Engineering Cyclodextrin Clicked Chiral Stationary Phase for High-Efficiency Enantiomer Separation

www.nature.com/scientificreports OPEN Engineering Cyclodextrin Clicked Chiral Stationary Phase for HighEfficiency Enantiomer Separation received: 17 March 2015 accepted: 28 May 2015 Published: 03 August 2015 Jian Tang*, Shapopeng Zhang*, Yuzhou Lin, Jie Zhou, Limin Pang, Xuemei Nie, Baojing Zhou & Weihua Tang The separation of racemic molecules is of crucial significance not only for fundamental research but also for technical application. Enantiomers remain challenging to be separated owing to their identical physical and chemical properties in achiral environments. Chromatographic techniques employing chiral stationary phases (CSPs) have been developed as powerful tools for the chiral analysis and preparation of pure enantiomers, most of which are of biological and pharmaceutical interests. Here we report our efforts in developing high-performance phenylcarbamated cyclodextrin (CD) clicked CSPs. Insights on the impact of CD functionalities in structure design are provided. High-efficiency enantioseparation of a range of aryl alcohols and flavanoids with resolution values (Rs) over 10 were demonstrated by per(3-chloro-4-methyl)phenylcarbamated CD clicked CSP. Comparison study and molecular simulations suggest the improved enantioselectivity was attributed to higher interactions energy difference between the complexes of enantiomers and CSPs with phenylcarbamated CD bearing 3-chloro and 4-methyl functionalities. One of the essential attributes in nature is chirality. The existence of stereoisomers for a great variety of substances is a critical issue to be addressed, especially for those molecules of biological and pharmaceutical interests. The importance of chirality determination and chiral resolution of racemates has thus been emphasized, leading to the extensive research efforts devoted for chiral separation1. Chromatographic techniques and other resolution methods have been developed rapidly. High performance liquid chromatography (HPLC) coupled with chiral stationary phases (CSPs) has become as a robust technology for both analytical and preparative purposes2. The library of CSPs spans from Pirkle types to chiral crown ethers, macrocyclic glycopeptides antibiotics, ligand exchange, polysaccharides, proteins and cyclodextrins (CDs)3–11. CDs derived CSPs are advantageous for enantioseparation due to the versatile chemical modification to construct π -π interactions, dipole-dipole, ion-pairing, hydrogen bonding, electrostatic and steric repulsion interactions besides inclusion complexation to promote the chiral recognition process12,13. Moreover, CD CSPs are especially attractive for their compatibility and durability to be operated under different separation modes such as normal phase, reversed phase, polar organic and hydrophilic interaction modes. The manner of CD immobilization onto silica support contributes to mechanical properties and chiral selectivity of CSPs14. Click chemistry has estabilished as a facile method for the selective immobilization of functional molecules onto substrate surfaces under mild reaction conditions15. Click preparation of separation materials has thus generated a large spectrum of stationary phases for liquid chromatography16. Click saccharides was first developed via Cu(I)-catalyzed azide-alkyne cycloaddition for the separation of polar compounds in hydrophilic interaction LC17. The enantioseparation of flavanone and selected racemates was further achieved with native β -CD clicked CSP in different mode HPLC18,19. Relatively low resolutions were observed due to the lack of π -π interactions. The development of both Key Laboratory of Soft Chemistry and Functional Materials (Ministry of Education), Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China. *These authors contributed equally to this work. Correspondence and requests for materials should be addressed to W.T. (email: ) Scientific Reports | 5:11523 | DOI: 10.1038/srep11523 1 www.nature.com/scientificreports/ Figure 1. Structure of CD, perphenylcarbamated CD, per(3-chloro-4-methyl)phenylcarbamated CD and per(4-chloro-3-methyl)phenylcarbamated CD clicked CSPs (denoted as CCN-CSP, CCP-CSP, CCC3M4CSP and CCM3C4-CSP, respectively). native and functionalized CDs clicked CSP was also explored in Ng’s group20–22. The native CD clicked CSP (CCN-CSP) afforded good enantioselectivities towards dansyl amino acids and flavonoids20. The comparison study of perphenylcarbamated β -CD CSP (CCP-CSP) and permethylated β -CD one revealed their enantioselectivities strongly depended on a complex interplay of ‘host–guest’ inclusion, hydrogen bonding, π –π and hydrophobic interactions21. Heptakis(6-deoxy-6-azido)-β -CD and heptakis(6-deoxy -6-azido-phenylcarbamoylated)-β -CD clicked CSPs were found to show better resolution for selected racemates than the mono-clicked counterpart22. Our recent studies found the surface loading of clicked CDs and column efficiency could also significantly improve the enantioselectivities of both native and perphenylcarbamated CD clicked CSPs12,23. Except for CD modification and multiple clicking on the development of CD clicked CSPs, the impact of functionalities on phenyl group of perphenylcarbamated CDs has seldom been explored on the enantioselectivity of CD clicked CSPs so far. As known, the introduction of an electron-donating methyl group or an electron-withdrawing halogen at the 3- and/or 4-position of the phenyl ring on phenylcarbamated polysaccharide (cellulose and amylose) CSPs was found to improve their chiral recognition ability towards many racemates11,24,25. The nature and position of the substituents on the phenyl ring was explored with dichroro, dimethyl and chloromethylphenylcarbamate CDs CSPs, where the perphenylcarbamate CDs were immobilized onto silica at 2,3-positions or even 6-position of CDs using hexamethylene diisocyanate or 4,4’-diphenylmethane diisocyanate (corresponding to 15–20% of hydroxyl groups of the CD) as spacer26. All CSPs showed rather low enantiomeric resolving abilities, probably due to uncontrolled immobilization and low conlumn efficiency. NMR studies have, however, shown the interactions on the outer surface of chemically-modified CDs played an important role in the chiral recognition process27. It is thus noted that fine-tuned substitution of phenylcarbamate CD and well-controlled immobilization is the key to achieve high enantioselectivity. The preparation of perphenylcarbamate CD CSPs via mono-click approach is attractive to resolve the above-mentioned key challenges. The functionalities on CD rims are crucial to create the driving forces efficiently and construct largest interaction energy between the two complexes of CSP and two enantiomers to discriminate the enantiomers in the inclusion process. To elucidate the impact of functionalities on CD rims on the chiral recognition ability of CD clicked CSPs, chloro and methyl substituents are introduced onto the 3 and 4 position (...truncated)