Methods for determination of electrophoretic mobility and stability of complexes originating in solutions during the chiral discrimination process.

Abstract

An equation for the calculation of electrophoretic mobility of kinetically labile complexes originating in solutions during the chiral discrimination process is derived. The mobility of the complex is calculated from that of a fully ionized racemic compound, measured in absence of the chiral selector, and from the effective mobilities of its enantiomers, corresponding to the concentration of the chiral selector causing their maximum difference. Correct values of stoichiometric stability constants of both enantiomers may be calculated from the mobility of the complex obtained in this way. Both the mobility and the stability values hold only for the experimental conditions used and the selected background electrolyte. The proposed method is demonstrated for the separation of the fully ionized N-t-BOC-DL-tryptophan with beta-cyclodextrin in 20 mM aqueous solution of alpha-hydroxyisobutyric acid, adjusted with NaOH to pH 4.5. Mobility of the complex is 8.4 X 10(-9) m2V[-1]s(-1) at 25 degrees C. The stability constants of D- and L-enantiomers of N-t-BOC-DL-tryptophan with beta-cyclodextrin, KD and KL obtained from migration data using this mobility of the complex, are 374+/-37 M(-1) and 336+/-31 M(-1), respectively. The geometrical mean value of calculated stability constants, 355 M(-1), agrees perfectly with the value of 350 M(-1), calculated from the same experimental data by another procedure recently.