Computer simulation and enantioselective capillary electrophoresis to characterize isomer mixtures of sulfated β‐cyclodextrins

Abstract

The enantiomeric separation of methadone in the presence of multiple isomer mixtures of sulfated β-cyclodextrin (S-β-CD) was studied experimentally with CZE and theoretically using computer simulation. Experiments were performed over many years with several lots of S-β-CD from the same manufacturer with a specified degree of substitution of 7-11. Large differences in the migration patterns were observed between certain lots and it was concluded that the extent of labelling in lots released after a transition time was higher than originally specified. The migration pattern was observed to be associated with (i) the ionic strength increase resulting from using S-β-CDs with a higher charge state and (ii) differences in buffer composition. Apparent binding constants between methadone and the S-β-CD and complex mobilities were determined for different lots of S-β-CD at varying ionic strength using phosphate and 3-morpholino-2-hydroxypropanesulfonic acid buffers. The obtained values were used as input for simulations. For a given ionic strength, agreement between predicted and experimentally observed behavior was obtained for different buffers. R-methadone has a stronger interaction with S-β-CD than S-methadone. For any given configuration there is a distinct S-β-CD concentration range which results in the cationic migration of S-methadone while the migration direction of R-methadone is reversed. This configuration was demonstrated to be applicable for micropreparative CZE separations.

Inverse cationic ITP for separation of methadone enantiomers with sulfated β‐cyclodextrin as chiral selector

“…Unless otherwise stated, the sample consisted of 0.4 mM each of S‐ and R‐methadone together with 0.8 mM Cl − in LE and occupied a 1% section of the column. All input parameters were taken from using those for S‐β‐CD with a degree of substitution (DS) of 9. They were determined with S‐β‐CD lot 07222HO which has also 7–11 sulfated groups per CD.…”

“…They are able to describe 1:1 equilibria between solutes and a buffer additive and have been used to study enantiomer separation dynamics and the formation of boundaries and zone stability during chiral ITP in presence of neutral CDs . GENTRANS is restricted to complexation equilibria with monovalent ligands whereas SIMUL5 can also be employed with polyvalent additives, such as S‐β‐CD . Sulfated CDs contain between four and 16 sulfates per CD molecule and are particularly useful for the enantioseparation of cationic pharmaceuticals .…”

Computer simulation of the enantioselective separation of weak bases in an online capillary electrophoresis based microanalysis configuration comprising sulfated cyclodextrin as selector

“…Furthermore, based on initial experiments with Tris buffers of about 15 mM, K values were estimated to be about fourfold higher compared to those given in Table . Thus, since the complexation constants are increasing with decreasing ionic strength , the constants determined in 50 mM Tris‐phosphate were not able to correctly describe the migration behavior under the lower ionic strength conditions within the incubation zone at which complexation took place. Increasing all input complexation constants by a factor of four generated simulation data agreeing with the experimentally observed behavior.…”

“…Thus, electrophoresis proceeds in a discontinuous buffer system . The discontinuities of the buffer system are accompanied by changes in pH, conductivity, and ionic strength, in which the latter has a large impact on chiral separations by influencing the effective mobilities of all components, and potentially also the values of binding constants . The electrophoretic processes occurring in this complex system are herein described utilizing computer simulation with the software SIMUL5complex.…”

“…Complexation constants, K , between the interacting species and the chiral selector and mobilities of the formed complexes, u c , are required as input. These parameters were determined using ACE as described previously for methadone , employing a ProteomeLab PA 800 CE system (Beckman Coulter, Fullerton, CA, USA) with a 50 μm id fused‐silica capillary (Polymicro Technologies, Phoenix, AZ, USA) of 60.2 cm total (50 cm effective) length at 25°C. A sample consisting of 10 μg/mL each of racemic ketamine and norketamine (Cerilliant, Round Rock, TX, USA) with 30 μg/mL caffeine (Inselspital Pharmacy, Bern, Switzerland) as EOF marker was injected by application of 1 psi for 6 s, and normal polarity runs were performed at 10 kV with UV detection at 195 nm.…”