CAPILLARY ELECTROPHORESIS WITH CHIRAL SELECTORS - OPTIMIZATION OF SEPARATION AND DETERMINATION OF THERMODYNAMIC PARAMETERS FOR BINDING OF TIOCONAZOLE ENANTIOMERS TO CYCLODEXTRINS
Sg. Penn et al., CAPILLARY ELECTROPHORESIS WITH CHIRAL SELECTORS - OPTIMIZATION OF SEPARATION AND DETERMINATION OF THERMODYNAMIC PARAMETERS FOR BINDING OF TIOCONAZOLE ENANTIOMERS TO CYCLODEXTRINS, Analytical chemistry, 66(18), 1994, pp. 2866-2873
A systematic approach is outlined for optimization of enantiomeric sep
arations in free solution capillary electrophoresis using chiral mobil
e-phase additives. Maximum electrophoretic mobility difference between
the enantiomers occurs when the concentration of free selector is equ
al to the reciprocal of the average binding constant. General equation
s and data analysis methods are presented to relate mobilities to equi
librium constants in simple and competitive binding equilibria and use
d to determine thermodynamic parameters for host-guest complexation of
tioconazole enantiomers with a range of cyclodextrin selectors. Selec
tivities are found to be in the reverse order of binding constants in
the series dimethyl-beta-cyclodextrin (K-1 = 6.9 X 10(3) M(-1), alpha
= 1.10) to hydroxypropyl-beta-cyclodextrin (K-1 = 0.72 x 10(3) M(-1),
alpha = 1.29). For beta-cyclodextrin (K-1 = 1.32 x 10(3) M(-1), alpha
= 1.20), Delta H degrees provides the dominant contribution to binding
but Delta Delta H degrees and T Delta Delta S degrees terms give comp
arable contributions to the selectivity. Addition of alcohol does not
affect the selectivity, but allows displacement of the optimum separat
ion conditions to higher cyclodextrin concentration through either com
petitive binding (with cyclohexanol) or preferential solvation of reac
tants (with methanol).