Diquat Enantiomers Separated by Chiral Capillary Electrophoresis with Random Sulfated Cyclodextrin Selectors

A team of researchers at the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences in Prague, Czech Republic, recently released a study examining the separations of 11 new derivatives of diquats (DQs) synthesized at their facility (1). Capillary electrophoresis (CE) is thought to be suitable for this purpose, and in this instance used randomly highly sulfated cyclodextrins (CDs) as chiral selectors.

The eight-author study published in the Journal of Separation Science briefly summarized the history of DQs, derivatives of the herbicide diquat that are N-heteroaromatic dications with the structure 2,2´-bipyridyl (1). Noncoplanarity of two pyridinium rings linked by the axis of chirality causes P and M atropoisomeric conformations, and the researchers go on to say that the configurations of the P and M isomers are not stable. But for control of enantiomeric purity of these isomers, the researchers said suitable separation is necessary.

Capillary electrophoresis (CE) is often used for chiral analysis, according to this study, with enantioseparation achieved by adding a chiral selector to the background electrolyte (BGE). The selector helps form transient diastereomeric complexes, which in turn enables separation. CE is well-known for high efficiency and sensitivity, low chemical consumption, and a need for only a small sample size and short analysis time (1).

The use of diquat as herbicide dates back at least to 1958, the authors said, when dibromide salt was first promoted for such purposes by Imperial Chemical Industries. DQ spray acts as a dessicant and defoliant, slowing down photosynthetic activity and causing the cell membranes of plants to eventually degrade (1). The US and other countries still allow the use of DQ in some form, though it has been banned by the European Union (EU). Previous methods of separation and identification have included high performance liquid chromatography coupled to mass spectrometry (HPLC–MS).

However, when it comes to DQs, these researchers found that prior approaches to the separation of helquats, or helical extended diquats, proved instructive. CE using α-, β-, and γ-cyclodextrins (CDs) had been able to separate helquat enantiomers, and so for the separation of the enantiomers of the 11 newly synthesized DQs, chiral selectors of the same class—meaning CDs that were randomly highly sulfated—were deployed (1).

The results of the study showed what the authors termed “better than baseline” CE separation of the enantiomers of all 11 diquats, within a short (5–7 min) analysis time as advertised (1). The sulfated γ-cyclodextrin baseline separated enantiomers in all 11, followed by β-cyclodextrin which successfully separated 10 DQs and α-cyclodextrin with 9. Furthermore, for isolated P and M enantiomers of DQ 4 and DQ 9, the researchers said a high degree of enantiopurity was confirmed, and for DQs 4, 7, and 9 for which an absolute configuration of the enantiomers was determined, a migration order was also identified. Further classification of the properties within DQs, according to this study, could eventually lead to practical applications not only within chiral analysis and catalysis, but also supramolecular chemistry on a broader scale.

Reference

(1) Bílek, J.; Koval, D.; Sázelová, P.; et al. The Separation of the Enantiomers of Diquats by Capillary Electrophoresis Using Randomly Sulfated Cyclodextrins as Chiral Selectors. J. Sep. Sci. 2023, 2300417. DOI: 10.1002/jssc.202300417