Study On The Use Of Deep Eutectic Solvents In Capillary Electrophoresis For Enantioseparations

Chiral drugs are of great significance in the pharmaceutical industry.The development of efficient enantioseparation methods is an important and difficult point in chiral drug research.Capillary electrophoresis(CE)has shown application potential in the field of chiral drug analysis.Chiral CE separation is usually achieved by adding a chiral selector to a CE background electrolyte.However,with the development of modern pharmaceutical science,in some cases it is rather difficult to achieve satisfactory enantioseparations when facing numerous and complex chiral analytes,and the constant pursuit of high efficiency.Therefore,modifying traditional separation systems and exploring new chiral CE separation systems have important scientific significance and wide application prospects.Deep eutectic solvents(DESs),as a new generation of functional liquid materials after ionic liquids,have shown great applications in many fields such as extraction,catalysis,and electrochemistry.In CE,however,DESs have only been used sporadically and a search of the literature identified only a few examples.Considering that this is a nascent but highly promising research field,this thesis aims to systematically investigate the use of DESs in chiral CE,identifies and addresses the problems in the current application strategy of DESs in this field,and propose solutions.On this basis,DESs-based CE systems with superior chiral separation performance will be established,in order to promote the development of DESs technology in the field of CE separation science.The specific contents are as follows:(1)Raising Questions: Since DESs were first applied to CE in 2019,in subsequent literature DESs have been limited to a single application-strategy: adding them directly to cyclodextrins(CDs)-based CE systems as additives to affect chiral separation.The improvement of enantioselectivity was subsequently attributed to the “effect of DESs”.Herein,a total of ten DESs with different characteristics were prepared and applied as additives in an α-CD-based electrokinetic chromatography(EKC)system.A series of comparative experiments were performed to demonstrate the issues with the current application strategy in this field: To date,the conventional DESs selected in the literature were all hydrophilic.A series of comparative studies demonstrated that the supramolecular hydrogen bond network of such hydrophilic DESs could be completely disrupted in aqueous solutions,resulting in a dissociation into their initial components(HBA + HBD).This indicates that there may be no real DESs in the CE separation media obtained by the current strategy,but completely dissociated HBA and HBD.Therefore,it is urgent to explore DESs-based new CE systems,in which DESs can function as an entity,as to fully exhibit their superior physicochemical properties.(2)Solution: This study demonstrates that hydrophobic DESs(HDES)can form stable heterogeneous “nanoscale droplet dispersion” in aqueous CE background electrolyte by a simple ultrasonic-assisted dispersion method.The unique HDES phase could act as a functional “pseudostationary phase(PSP)”,fully exerting the hydrophobic partition function in CE separation process,thus constructing a high-performance EKC system.As a proof-of-concept study,13 structurally similar aromatic acids(including isomers,homologues,etc.)were employed as model analytes.The hydrophobic partitioning performance of an HDES,menthol:octanoic acid(Men:Oct A)in an EKC system was investigated.The results showed that Men:Oct A can either be used as a sole PSP for direct EKC separation,or be used in combination with traditional CD-type PSPs to form dualPSP systems.As an example,the “Men:Oct A + HP-γ-CD” system allowed the simultaneous baseline separation of a simulated complex mixture consisting of a total of 13 structurally similar aromatic acids.(3)Chiral Separation: As a follow-up study,we further explored and successfully demonstrated the feasibility of applying HDES to CE enantioseparation.By using carboxymethyl-β-cyclodextrin(CM-β-CD)as a chiral selector,and six chiral drugs with different structural types as model analytes,the results showed that during the CE chiral separation process,the HDES-type PSP(Men:Oct A)exhibited strong hydrophobic partitioning effect toward the transient diastereomeric complex formed by the enantiomers and the chiral selector.In the present EKC system,the protonated enantiomers would be involved in two main separation mechanisms:(i)The original electrophoretic separation mechanism.(ii)A high-selective distribution mechanism similar to classical chromatography that exists between the “HDES-type PSP” and the enantiomers.With the synergistic effect of these two separation mechanisms,the enantioseparations could be significantly improved(including increased enantioresolutions and shortened analysis time).These results,together with the fact that HDESs are low-cost,ease of preparation,low toxicity,and are “designer solvents”,suggest that HDESs may be a breakthrough in this field to establish high-performance chiral CE systems.