Molecular modifications, biological activities, and applications of chitosan and derivatives: A recent update

Polysaccharides arouse great interest due to their structure and unique properties, such as biocompatibility, biodegradability, and absence of toxicity. Polysaccharides from marine sources are particularly useful due to the wide variety of applications and biological activities. Chitosan, a deacetylated derivative of chitin, is an example of an interesting bioactive marine-derived polysaccharide. Moreover, a wide variety of chemical modifications and conjugation of chitosan with other bioactive molecules are responsible for improvements in physicochemical properties and biological activities, expanding the range of applications. An overview of the synthetic approaches for preparing chitosan, chitosan derivatives, and conjugates is described and discussed. A recent update of the biological activities and applications in different research fields, mainly focused on the last 5 years, is presented, highlighting current trends.

Chiral separation materials based on derivatives of 6-amino-6-deoxyamylose

In order to develop new type of chiral separation materials, in this study, 6-amino-6-deoxyamylose was used as chiral starting material with which 10 derivatives were synthesized. The amino group in 6-amino-6-deoxyamylose was selectively acylated and then the hydroxyl groups were carbamoylated yielding amylose 6-amido-6-deoxy-2,3-bis(phenylcarbamate)s, which were employed as chiral selectors (CSs) for chiral stationary phases of high-performance liquid chromatography. The resulted 6-amido-6-deoxyamyloses and amylose 6-amido-6-deoxy-2,3-bis(phenylcarbamate)s were characterized by IR, ¹ H NMR, and elemental analysis. Enantioseparation evaluations indicated that most of the CSs demonstrated a moderate chiral recognition capability. The 6-nonphenyl (6-nonPh) CS of amylose 6-cyclohexylformamido-6-deoxy-2,3-bis(3,5-dimethylphenylcarbamate) showed the highest enantioselectivity towards the tested chiral analytes; the phenyl-heterogeneous (Ph-hetero) CS of amylose 6-(4-methylbenzamido)-6-deoxy-2,3-bis(3,5-dimethylphenylcarbamate) baseline separated the most chiral analytes; the phenyl-homogeneous (Ph-homo) CS of amylose 6-(3,5-dimethylbenzamido)-6-deoxy-2,3-bis(3,5-dimethylphenylcarbamate) also exhibited a good enantioseparation capability among the developed CSs. Regarding Ph-hetero CSs, the enantioselectivity depended on the combination of the substituent at 6-position and that at 2- and 3-positions; as for Ph-homo CSs, the enantioselectivity was related to the substituent at 2-, 3-, and 6-positions; with respect to 6-nonPh CSs, the retention factor of most analytes on the corresponding CSPs was lower than that on Ph-hetero and Ph-homo CSPs in the same mobile phases, indicating π-π interactions did occur during enantioseparation. Although the substituent at 6-position could not provide π-π interactions, the 6-nonPh CSs demonstrated an equivalent or even higher enantioselectivity compared with the Ph-homo and Ph-hetero CSs.

The interactions between chiral analytes and chitosan-based chiral stationary phases during enantioseparation

The goal of the present study was to disclose the interactions between chitosan-type chiral selectors (CSs) and chiral analytes during enantioseparation. Hence, six chitosan 3,6-bis(phenylcarbamate)-2-(cyclohexylmethylurea)s were synthesized and characterized. These chitosan derivatives were employed as CSs with which the corresponding coated-type chiral stationary phases (CSPs) were prepared. According to the nature and position of the substituents on the phenyl group, the CSs and CSPs were divided into three sets. The counterparts of the three sets were 3,5-diMe versus 3,5-diCl, 4-Me versus 4-Cl and 3-Me versus 3-Cl. The enantioseparation capability of the CSPs was evaluated with high-performance liquid chromatography. The CSPs demonstrated a good enantioseparation capability to the tested chiral analytes. In enantioselectivity, the CSs with 3,5-diCl and with 4-Me roughly were better than the counterparts with 3,5-diMe and with 4-Cl respectively. The CS with 3-Me enantiomerically recognized more analytes than the one with 3-Cl, but showed lower separation factors in more enantioseparations. The acidity of the amide hydrogen in the phenylcarbamates was investigated with density functional theory calculations and ¹H NMR measurements. The trend of the acidity variation with different substituents on the phenyl group was confirmed by the retention factors of acetone on the CSPs. Compared the retention factors of analytes on every set of the counterparts, the formation of hydrogen bond (HB) in enantioseparation could be outlined as follows: when the CSs interacted with chiral analytes without reactive hydrogen but with lone paired electrons, the carbamate N‒Hs in the CSs were HB donors and the analytes were HB acceptors; if the CSs interacted with analytes with a reactive hydrogen, the role of the CSs in HB formation was related to the acidity of the reactive hydrogen; the patterns of HB formation between the CSs and analytes were also impacted by compositions of mobile phases, in addition to the nature, number and position of substituents on the phenyl group. Based on the discussion, chiral recognition mechanism could be understood in more detail. Besides, the strategy to improve enantioseparation capability of a CSP by introducing a substituent onto phenyl group was clarified and further comprehended.

Collective Solvation and Transport at Tetrahydrofuran-Silica Interfaces for Separation of Aromatic Compounds: Insight from Molecular Dynamics Simulations

We have performed umbrella sampling molecular dynamics simulations to study the separation mechanism of aromatic compounds at the tetrahydrofuran (THF)-methanol-silica interface by liquid chromatography. Solute molecules with different polarities (naphthol and naphthalene) are selected as representative aromatic compounds. For the polar solute (naphthol), the free energy profile shows a deep minimum near the THF-silica interface, suggesting strong interactions with the polar surface. When methanol is added to the interface, there is a sharp increase in naphthol's free energy minimum, and the corresponding diffusion dynamics also undergoes a dramatic change. These findings explain the fast separation mechanism in recent experiments of separating fused ring compounds in asphaltenes with liquid chromatography. Further solvation structure and orientation analysis suggest that apolar and polar solutes may find their own comfort zones several angstroms away from the interface, and their phenyl ring's orientations would undergo a parallel-to-perpendicular transition as the solute molecule moves away from the surface. Extending our simulation studies to systems with different solute concentrations reveals that there is a decrease in the adsorption free energy accompanied by enhanced surface diffusion as the solute concentration increases, which is related to the crowding in the interfacial layers. Our simulation analysis gives a detailed microscopic description of solute solvation and transport at the THF-silica chromatography interface and will be helpful for improving separation protocols in future applications.

Regioselectively substituted cellulose mixed esters synthesized by two-steps route to understand chiral recognition mechanism and fabricate high-performance chiral stationary phases

It is challenging to design and fabricate new and high-performance cellulose-based chiral stationary phases (CSPs), due to the indistinct chiral recognition mechanism and the inherent difficulty to control the structure of cellulose derivatives. Herein, taking advantage of the high regioselective benzoylation of cellulose in 1-allyl-3-methylimidazolium chloride, a series of regioselectively substituted cellulose mixed esters, cellulose 6-benzoate-2,3-phenylcarbamate, are directly obtained by a facile two-steps route without protecting and deprotecting process. The resultant cellulose mixed esters exhibit high chiral recognition capability. In particular, when the benzoate group has an electron-donating substituent on phenyl ring, such as 4-tert-butyl group, the corresponding regioselectively substituted cellulose mixed esters have much better enantioseparation capability than cellulose tri(3,5-dimethylphenylcarbamate), which is commercially available as Chiralcel OD column, one of the most powerful CSPs. More importantly, via adjusting the chemical structure of cellulose derivatives and adding a post-treatment process to optimize their chiral recognition properties, the chiral recognition mechanism is clearly revealed. The synergy of the hydrophobic helical conformation, weak hydrogen-bond donating ability and appropriate distribution of substituents of cellulose derivatives is essential to fabricate high-performance CSPs.

Structure screening and performance restoration of chiral separation materials based on chitosan derivatives

In order to screen chiral selector structure and find structure-property relationship, ten chitosan 3,6-bis(phenylcarbamate)-2-(cyclopropylformamide)s were synthesized from which corresponding chiral stationary phases were prepared. Enantioseparation capability and mobile phase tolerability of the chiral stationary phases were evaluated. The chiral selectors with 3-chloro-4-methylphenyl, 4-chlorophenyl, 3-chlorophenyl, 3,5-dichlorophenyl and 4-trifluoromethoxyphenyl groups demonstrated powerful enantioseparation capability. Enantioseparation capability was found to be dependent on a match between the substituent at C2 and the one attached to phenyl group at C3/C6 of glucose unit in the chitosan derivatives. Moreover, the tolerability tests revealed that the developed chiral stationary phases were highly tolerable to pure ethyl acetate, pure acetone and n-hexane/tetrahydrofuran of various ratios. In addition, n-hexane/tetrahydrofuran was found to be a modifier to adjust suprastructure of the chitosan derivatives, resulting in an improvement or restoration in enantioseparation. This observation implies n-hexane/tetrahydrofuran may make a damaged chiral selector restore its enantioseparation capability.

Chiral Stationary Phases for Liquid Chromatography: Recent Developments

The planning and development of new chiral stationary phases (CSPs) for liquid chromatography (LC) are considered as continuous and evolutionary issues since the introduction of the first CSP in 1938. The main objectives of the development strategies were to attempt the improvement of the chromatographic enantioresolution performance of the CSPs as well as enlarge their versatility and range of applications. Additionally, the transition to ultra-high-performance LC were underscored. The most recent strategies have comprised the introduction of new chiral selectors, the use of new materials as chromatographic supports or the reduction of its particle size, and the application of different synthetic approaches for preparation of CSPs. This review gathered the most recent developments associated to the different types of CSPs providing an overview of the relevant advances that are arising on LC.

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.

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.

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

Enantioseparation plays an important role for many fields and for pharmaceutical industry in particular. Chiral stationary phase (CSP) is the core of chiral liquid chromatography that effectively implements enantioseparation. In order to develop coated type CSPs with excellent enantioseparation capability and high tolerance against mobile phases, in this work, a series of chitosan bis(3,5-dimethylphenylcarbamate)-(alkyl urea)s were synthesized, which were coated on 3-aminopropyl silica gel to afford new CSPs. Owing to strong hydrogen bonds formed among the synthesized derivatives, the supra-structure of the derivatives should be highly ordered. Hence, these CSPs could provide excellent separation capability and could tolerate common organic solvents that are usually prohibited for coated type CSPs of cellulose and amylose derivatives. Therefore the newly prepared CSPs exhibited promising prospects for enantioseparation of chiral compounds.