Characterization of complexes between phenethylamine enantiomers and β‐cyclodextrin derivatives by capillary electrophoresis—Determination of binding constants and complex mobilities

Abstract

To optimize chiral separation conditions and to improve the knowledge of enantioseparation, it is important to know the binding constants K between analytes and cyclodextrins and the electrophoretic mobilities of the temporarily formed analyte-cyclodextrin-complexes. K values for complexes between eight phenethylamine enantiomers, namely ephedrine, pseudoephedrine, methylephedrine and norephedrine, and four different β-cyclodextrin derivatives were determined by affinity capillary electrophoresis. The binding constants were calculated from the electrophoretic mobility values of the phenethylamine enantiomers at increasing concentrations of cyclodextrins in running buffer. Three different linear plotting methods (x-reciprocal, y-reciprocal, double reciprocal) and nonlinear regression were used for the determination of binding constants with β-cyclodextrin, (2-hydroxypropyl)-β-cyclodextrin, methyl-β-cyclodextrin and 6-O-α-maltosyl-β-cyclodextrin. The cyclodextrin concentration in a 50 mM phosphate buffer pH 3.0 was varied from 0 to 12 mM. To investigate the influence of the binding constant values on the enantioseparation the observed electrophoretic selectivities were compared with the obtained K values and the calculated enantiomer-cyclodextrin-complex mobilities. The different electrophoretic mobilities of the temporarily formed complexes were crucial factors for the migration order and enantioseparation of ephedrine derivatives. To verify the apparent binding constants determined by capillary electrophoresis, a titration process using ephedrine enantiomers and β-cyclodextrin was carried out. Furthermore, the isothermal titration calorimetry measurements gave information about the thermal properties of the complexes.

Studying molecular interactions via capillary electrophoresis and microscale thermophoresis: A review

“…The requirement of this method is fast on and off kinetics; therefore, ligand concentrations 1-2 orders of magnitude higher than the analyte are usually used [69,70]. The most studied interactions are systems with cyclodextrins [14,45,[71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90]. They are suitable model systems for the development of new approaches in data evaluation and other theoretical models.…”

Thermodynamic properties of hydroxypropyl-β-cyclodextrin/guest interaction: a survey of recent studies

“…In a couple of papers, HP-β-CD was checked for its capacity to differently capture phenethylamine enantiomers [32] or two geometrical isomers: ( +)-catechin and (-)-epicatechin [33]. In the first paper, the binding constants for a set of eight phenethylamine enantiomers and four cyclodextrins at pH 3.0 were calculated by ITC and electrophoretic mobility data.…”

Determining binding constants for 1:1 and 1:2 inclusion complexes of ester betulin derivatives with (2‐hydroxypropyl)‐β‐cyclodextrin by affinity capillary electrophoresis

“…CDs usually form 1:1 complexes with guest compounds according to the following equation [14–30]: where D is the drug (guest compound) under study, is the apparent binding (stability) constant of the complex D/CD, and […] is the equilibrium concentrations of corresponding species. The effective electrophoretic mobility of an analyte (guest compound) for the case of 1:1 complexation is dependent on the ligand (host compound) concentration in BGE as follows: where is the coefficient allowing viscosity change correction for the i th BGE containing CD, is the effective electrophoretic mobility, μ D is the ionic mobility of D (effective electrophoretic mobilities of the analyte when CD concentration in BGE is zero), μ 11 is the ionic mobility of the complex D/CD, [ CD ] i is the CD (ligand) concentration in BGE, μ analyte, i and μ marker, i are the electrophoretic mobilities of the analytes and EOF marker, respectively [14].…”