High performance chiral separation materials based on chitosan bis(3,5-dimethylphenylcarbamate)-(alkyl urea)s

Regioselective modification at the 2,3- and 6-positions of chitosan with phenylcarbamates for chromatographic enantioseparation

Chitosan derivatives with two different phenylcarbamate pendants at the 6-position and 2,3-positions of the glucosamine unit were synthesized by triphenylmethyl as a protective group. The regioselective chitosan derivatives were prepared corresponding to coated-type chiral packed materials (CPMs), which were evaluated with thirteen chiral compounds by high-performance liquid chromatography (HPLC). The regioselective chitosan derivatives (4aⅠ/4aⅡ, 4bⅠ/4bⅡ) bearing electron-withdrawing 3,5‑chloro or 4‑chloro at the 6-position can recognize 7 or 8 of the 13 enantiomers and achieve baseline separation for enantiomers 5 and 7. They exhibited better chiral recognition abilities than the other derivatives with different substituents at the 6-position and the same 3,5-dimethylphenyl substituent at the 2,3-postion. In comparison to Chit-1 featuring a 3,5-dimethylphenyl substituent at the 2,3- and 6-positions, it was observed that the combination of both an electron-withdrawing and an electron-donating substituent of the regioselective chitosan derivatives (4aⅠ/4aⅡ, 4bⅠ/4bⅡ) showed better or similar enantioseparation abilities for racemic Compounds 7 and 6, respectively. The molecular weight-performance relationship of the regioselective chitosan derivatives was investigated in detail. It was found that with increasing molecular weight, the derivatives 4aⅡ and 4bⅡ all possessed greater enantioseparation power for 4 enantiomers, such as enantiomers 4, 7, 11, and 15, than the corresponding derivatives with low molecular weights. The molecular docking simulation results showed that excellent enantioseparation power significantly depended on the combination and interaction of multiple factors, such as steric hindrance, and polarity of the substituents on the CPMs and enantiomers.

Multiple chiroptical switches and logic circuit based on salicyl‒ imine‒chitosan hydrogel

A chitosan-based chiral hydrogel was fabricated by grafting achiral salicylaldehyde (SA) on chitosan chains, followed by supramolecular assembly (CS-SA hydrogel hereafter). The structures and properties of the CS-SA hydrogel were characterized and investigated. The results indicated that the swelling ability of the CS-SA hydrogel depended on the medium pH and crosslinking degree. Circular dichroism measurements revealed that the chiral information of the chitosan was successfully transcribed to the achiral salicylic chromophores through imine bonds. Chiroptical switches based on acid-base responses of the imine bond and the OH fragment of SA and the swelling properties of the CS-SA hydrogel were fabricated, which is first reported for a chitosan-based hydrogel. In addition, a gel film showed good fatigue resistance under external stimuli. IMPLICATION, INHIBIT, and PASS logic gates and a logic circuit based on the chiroptical switches were successfully designed. This study suggests a new method of constructing biobased chiral functional materials.

Chiral Recognition for Chromatography and Membrane-Based Separations: Recent Developments and Future Prospects

With the rapid development of global industry and increasingly frequent product circulation, the separation and detection of chiral drugs/pesticides are becoming increasingly important. The chiral nature of substances can result in harm to the human body, and the selective endocrine-disrupting effect of drug enantiomers is caused by differential enantiospecific binding to receptors. This review is devoted to the specific recognition and resolution of chiral molecules by chromatography and membrane-based enantioseparation techniques. Chromatographic enantiomer separations with chiral stationary phase (CSP)-based columns and membrane-based enantiomer filtration are detailed. In addition, the unique properties of these chiral resolution methods have been summarized for practical applications in the chemistry, environment, biology, medicine, and food industries. We further discussed the recognition mechanism in analytical enantioseparations and analyzed recent developments and future prospects of chromatographic and membrane-based enantioseparations.

Anti-β-amyloid aggregation activity of enantiomeric furolactone-type lignans from Archidendron clypearia (Jack) I.C.N

The phytochemical investigation on the twigs and leaves of Archidendron clypearia (Jack) I.C.N. led to the isolation of three pairs of furolactone-type lignans enantiomers, including a pair of new compounds (1R,5S,6S)-Kachiranol (1a) and (1S,5R,6R)-Kachiranol (1b) and four known compounds (2a/2b and 3a/3b). Separation of the furolactone-type lignans enantiomeric mixtures was achieved using chiral HPLC for the first time. Their structures were determined by spectroscopic analysis and comparison between the experimental and calculated electronic circular dichroism (ECD) spectra. All optical pure compounds were evaluated for their inhibitory effects on β-amyloid aggregation by ThT assay. Among them, the inhibitory activity of the compound 1b (71.1%) was higher than the positive control (61.0%) and other compounds. In addition, molecular dynamics and molecular docking were employed to explore the binding relationship between the ligand and the receptor.

Preparation of 2,6-diurea-chitosan oligosaccharide derivatives for efficient antifungal and antioxidant activities

In this study, 2-urea-chitosan oligosaccharide derivatives (2-urea-COS derivatives) and 2,6-diurea-chitosan oligosaccharide derivatives (2,6-diurea-COS derivatives) were successfully designed and synthesized via intermediate 2-methoxyformylated chitosan oligosaccharide. All samples were characterized and compared based on FT-IR, ¹H NMR spectroscopy, and elemental analysis. The antifungal effects of COS derivatives were tested against Fusarium oxysporum f. sp. niveum, Phomopsis asparagus, and Botrytis cinereal. Their antioxidant properties, including superoxide radicals' scavenging activity, hydroxyl radicals' scavenging activity, and DPPH radicals' scavenging activity were also explored within different concentrations. COS derivatives bearing urea groups showed improved bioactivity compared with pristine COS and 2,6-diurea-COS derivatives had a higher biological activity than 2-urea-COS derivatives in tested concentrations. Additionally, L929 cells were used to carry out cytotoxicity test of COS and COS derivatives by CCK-8 assay. The results indicated that some of samples showed low cytotoxicity. These findings offered a suggestion that COS derivatives bearing urea groups are promising biological materials.

Synthesis, Characterization, and Antioxidant Evaluation of Novel Pyridylurea-Functionalized Chitosan Derivatives

In order to improve the bioactivity of chitosan, we synthesized a novel series of chitosan derivatives: firstly, chitosan was reacted with methylclhlorofonmate obtaining N-methoxyformylated chitosan (1), which was then converted into N-pyridylurea chitosan derivatives (2a-2c) by amine-ester exchange reaction. In addition, N-pyridylurea chitosan derivatives were conducted by reacting with iodomethane to obtain quaternized N-pyridylurea chitosan derivatives (3a-3c). The structural characteristics of as-prepared chitosan derivatives were confirmed by fourier transform infrared (FT-IR), ¹H nuclear magnetic resonance (¹H NMR), elemental analysis, and scanning electron microscope (SEM). Meanwhile, the antioxidant activity of the chitosan derivatives was assessed in vitro. As shown in this paper, the antioxidant activity decreased in the order: c > b > a. Moreover, after the quaternization with iodomethane, quaternized N-pyridylurea chitosan derivatives immediately exhibited enhanced antioxidant capacity compared with N-pyridylurea chitosan derivatives. For example, in 1,1-Diphenyl-2-picrylhydrazyl (DPPH) radical scavenging assay, the scavenging activities of 3a-3c were 91.75%, 93.63%, and 97.63% while 2a-2c were 42.32%, 42.97%, and 43.07% at 0.4 mg/mL. L929 cells were also adopted for cytotoxicity test of chitosan and synthesized derivatives by CCK-8 assay and all samples showed decreased cytotoxicity. These results suggested that the novel pyridylurea-functionalized chitosan derivatives could be an ideal biomaterial.

Chiral, thermal-responsive hydrogels containing helical hydrophilic polyacetylene: preparation and enantio-differentiating release ability

Chiral hydrogels constructed from helical hydrophilic polyacetylene demonstrate chirality, thermo-responsivity, biocompatibility and enantio-selective release ability towards chiral drugs.

Recent development trends for chiral stationary phases based on chitosan derivatives, cyclofructan derivatives and chiral porous materials in high performance liquid chromatography

The separation of enantiomers by chromatographic methods, such as gas chromatography, high-performance liquid chromatography and capillary electrochromatography, has become an increasingly significant challenge over the past few decades due to the demand of pharmaceutical, agrochemical, and food analysis. Among these chromatographic resolution methods, high-performance liquid chromatography based on chiral stationary phases has become the most popular and effective method used for the analytical and preparative separation of optically active compounds. This review mainly focuses on the recent development trends for novel chiral stationary phases based on chitosan derivatives, cyclofructan derivatives, and chiral porous materials that include metal-organic frameworks and covalent organic frameworks in high-performance liquid chromatography. The enantioseparation performance and chiral recognition mechanisms of these newly developed chiral selectors toward enantiomers are discussed in detail.

Chiral betulin-imino-chitosan hydrogels by dynamic covalent sonochemistry

A series of chiral hydrogels was prepared from a homogeneous mixture of chitosan and betulinic aldehyde in different molar ratios, under the effect of ultrasound. The hydrogelation mechanism has been investigated by FTIR and CD spectroscopy, wide angle X-ray diffraction and polarized light microscopy. The morphology of hydrogels was examined by SEM. The swelling ability has been tested in three media of different pH. It was concluded that hydrogelation occurred by different pathways, closely related to the peculiarities of the chitosan-betulin systems. Circular dichroism measurements revealed chiroptical properties of the hydrogels, correlated to their content and crosslinking pathway.

A high-performance chiral selector derived from chitosan (p-methylbenzylurea) for efficient enantiomer separation

N-Methoxycarbonyl chitosan was prepared by selectively modifying the amino group at the 2-position of chitosan with methyl chloroformate, which was further functionalized with p-methylbenzylamine to produce chitosan (p-methylbenzylurea). Then, the hydroxyl groups at the 3- and 6-positions of the glucose skeleton were modified with various phenyl isocyanates, affording a series of chitosan 3,6-bis(arylcarbamate)-2-(p-methylbenzylurea)s, which were characterized and proposed as chiral selectors for enantiomer separation. Nineteen racemates, most of which are drugs or intermediates for drugs, were selected as the model analytes to evaluate the enantioseparation performance. The structure-performance relationship of the chiral selectors was investigated in detail. It was found that the methyl-substituted chiral selectors possessed more preferable enantioseparation performance compared with the chloro-substituted ones, and the chiral selectors containing a methyl substituent at the 4-position of the benzene ring showed the best chiral recognition and separation ability with 17 racemates being recognized and 13 racemates being baseline separated. The prepared chiral separation materials derived from these chiral selectors exhibited favorable solvent tolerance towards ethyl acetate, acetone, chloroform and a low proportion of tetrahydrofuran in normal phase. To sum up, this work provided a useful reference for the design and preparation of high-performance chiral separation materials for efficient enantiomer separation.

Performance comparison of chiral separation materials derived from N-cyclohexylcarbonyl and N-hexanoyl chitosans

Chitosan bis(phenylcarbamate)-(N-cyclohexylformamide)s and chitosan bis(phenylcarbamate)-(N-hexanamide)s were synthesized as chiral selectors for enantiomeric separation. Since two types of substituents with different structures were, respectively, introduced onto the 2-position and the 3-/6-positions of the glucose skeleton in the chitosans through a "heterogeneous" modification pathway, the enantioseparation performances of the chiral selectors could be improved. Influence of the type and position of the substituents on chiral recognition and enantioseparation abilities was studied in detail, and the structural dependence on enantioseparation performance was particularly demonstrated. It was found that methyl- and chloro-substituted chitosan bis(phenylcarbamate)-(N-hexanamide)s possessed comparable enantioseparation performances, whereas chloro-substituted chitosan bis(phenylcarbamate)-(N-cyclohexylformamide)s exhibited much more powerful chiral recognition and enantioseparation abilities than the methyl-substituted ones. Among all the prepared chiral selectors, those with the combination of the cyclohexyl group at the 2-position of the glucose skeleton in the chitosan derivatives and the chlorophenyl group at the 3-/6-positions seemed to be more preferable for enantiomeric separation. As a result, the chitosan bis(3,4-dichlorophenylcarbamate)-(N-cyclohexylformamide) possessed the best enantioseparation performance. The solvent tolerability of the prepared chiral selectors was also investigated in the present study. Compared with the classical coated-type chiral separation materials derived from cellulose/amylose derivatives, the N-cyclohexylcarbonyl and N-hexanoyl chitosans based chiral stationary phases were observed to possess more favorable solvent tolerability, thus possibly widening their applications for various practical enantioseparations.

Structural dependence on the property of chiral stationary phases derived from chitosan bis(arylcarbamate)-(amide)s

The goal of present study was to investigate the structural dependence of chitosan derivatives on enantioseparation and mobile phase tolerance of the corresponding chiral packing materials for liquid chromatography. Hence, a series of chitosan bis(arylcarbamate)-(n-pentyl amide)s and the related chiral stationary phases (CSPs) were prepared from chitosans with different molecular weights. Because of the H-bond formed via CH₃-π interaction, the CSP bearing methyl substituent exhibited high tolerance than the ones bearing dichloro substituents. The CSP derived from the chitosan bis(3,5-dichlorophenylcarbamate)-(n-pentyl amide) with a higher molecular weight possessed high tolerance to mobile phases, whereas the enantioseparation capability of this CSP was not as good as that of the one prepared from the chitosan derivative with a lower molecular weight. Therefore, enantioseparation capability and mobile phase tolerance have to be counterbalanced in designing chiral selectors for the CSPs derived from chitosan bis(arylcarbamate)-(amide)s.

Evaluation and comparison of N-cycloalkylformylated chitosan bis(arylcarbamate)s as chiral selectors for enantioseparation

Chiral separation materials were prepared by coating chitosan bis(3,5-dimethylphenylcarbamate)-(N-cycloalkylformamide)s onto silica gel for enantiomeric separations.

Eluent Tolerance and Enantioseparation Recovery of Chiral Packing Materials Based on Chitosan Bis(Phenylcarbamate)-(n-Octyl Urea)s for High Performance Liquid Chromatography

The goal of the present work was to study the influence of the swelling of chitosan derivatives on the enantioseparation and the separation performance recovery of chiral stationary phases (CSPs) based on these derivatives. Therefore, six chitosan bis(phenylcarbamate)-(n-octyl urea)s were synthesized, which were coated on macroporous 3-aminopropyl silica gel affording new CSPs. Most of the CSPs demonstrated strong enantioseparation capability for the tested chiral compounds. The swelling capacity of the chitosan bis(phenylcarbamate)-(n-octyl urea)s in ethyl acetate, acetone and tetrahydrofuran (THF) was evaluated. Among the chitosan derivatives, the chitosan bis(3,5-dichlorophenylcarbamate)-(n-octyl urea) polymer showed the highest swelling capacity in ethyl acetate and THF. The polymer-based CSPs could be utilized with pure ethyl acetate and a normal phase containing 70% THF, but was damaged by pure THF. On the other hand, the separation performance of the damaged CSP could be recovered after it was allowed to stand for a period of time. The observations are important for the development and application of polysaccharide derivative-based CSPs.