Dynamic micellar electrokinetic chromatography. Determination of the enantiomerization barriers of oxazepam, temazepam, and lorazepam

The temperature-dependent enantiomerization barriers of oxazepam, temazepam , and lorazepam have been determined between 0 and 30 degrees C by dynamic micellar electrokinetic chromatography (DMEKC) in an aqueous 20 mM borate/p hosphate buffer system at pH 8 with 60 mM sodium cholate as chiral surfacta nt. Interconversion profiles featuring plateau formation and peak broadenin g were observed and simulated by the new program ChromWin based on the theo retical plate as well as on the stochastic model using the experimental dat a plateau height, h(plateau), peak width at half-height, w(h), total retent ion times, t(R), and electroosmotic breakthrough time, to. Peak form analys is yielded rate constants k and kinetic activation parameters, Delta G(doub le dagger), Delta H-double dagger, and Delta S-double dagger, of the enanti omerization of oxazepam, temazepam, and lorazepam, At 25 degrees C, the ena ntiomerization barrier, Delta G(double dagger), was determined to be simila r to 90 kJ mol(-1) and the half-lives, tau, were determined to be approxima tely 21 min. The new approach allows the fast and precise determination of enantiomerization barriers in a biogenic environment and it mimics physiolo gical conditions, as no organic modifiers or abiotic chiral stationary phas es (CSP) are employed.