neutral enantiomers and therefore identification of the potential of neutral chiral agents for neutral enantiomers.

Histamine-modified cationic β-cyclodextrins as chiral selectors for the enantiomeric separation of hydroxy acids and carboxylic acids by capillary electrophoresis

“…A -methylamino-and 6 A ,6 D -dimethylamino-b-CD studied by Fanali et al [42] and the 6-deoxy-6-amino-b-CD used by Lelievre et al [43], besides polycationic derivatives proposed to obtain a wider separation window for acidic racemates [44], have been used. However, the role of electrostatic interactions in the recognition process and in the complex formation is a matter of discussion [45±47, 25,26,42].…”

Chiral capillary electrophoretic separation of amino acids derivatized with 9‐fluorenylmethylchloroformate using mixed chiral selectors of β‐cyclodextrin and sodium taurodeoxycholate

“…According to literature [13,17,23,24] and Eq. (1), the mobility difference of a pair of enantiomers, Dm, is the most critical to obtain chiral resolution.…”

“…Fillet et al [13,17] proposed that the mobility difference between a pair of enantiomers, Dm, was enlarged by the addition of a second chiral selector, which acted in a cooperative way. Slightly differently, Leliever et al [23] suggested a concept of intrinsic selectivity for binary CD, which was originally defined as the ratio of the affinity factors of the enantiomers for a single chiral selector [24]. Both of the concepts are undoubtedly helpful for better understanding of the improvement of separation selectivity in dual chiral selector systems.…”

Transient state of chiral recognition in a binary mixture of cyclodextrins in capillary electrophoresis

“…the running buffer to resolve the enantiomers of voltage for all separations was 15 kV, and samples interest, a dual CD system became increasingly were injected by electromigration, 15 kV for 3 s. popular for this purpose; thus, neutral CDs were Prior to use, the capillary was rinsed with deionized employed in combination with either other neutral and distilled water for 5 min, and subsequently for 5 CDs [6,7] or with charged CDs [8][9][10][11][12][13][14][15], in order to min with plain buffer. Between sample runs, the adjust the enantioselectivity.…”