Crown Ethers

Crown ethers act in a similar manner of enantiomer inclusion as CDs and contain a central cavity, although the mechanism is based on ionic and hydrogen bonding. They are macrocyclic polyethers, soluble in both aqueous and organic solvents, which form stable complexes with enantiomers, which have a primary amine or alkylamine functionality. For resolution, differential stability of the host-guest complex is required. This is reliant on the spatial arrangement between the amine and its hydrogen bonds with the ether oxygens. 18-Crown-6-tetracar-boxylic acid is the easiest to obtain commercially and has been used to resolve a number of primary amines and racemic amino acids on a silica capillary, by addition of the crown ether into the buffer phase at low concentration, under CZE conditions. By this means a few chiral separations have been achieved which have not been fully successful by other means, such as chiral peptides. Crown ethers have also been used in combination with CDs for complex mixtures. In general, however, crown ethers have not been

Figure 2 The resolution of oxamniquine by capillary-zone electrophoresis with neutral cyclodextrins. Resolution was at pH 2.25 with hydroxypropyl p-CD, but there was no resolution with p-CD at this pH. The p-CD gives resolution at pH 12, but with reversed migration order. (A) 50mmol L~1 disodium hydrogen phosphate (pH 12) with 25mmolL~1 p-CD; applied voltage 15 kV; (B) 50 mmol L~1 sodium dihydrogen phosphate (pH 2.25) containing 40mmolL~1 hydroxypropyl p-CD; applied voltage 20 kV. The temperature was 30°C and detection wavelength 246 nm.

the presence of the l-enantiomer. During these runs the polarity is reversed over the conventional direction of operation. The coated capillaries mentioned above are commercially available and have been shown to be stable if used within the suggested limits. The anionic (sulfonic acid) coated capillaries are of particular interest in chiral and conventional separations as they give a consistent EOF over the range pH 3-9 and therefore are more controlled in operation.

Figure 3 Detection of D-tryptophan at 0.05% m/m in L-tryp-tophan (LOD (3 x s/n) 0.01 % m/m) with a polyamine coated capillary (eCAP™ with polyamine regeneration solution) to give a positive charge on the capillary. Capillary was 37 cm (30 cm to detector) x 50 ^m fused silica and buffer 40 mmol L~1 tris-phos-phoric acid containing 75 mmol L-1 a-CD. The applied voltage was - 10 kV, detection wavelength 241 nm, injection time 2 s and operating temperature 30°C.

Figure 3 Detection of D-tryptophan at 0.05% m/m in L-tryp-tophan (LOD (3 x s/n) 0.01 % m/m) with a polyamine coated capillary (eCAP™ with polyamine regeneration solution) to give a positive charge on the capillary. Capillary was 37 cm (30 cm to detector) x 50 ^m fused silica and buffer 40 mmol L~1 tris-phos-phoric acid containing 75 mmol L-1 a-CD. The applied voltage was - 10 kV, detection wavelength 241 nm, injection time 2 s and operating temperature 30°C.

extensively used because they need to be applied under controlled conditions as they are highly toxic and they are limited to amines and alkylamines. However, in the applications reported they give highly efficient resolutions of certain chiral compounds.

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