Effects of the limited analyte solubility on its mobility and zone shape: Electrophoretic behavior of sanguinarine and chelerythrine around pH 7

Abstract

Electrophoretic mobilities and shapes of zones of sanguinarine and chelerythrine in aqueous media around pH 7 are affected by limited solubility of their uncharged forms and by the pH-dependent chemical equilibrium between cationic and uncharged forms of these alkaloids. The sanguinarine solubility in sodium MOPS of pH 7.4 was estimated at 50 micromol x L(-1). Sanguinarine zones in this buffer have the shape of tailed peak with concentration-independent mobility if the injected sanguinarine concentration exceeds this solubility limit only slightly. The chelerythrine solubility is higher because of lower dissociation constants of its cations. Precipitation of sanguinarine and chelerythrine with the phosphate anions decelerates their electrophoretic transport in phosphate buffer. Sanguinarine solubility is 5 micromol x L(-1) at the most in 13 mmol x L(-1) sodium phosphate buffer of pH 7.4. Acidifying of the sample up to pH 3 decreases the tailing of the peaks of sanguinarine and chelerythrine and contributes to the rise of sharp maxima of their migrating zones. Any capillary coating deteriorates the peak shape.

Are the asserted advantages of organic solvents in capillary electrophoresis real? A critical discussion

“…If the kinetics is too slow, precipitated HA or Q 1 A 2 will be left behind, and will either be the cause for a distorted peak, or will remain in part at the top of the capillary. These effects have clearly been demonstrated by Vespalec et al [52] for aqueous solutions, but their results are not restricted to a specific solvent. In both cases, we see that it is a quite complex sequence of reactions that determine the overall state.…”

Capillary electrophoretic studies of acid–base properties of sanguinarine and chelerythrine alkaloids

“…Such detection sensitivity is insufficient for the measurements with alkaloids concentrations below their solubility at higher pH. However, the precipitation of the alkaloids induced by injecting samples of pH 5 and lower into the sodium MOPS buffer of pH 7.4 does not influence the mobility of such a heterogeneous zone in concentration 50 M and lower according to our previous experiments [13]. The decrease of the sample pH up to 2 made the alkaloid peak narrower and higher but the mobility of its zones did not change measurably in these experiments [13].…”

“…However, first estimations of the solubility of the alkaloids given in the study on electrophoretic behavior of SA and CHE in neutral solutions [13] are in the order of tenth of micromoles in solutions of pH 7.4 being somewhat higher for CHE than for SA. With regard to the reported pK R + values of SA and CHE [6][7][8][9] the solubility of the uncharged form (S 0 ) is expected to be several micromoles or even lower.…”

“…CZE was already successfully used for the determination of equilibrium constants of interactions of SA and CHE with compounds containing mercapto group in our recent electrophoretic study [12] although an unusual electrophoretic behavior of both alkaloids was observed in neutral solutions. A detailed study regarding the electrophoretic behavior of SA and CHE in almost neutral and weakly alkaline solutions [13] identified limited solubility of the alkaloids in these solutions as the primary reason for their anomalous electrophoretic behavior and describes conditions at which reliable electrophoretic measurements with these alkaloids might be performed.…”

Chiral separability of hydrophobic boron cluster anions with native cyclodextrins in water–methanol background electrolytes

“…Only the complexes 7 and 8 exhibited weak chiral discrimination. Broad tailing peaks of the ions 7-15 indicated their solubility problems [37] in BGE containing 30% of methanol.…”