Determination of equilibrium constants from chromatographic and electrophoretic measurements

Chemical interactions, such as acid-base, complex-forming, ion association and other equilibria, are widely exploited to improve the separation efficiency in liquid chromatography as well as in electrophoresis. On the other hand, these techniques can be advantageously used to study the chemical equilibria affecting the separations. If the equilibium is sufficiently fast in comparison with the separation process, then the retention characteristics in chromatography (retention factors) or the migration characteristics in electrophoresis (effective mobilities) may be expressed as functions of the composition of mobile phase or background electrolyte (BGE), respectively. Using a proper experimental arrangement, the dependencies of retention (migration) characteristics on the mobile phase (background electrolyte) composition can be measured and utilized to calculate the equilibrium constants for equlibria taking place in the mobile phase (background electrolyte). Although principles of these measurements have been known for a long time, only more recent studies utilizing HPLC and capillary electrophoretic techniques are reviewed in this paper.

Chromatographic determination of the association constant between 8-methoxypsoralen and modified β-cyclodextrin: protective effect of hydroxypropyl-β-cyclodextrin on 8-methoxypsoralen toxicity in human keratinocytes

The retention of 8-methoxypsoralen (8-MOP) on an immobilised hydroxypropyl-beta-cyclodextrin (HP-beta-CD) column was analysed in HPLC by the determination of its Langmuir distribution isotherm. A such method was used to confirm the potential drug complexing role of this cyclodextrin. The 8-MOP/HP-beta-CD association constant (K) was equal to 29.5 and 18.7 M-1, respectively, at a temperature equal to 5 and 25 degrees C, respectively. These association constant values were used to determine the cytotoxicity profile of human keratinocyte cell line (HaCaT) in relation to the complex concentration. It was showed through these data that HP-beta-CD had a cytoprotective since a reverse effect of HP-beta-CD on 8-MOP cytotoxicity was observed.