Pulsed amperometric detection of calystegines separated by capillary electrophoresis

Calystegines are polyhydroxyalkaloids with a nortropane skeleton. They are oxidized by pulsed amperometry at a gold electrode due to their vicinal hydroxyl groups similar to monosaccharides, but at a slightly higher potential. Compared to carbohydrates, calystegines exhibit lower acidity, thus the effective electrophoretic mobility as anions in 0.1 M NaOH is lower, independent of their molecular mass. The acidity and mobility of calystegines increase with the number of hydroxyl groups. The influence of temperature and power dissipation in the capillary and changes of the inner surface on the migration times was eliminated by cooling and subtraction of the electroosmotic flow velocity. The high resolving power of capillary zone electrophoresis allows the separation of calystegines with the same number of hydroxyl groups. Detection is linear from 2 to 200 mg L−1 with a 1 nL injection volume. Calystegines were determined in crude plant sap after filtration without further sample purification.