Screening of cyclodextrins by nuclear magnetic resonance for the design of chiral capillary electrophoresis separations

Abstract

High-field one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy and capillary electrophoresis (CE) are examined to investigate the process of chiral recognition occurring between different cyclodextrins (CDs) and the racemic anti-schistosomiasis drug, oxamniquine. Five neutral CDs (α-CD, β-CD, γ-CD, hydroxypropyl-β-CD and hydroxyethyl-β-CD) and two anionic CDs, carboxymethyl-β-CD (CM-β-CD) and sulphobutyl ether-β-CD (SBE-β-CD) were selected for these NMR and CE studies. Three batches of the anionic CD [SBE-β-CD(7), SBE-β-CD(4) and SBE-β-CD(1)] differing in their degree of substitution (6.5, 4.5 and 1.0, respectively), were also examined in these studies. In the 1D NMR studies, the shift displacement values (Δδ Hz), after addition of each of the nine CDs in a 1:1 molar ratio, the diagnostic aromatic singlets indicated interaction between oxamniquine and each of the CDs. With regard to shift non-equivalence (Δδ*) for the two aromatic singlets, the anionic CDs were the only agents to show any interaction. It was also found that by manipulating the sample pH value, the ionic state of both selector and selectand had an influence on the enantiorecognition process. The complex between oxamniquine and the SBE-β-CD(7) was subsequently examined using 2D rotating frame nuclear Overhauser effect spectroscopy (ROESY), and the complexation process was found to involve an inclusion mechanism. Spin-lattice relaxation time (T1) measurements confirmed this observation. In the CE studies, each of the nine CDs was examined as a chiral electrolyte additive for the enantioseparation of racemic oxamniquine. Complete separation was not found when using any of the neutral CDs, but in contrast to the anionic CDs, CM-β-CD and the SBE-β-CD(4), displayed a 95 and 100% separation, respectively. Interestingly, the higher and lower derivatised SBE-β-CD selectors yielded 10% and zero separation, respectively. This is strong evidence that the degree of substitution has a critical effect on the enantiorecognition process. Good correlation between the NMR shift non-equivalence data for oxamniquine and the degree of enantioseparation was observed in CE. The neutral CDs displayed no shift non-equivalence for oxamniquine and no enantioseparation was found. However, for the anionic CDs a significant shift non-equivalence was observed and this corresponded to a significant enantioseparation in CE.