Theoretical study on chiral recognition mechanism of methyl mandelate enantiomers on permethylated β-cyclodextrin

Chiral Recognition of Amino Acids Using CC2 Porous Organic Cages

Enantiomers have the same physical properties but different chemical properties due to the difference in the orientation of groups in space and thus Chiral discrimination is quite necessary, as an enantiomer of drug can have lethal effects. In this study, we used the CC2 cage for chiral discrimination of amino acids using density functional theory. The results indicated the physisorption of amino acids in the central cavity of the cage. Among the four selected amino acids, proline showed maximum interactions with the cage and maximum chiral discrimination energy is also observed in the case of proline that is 2.78 kcal/mol. Quantum theory of atoms in molecules and noncovalent interaction index analyses showed that the S enantiomer in each case has maximum interactions. The charge transfer between the analyte and surface is further studied through natural bond orbital analysis. It showed sensitivity of cage for both enantiomers, but a more pronounced effect is seen for S enantiomers. In frontier molecular orbital analysis, the least E_H-L gap is observed in the case of R proline with a maximum charge transfer of -0.24 e^-. Electron density difference analysis is carried out to analyze the pattern of the charge distribution. The partial density of state analysis is computed to understand the contribution of each enantiomer in overall density of the complexes. Our results show that S-CC2 porous organic cages have a good ability to differentiate between two enantiomers. S-CC2 porous organic cages efficiently differentiated the S enantiomer from the R enantiomers of selected amino acids.

Exploring the inclusion interaction of estradiol with β-CD and HP-β-CD with the help of molecular dynamics simulation as well as multi-spectroscopic approaches

The inclusion behaviors of estradiol with β-CD and HP-β-CD were characterized using molecular dynamics simulation combined with multi-spectroscopic approaches. The findings revealed that estradiol enclosed into the cavity of β-CD and HP-β-CD and produced the estradiol-β-CD and estradiol-HP-β-CD complexes with the stoichiometry of 1:1. The association constants of the estradiol-β-CD and estradiol-HP-β-CD complexes were 3.14 × 10⁴ and 3.22 × 10⁴ M^-1 at 298 K, respectively, which declined with rising temperature. The analysis results of thermodynamic parameters confirmed that the dominate interaction forces were the hydrophobic and hydrogen-bonding interactions for stabilizing the estradiol-β-CD complex, and were the hydrogen bonding interaction and van der Waals forces for stabilizing the estradiol-HP-β-CD complex. Moreover, it was confirmed from the results of molecular modeling that estradiol inserted into the hydrophobic cavity of β-CD and HP-β-CD and form a stable estradiol-CD complexes. And, it is also observed that the phenyl moiety in estradiol is almost parallel to the central axis of β-CD and HP-β-CD, and the phenyl moiety was located on wider rim of β-CD and HP-β-CD.

Enantioseparation of mandelic acid and substituted derivatives by high-performance liquid chromatography with hydroxypropyl-β-cyclodextrin as chiral mobile additive and evaluation of inclusion complexes by molecular dynamics

The enantioseparation and resolution mechanism of mandelic acid (MA), 4-methoxymandelic acid (MMA), and 4-propoxymandelic acid (PMA) were investigated by reversed-phase high-performance liquid chromatography (HPLC) with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) as a chiral mobile-phase additive and molecular dynamics simulation. The suitable chromatographic conditions for the enantioseparation of MA, MMA, and PMA were obtained. Under the selected chromatographic conditions, these enantiomers could achieve baseline separation. The results of thermodynamic parameter analysis revealed that the main driven forces for the enantioseparation of MA, MMA, and PMA could be van der Waals forces and hydrogen-bonding interactions and the chromatographic retention of these chiral compounds was an enthalpy-driven process. The results of the molecular simulation revealed that their chiral resolution mechanism on HP-β-CD was responsible for the formation of inclusion complexes of enantiomers with HP-β-CD with different conformations and binding energies. And the binding energy of HP-β-CD with (S)-isomer was larger than that with (R)-isomer, which is consistent with the experimental results of the first elution of (S)-isomer. Additionally, it is also confirmed that the interaction energies included the van der Waals energy (∆E_vdw ), electrostatic energy (∆E_elec ), polar solvation energy, and SASA energy (∆E_sasa ), and the separation factor (α) was closely connected with the disparity in the binding energies of optical isomers and HP-β-CD complexes. Meanwhile, from molecular dynamics simulation, it can be found that the ∆(∆E_binding ), (∆(∆E_binding ) = ∆E_binding,R  - ∆E_binding,S ) value was in order of MA-HP-β-CD complex > MMA-HP-β-CD complex > PMA-HP-β-CD complex, which was consistent with the order of Δ(ΔG) values obtained from van't Hoff plot. This indicated that the molecular dynamics simulation has predictive function for chiral resolution.

Study of the enantioselectivity and recognition mechanism of chiral dual system based on chondroitin sulfate D in capillary electrophoresis

Several chiral reagents including cyclodextrins (CDs) and derivatives, crown ethers, proteins, chiral surfactants, and polymers have been involved in dual-selector systems for enantioseparation of a series of compounds by capillary electrophoresis (CE). In this paper, chondroitin sulfate D-based dual-selector system (CSD/CM-β-CD) was firstly established and investigated for the enantioseparation of six basic racemic drugs in capillary electrophoresis. Compared to the single-selector systems, synergistic effect and significantly improved separations for all tested analytes were observed in CSD/CM-β-CD system. The effect of several parameters, such as buffer pH, chiral selector concentration, and applied voltage, was systematically optimized. Meanwhile, to investigate the possible chiral recognition mechanisms in CSD/CM-β-CD synergistic system, we tried to apply the molecular docking method to simulate the host-guest binding procedures of the polysaccharide-based dual system for the first time. The difference in binding free energy was found to correspond to the chiral selectivity factor. The existence of CSD-CM-β-CD complex may give rise to a higher discriminatory ability against the enantiomers, indicating the synergistic effect in CSD/CM-β-CD system. Graphical abstract ᅟ.

Chiral recognition in separation science - an update

Stereospecific recognition of chiral molecules is an important issue in various aspects of life sciences and chemistry including analytical separation sciences. The basis of analytical enantioseparations is the formation of transient diastereomeric complexes driven by hydrogen bonds or ionic, ion-dipole, dipole-dipole, van der Waals as well as π-π interactions. Recently, halogen bonding was also described to contribute to selector-selectand complexation. Besides structure-separation relationships, spectroscopic techniques, especially NMR spectroscopy, as well as X-ray crystallography have contributed to the understanding of the structure of the diastereomeric complexes. Molecular modeling has provided the tool for the visualization of the structures. The present review highlights recent contributions to the understanding of the binding mechanism between chiral selectors and selectands in analytical enantioseparations dating between 2012 and early 2016 including polysaccharide derivatives, cyclodextrins, cyclofructans, macrocyclic glycopeptides, proteins, brush-type selectors, ion-exchangers, polymers, crown ethers, ligand-exchangers, molecular micelles, ionic liquids, metal-organic frameworks and nucleotide-derived selectors. A systematic compilation of all published literature on the various chiral selectors has not been attempted.

Rotational spectroscopy of methyl benzoylformate and methyl mandelate: structure and internal dynamics of a model reactant and product of enantioselective reduction

Pure rotational spectra of a prototypical prochiral ester, methyl benzoylformate (MBF), and the product of its enantioselective reduction, (R)-(-)-methyl mandelate (MM), were measured in the range of 5-16 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. Potential conformers were located using density functional theory calculations, and one conformer of each species was identified experimentally. The minimum energy conformer of MBF, in which the ester group is in a Z orientation, was observed for the first time. Based on an atoms-in-molecules analysis, MBF contains a weak CH···O=C hydrogen bond between the carbonyl oxygen atom of the ester group and the nearest hydrogen atom of the aromatic ring. In the minimum energy conformer of MM, the ester group is oriented to accommodate a hydrogen bond between the hydrogen atom of the hydroxyl group and the carbonyl oxygen atom (OH···O=C), rather than the sp(3) oxygen atom (OH···O-C). For both species, splittings of the rotational transitions were observed, which are attributed to methyl internal rotation, and the orientations and barrier heights of the methyl tops were determined precisely. The barrier heights for MBF and MM are 4.60(2) and 4.54(3) kJ mol(-1), respectively, which are consistent with values predicted by high-level wavefunction-based calculations. On the basis of an atoms-in-molecules analysis, we propose that destabilization of the sp(3) oxygen atom of the ester group most directly dictates the barrier height.

Molecular modeling-based inclusion mechanism and stability studies of doxycycline and hydroxypropyl-β-cyclodextrin complex for ophthalmic delivery

The aim of the present study was to prepare a stable complex of doxycycline (Doxy) and hydroxypropyl-β-cyclodextrin (HPβCD) for ophthalmic delivery and investigate the inclusion mechanism and the inclusion effects on the stability of Doxy. The Doxy/HPβCD complex was prepared by solution stirring and then characterized by scanning electron microscopy and ultraviolet spectroscopy. Based on results of nuclear magnetic resonance, molecular model of Doxy/HPβCD complex was established using computational simulation of PM3 method implemented in Gaussian 03. Stabilities of Doxy/HPβCD complex in both aqueous solution and solid state at 25°C were evaluated by HPLC. Finally, in vitro antibacterial activity of the Doxy/HPβCD complex was evaluated by disk diffusion test. It was found that the stabilities of Doxy/HPβCD complex in both aqueous solution and solid state were improved obviously as compared with Doxy alone. This stability enhancement is consistent with the inclusion mechanism between HPβCD and Doxy, which showed that the unstable site of Doxy molecule at 6-CH3 was protected in the hydrophobic cavity of HPβCD, additionally, the chelation of Mg2+ provided a synergetic protection of the other unstable site of Doxy at 4-N(CH3)2. The antibacterial activity results indicated that Doxy/HPβCD complex might have potential for clinical applications.

Enantiomeric separation and determination of the enantiomeric impurity of armodafinil by capillary electrophoresis with sulfobutyl ether-β-cyclodextrin as chiral selector

A selective capillary electrophoresis method using sulfobutyl ether-β-cyclodextrin as a chiral selector was developed and validated for the determination of the enantiomeric impurity of (R)-modafinil, i.e., armodafinil. Several parameters were optimized for a satisfactory enantioresolution, including the type and concentration of chiral selector and organic modifier, pH of background electrolyte (BGE), capillary temperature. The finally adopted condition was: 20 mmol/L phosphate buffer at pH 7.5, containing 20 mmol/L sulfobutyl ether-β-cyclodextrin and 20% methanol, at temperature of 25 °C. A good resolution of 3.3 for the two enantiomers of modafinil was achieved by applying the optimal conditions. The limit of detection (LOD) and limit of quantification (LOQ) of (S)-modafinil were 1.25 μg/mL and 2.50 μg/mL, respectively. The established method was also proven to display good selectivity, repeatability, linearity and accuracy. Finally, the method was used to investigate the enantiomeric purity of armodafinil in bulk samples.