Direct calculation and computer simulation of the enantiomerization barrier of oxazepam in dynamic HPLC experiments--a comparative study

Dynamic chromatographic methods constitute a versatile approach to the rapid and precise determination of enantiomerization barriers of stereolabile drugs. In the present study enantioselective dynamic high-performance liquid chromatography (DHPLC) was employed to determine the enantiomerization barrier of oxazepam. Dynamic elution profiles, exhibiting plateau formation and/or peak broadening between 20 and 60 degrees C at pH 2.6 and pH 8 were obtained in the presence of the chiral stationary phase (CSP) Nucleodex-beta-PM (permethylated beta-cyclodextrin chemically bonded to silica) using a 6:4 mixture of phosphate buffer and methanol as mobile phase. Evaluation of the experimental chromatograms was performed by the novel approximation function (AF) (without computer simulation), and by the stochastic model implemented in the ChromWin simulation software (with computer simulation) furnishing the respective apparent forward rate constants, k(1)(app)(T). From the rate constants, k(1)(app)(T), measured at variable temperatures, the kinetic Eyring activation parameters, deltaG(T)(#), deltaH(#) and deltaS(#), of the enantiomerization of oxazepam were obtained. By variation of the flow rate of the mobile phase, the expected independence of the enantiomerization barrier from the chromatographic time scale was demonstrated for the first time.