Separation of propranolol enantiomers by CE using sulfated β‐CD derivatives in aqueous and non‐aqueous electrolytes: Comparative CE and NMR study

Abstract

Separations using CE employing non-aqueous BGE are already as well established as separations in aqueous buffers. The separation mechanisms in achiral CE with non-aqueous BGEs are most likely similar to those in aqueous buffers. However, for the separation of enantiomers involving their interaction with chiral buffer additives, the interaction mechanisms might be very different in aqueous and non-aqueous BGEs. While the hypothesis regarding distinct mechanisms of enantiomer separations in aqueous and non-aqueous BGEs has been mentioned in several papers, no direct proof of this hypothesis has been reported to date. In the present study, the enantiomers of propranolol were resolved using CE in aqueous and non-aqueous methanolic BGEs with two single isomer sulfated derivatives of beta-CD, namely heptakis (2,3-diacetyl-6-sulfo)-beta-CD and heptakis (2,3-dimethyl-6-sulfo)-beta-CD. The enantiomer migration order of propranolol was inverted when an aqueous BGE was replaced with non-aqueous BGE in the case of heptakis (2,3-dimethyl-6-sulfo)-beta-CD but remained the same in the case of heptakis (2,3-diacetyl-6-sulfo)-beta-CD. The possible molecular mechanisms leading to this reversal of enantiomer migration order were studied by using nuclear overhauser effect spectroscopy in both aqueous and non-aqueous BGEs.

Comparative NMR and MS studies on the mechanism of enantioseparation of propranolol with heptakis(2,3‐diacetyl‐6‐sulfo)‐β‐cyclodextrin in capillary electrophoresis with aqueous and non‐aqueous electrolytes

“…The significant signal splittings were observed not only for the aromatic but also for some alkyl chain protons of propranolol (data not shown). Thus, in confirmation to our previous results [10,14], not only can enantioseparation in CE but also signal splitting in 1 H NMR spectra be observed without inclusion (but external) complex formation between CDs and their guests.…”

“…2A). This was quite surprising, as according to our previous study [10] and to the results reported in Section 3.5, propranolol does not form an inclusion complex with HDAS-b-CD in the aqueous buffer used in this study. Hence, the aromatic (as well as other) protons of propranolol are not included into the cavity of HDAS-b-CD.…”

“…3). This result (but only in combination with ROESY data) may indicate an involvement of alkyl chain of propranolol in inclusion complex formation with HDAS-b-CD in nonaqueous electrolyte, as noticed in [10].…”

“…As reported in our recent studies, the enantiomers of the chiral b-blocking drug propranolol exhibit opposite affinity patterns toward different CDs in aqueous [9] and nonaqueous [10] buffers. The same studies illustrated that multivariate inclusion and/or external complexes can be formed between CDs and propranolol depending on the chemistry and structure of chiral selector and the medium in which the complexes are formed.…”

“…The switch between inclusion and external complex took place in the case of the propranolol/ heptakis (2,3-dimethyl-6-sulfo)-b-CD pair when aqueous buffer was changed to a nonaqueous methanolic electrolyte and this led to the reversal of the enantiomer migration order (EMO). The opposite switch from external to inclusion complex occurred in the case of the propranolol/heptakis (2,3-diacetyl-6-sulfo)-b-CD (HDAS-b-CD) pair although this did not lead to the reversal of the EMO [10]. In order to follow the fine mechanisms of EMO of propranolol in CE with Abbreviations: EMO, enantiomer migration order; HDASb-CD, heptakis (2,3-diacetyl-6-sulfo)-b-CD; HRMS, high resolution mass spectrometry; NOE, nuclear Overhauser effect; 1-D ROESY, one-dimensional rotating frame nuclear Overhauser effect spectroscopy Correspondence: Professor Bezhan Chankvetadze,…”