Amino acid modified carbon nanotubes with optimal pore size for chiral separation

Precisely controlling pore size of porous materials is of great importance for chiral separation, but a great challenge in practical applications. In contrast, the molecular dynamics (MD) simulation can be quite a convenient way to determine the effect of the pore dimension on the chiral resolution performances and thus to define the optimal pore size. In this work, inner-wall functionalised carbon nanotubes (CNTs) were used as porous materials and D- and L-phenylalanine were selected as chiral probes. The enantioseparation behaviour was investigated via varying the pore diameter of CNTs, controlling the grafting amount of chiral selectors and tuning the spacer length. Results show that varying the pore size has a significant effect on the enantioselectivity. Additionally, the effect of the introduction of varying the grafting ratio and tuning the spacer length on the chiral separation performance was also examined in this work. It was found that varying the grafting ratio, especially the spacer length between substrates and selectors, could also be one of the most effective alternatives to improving enantioselectivity. Our findings can provide a guidance for the practical applications in the chiral separation.     

Chiral separation of Frovatriptan isomers by HPLC using amylose based chiral stationary phase

A stereospecific HPLC method for separation of Frovatriptan enantiomers in bulk drug and pharmaceutical formulations was developed and validated on a normal-phase amylose derivertized chiral column. The effects of the organic modifiers namely 2-propanol, ethanol and diethyl amine (DEA) in the mobile phase were optimized to obtain the best enantiomeric separation. Calibration curves were linear over the range of 200-6150 ng/mL, with a regression coefficient (R(2)) of 0.9998. The limit of detection (LOD) and limit of quantification (LOQ) were 65 ng/mL and 200 ng/mL, respectively. The method was accurate and precise and suitable for the intended purpose. Analysis results were compared with the results obtained by using a validated chiral CE method and found to be in very good agreement. This method can be successfully applied to the enantiomeric purity analysis of Frovatriptan in pharmaceutical bulk drug samples and formulations.     

Asymmetric polymerizations of chiral 4-benzyl-2-ethynyloxazoline with rhodium catalyst and chiroptical properties of the polymers

Optically active 4-alkyl-2-ethynyloxazoline derivatives (BnEOx) were polymerized with rhodium catalysts. The polymerization in toluene produced polymer with the highest absolute values of specific rotation ([α](D) = -77.3°). The yields, molecular weights, and specific rotations of poly(BnEOx)s were influenced by polymerization conditions. The copolymerization with phenylacetylene (PA) was effective to increase the molecular weight of the copolymer. It is interesting to note that the copolymers exhibited positive specific rotations ([α](D) = +4.7° to +62.5°) despite the fact that [α](D) s of BnEOx and the homopolymer are negative sign. The chiroptical properties were investigated by the chiral/achiral copolymerization of BnEOx with PA. The copolymerizations of BnEOx with PA gave copolymers containing higher order structure such as one-handed helical conformation. Furthermore, induced Cotton effects were observed in the π-π* transition region of conjugated main chain depending a complex of these polymers with zinc triflate salt in tetrahydrofuran solution, indicating the formation of chiral supramolecular aggregates.     

Enantiomer separation of imidazo-quinazoline-dione derivatives on quinine carbamate-based chiral stationary phase in normal phase mode

The normal phase mode liquid chromatographic enantiomer separation capability of a quinine tert-butyl-carbamate-type chiral stationary phase (CSP) has been investigated for a set of polar [1,5-b]-quinazoline-1,5-dione derivatives. This class of chiral heterocycles is currently under development as potential alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and/or N-methyl-D-aspartic acid (NMDA) receptor antagonists. The effect of the nature and concentration of polar modifier, i.e., ethanol and isopropanol, in n-hexane-based mobile phases, as well as the substituent pattern of the phenyl ring attached to the quinazolone framework on retention factor, enantioselectivity, and resolution was investigated. The Soczewiński competitive adsorption model was used to describe the relationship between the retention and the binary mobile phase compositions. According to this model, linear plots of the logarithms of retention factor versus molar fractions of the polar modifiers were obtained over a wide concentration range (X(B) between 0.15 and 0.35). Addition of equimolar ethanol yields higher resolution than isopropanol, R(S) values ranging between 1.54 and 2.75, whereas the latter allows to achieve moderately increased enatioselectivity. The resolution was further improved by using a ternary mixture of n-hexane:methanol:isopropanol/85:5:10 (v/v). The most pronounced selectivity factor alpha and resolution R(S) values were obtained for the para-hydroxy substituted compound, indicating that chiral recognition is sensitive to steric and stereoelectronic factors. In the course of optimization, the temperature-dependence on the chiral separation was also investigated. It turned out that the enantiomer separation is predominantly enthalpically driven in normal phase mode.     

Vegetable and Mineral Oil Organogels Based on Monoglyceride and Lecithin Mixtures

We investigated the development of vegetable (VO) and mineral (MO) oil organogels using mixtures of a commercial monoglycerides (MG_C) and saturated lecithin (LC). The MG_C (2% wt/wt) and LC (0.25% to 2.5% wt/wt) concentrations used in the MG_C-LC mixtures were below the minimal gelator concentrations in the oils. At the corresponding Moles_MGC/Mole_LC studied (1.5, 4.0, 7.6, and 15.3) we achieved the development of well-structured organogels in both oils. The highest elasticity (G’) of the MG_C-LC organogels was achieved in each type of oil at different Moles_MGC/Mole_LC. Thus, in the MO we obtained the highest G’ at the lowest Moles_MGC/Mole_LC (i.e., the highest %LC) and in the organogels with the highest solid fat content (%SC). In contrast, in the VO we obtained the highest G’ at 15.3 Moles_MGC/Mole_LC (i.e., at the lowest %LC) corresponding to the organogels with the lowest %SC. This behavior suggested that a solvent dependent synergistic effect existed between the MG_C and the LC. Additional experiments showed that the addition of water (2.5% to 10% of water/total mass of gelator) resulted in organogels with higher G’, particularly in the 1.5 Moles_MGC/Mole_LC organogels developed in the MO. The DSC and X-ray results showed that in the MG_C-LC organogels the Lα to β polymorphic transition was limited, and thus the MG_C-LC organogels did not show phase separation even after 12 months of storage at 15 °C. This behavior was accentuated in MG_C-LC organogels developed in presence of water. Therefore, the use of MG_C-LC systems open the possibility of developing organogels at lower concentrations than the concentration needed just by the use of monoglyceride. Additionally, the MG_C-LC organogels achieve higher G’ with elastic recovery properties, and longer stability against phase separation than MG_C organogels. This, particularly in the 1.5 Moles_MGC/Mole_LC organogels developed in MO with at least 5% of water/total mass of gelator.

     

Reversible lysozyme immobilization onto N,N′-bis-(3-(4-morpholino)-propyl)-3,4,9,10-perylenetetracarboxylic acid dimide (MPPDI) attached polymeric nanospheres

Novel hydrophobic nanospheres with an average size of 200 nm utilizing N,N′-bis-(3-(4-morpholino)-propyl)-3,4,9,10-perylenetetracarboxylic acid dimide (MPPDI) as a monomer were prepared by surfactant free emulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) and MPPDI conducted in an aqueous dispersion medium. The nanospheres were used for the adsorption of lysozyme. The system parameters, such as effect of the adsorption conditions (i.e. enzyme concentration, medium pH, and temperature) and the reusability of the support were studied. Specific surface area of the nonporous nanospheres was found 664 m²/g. Poly(HEMA–MPPDI) nanospheres were characterized by Fourier transform infrared spectroscopy (FT-IR), elemental analysis and scanning electron microscopy (SEM). Then, poly(HEMA–MPPDI) nanospheres were used in the adsorption of lysozyme in batch system. Using an optimized adsorption protocol, 400 mg lysozyme/g nanosphere loading capacity was obtained. The adsorption phenomena appeared to follow a typical Langmuir isotherm. Lysozyme could be repeatedly adsorbed and desorbed with poly(HEMA–MPPDI) nanospheres without noticeable loss in the adsorption capacity.     

Enantioselective recognitions of chiral molecular tweezers containing imidazoliums for amino acids

Two kinds of novel chiral molecular tweezers containing imidazoliums were synthesized from L-alanine, L-phenylalanine, and L-glutamic acid. They are constructed by the chiral imidazolium pincers and two different spacers which are 1,3-bis (bromomethyl)benzene and 2,6-bis(bromomethyl)pyridine, respectively. The enantioselective recognition of L- and D-amino acid derivatives by these molecular tweezers was investigated by UV spectroscopic titration experiments and good enantioselectivities were obtained, which are highly sensitive to whether the spacer has the binding site and the pincers has the other aromatic rings besides imidazolium ring. The host molecular 3b.2PF6- showed remarkable enantioselectivity for N-Boc protected histidine methyl ester, affording K(L)/K(D) of 5.10.     

Enantiomeric separation of imidazolinone herbicides using chiral high-performance liquid chromatography

Chiral high-performance liquid chromatography (HPLC) is one of the most powerful tools to prepare enantiopure standards of chiral compounds. In this study, the enantiomeric separation of imidazolinone herbicides, i.e., imazethapyr, imazapyr, and imazaquin, was investigated using chiral HPLC. The enantioselectivity of Chiralpak AS, Chiralpak AD, Chiralcel OD, and Chiralcel OJ columns for the three analytes was compared under similar chromatographic conditions. Chiralcel OJ column showed the best chiral resolving capacity among the test columns. The resolved enantiomers were distinguished by their signs of circular dichroism detected at 275 nm and their structures confirmed with LC-mass spectrometric analysis. Factors affecting the chiral separation of imidazolinones on Chiralcel OJ column were characterized. Ethanol acted as a better polar modifier than the other alcohols including 2-propanol, 1-butanol, and 1-pentanol. Although the acidic modifier in the mobile phase did not influence chiral recognition, it was necessary for reducing the retention time of enantiomers and suppressing their peak tailing. Thermodynamic evaluation suggests that enantiomeric separation of imidazolinones on Chiralcel OJ column is an enthalpy-driven process from 10 to 40 degrees C. This study also shows that small amounts of pure enantiomers of imidazolinones may be obtained by using the analytical chiral HPLC approach.     

Noncovalent chiral domino effect on one-handed helix of nonapeptide containing a midpoint L-residue

We here clarify whether noncovalent chiral domino effect characterized by the terminal interaction of a helical peptide with a chiral small molecule can alter the helical stability of N-deprotected peptides containing an L-residue covalently incorporated into the inner position. Two nonapeptides consisting of the midpoint L-leucine (1) or L-phenylalanine (2) and the achiral helix-forming residues were employed. NMR and IR spectroscopy and energy calculation indicated that both peptides adopt a 3(10)-helical conformation in chloroform. They strongly preferred a right-handed screw sense because of the presence of the midpoint L-residue. These original right-handed screw senses were retained on addition of chiral Boc-amino acid, but their helical stabilities clearly depended on its added chirality. Here, Boc-L-amino acid stabilizes the original right-handed helix, whereas the corresponding Boc-D-amino acid tends to less stabilize or destabilize it. This tendency was not observed for the corresponding N-Boc-protected peptides 1 and 2, strongly suggesting that the N-terminal amino group is required for controlling the stabilization of the original right-handed helix. Therefore, noncovalent chiral domino effect in peptides 1 and 2 can contribute even to the helical stability of a chiral peptide prevailing one-handed helix strongly through the midpoint L-residue. In addition, the N-terminal moiety of a 3(10)-helical peptide was found to generate chiral discrimination in complexation process with racemic additives.     

Preparation and adsorption characteristic of polymeric microsphere with strong adsorbability for creatinine

Cross-linking terpolymer microspheres (HEMA/NVP/MBA; it can also be designated as HEMA/NVP because HEMA and NVP are main components) with an average diameter of 180 microm, were prepared via inverse suspension copolymerization by using 2-hydroxyethyl methacrylate (HEMA) and N-vinylpyrrolidone (NVP) as monomers and N,N'-methylene bisacrylamide (MBA) as cross-linked agent. The microsphere HEMA/NVP was chemically modified with 3,5-dinitrobenzoyl chloride (DNBC), and the functional microsphere DNBZ-HEMA/NVP, on which a great number of 3,5-dinitrobenzoate group (DNBZ) were bound, was obtained. The functional microsphere DNBZ-HEMA/NVP were characterized with FTIR and the chemical analysis method. The adsorption characteristics and mechanism of the absorption of DNBZ-HEMA/NVP for creatine was mainly studied. The results of static adsorption experiments show that the functional microsphere DNBZ-HEMA/NVP has very strong adsorption ability for creatinine, and the saturated adsorption amount is 25 mg/g. The adsorption capacity of the functional microsphere DNBZ-HEMA/NVP for creatinine is enhanced 20 times as against unmodified microsphere HEMA/NVP. The adsorption capacity is smaller, at lower and higher pH, and has a maximum as pH 8.5. The higher the salinity of the medium, the smaller the adsorption capacity. The adsorption capacity decreases with increasing temperature. The study results show that the adsorption of the microsphere DNBZ-HEMA/NVP for creatinine is ascribed to a chemical adsorption by driving of electrostatic interaction.     

Enantioselective separation of chiral arylcarboxylic acids on an immobilized human serum albumin chiral stationary phase

A series of 12 chiral arylcarboxylic acids were chromatographed on an immobilized human serum albumin chiral stationary phase (HSA-CSP). The effects of solute structure on chromatographic retentions and enantioselective separations were examined by linear regression analysis and the construction of quantitative structure-enantioselective retention relationships. Competitive displacement studies were also conducted using R-ibuprofen as the displacing agent. The results indicate that the enantioselective retention of the solutes takes place at the indole-benzodiazepine site (site II) on the HSA molecule and that chiral recognition is affected by the hydrophobicity and steric volume of the solutes. The displacement studies also identified a cooperative allosteric interaction induced by the binding of R-ibuprofen to site II. Chirality 9:178–183, 1997. © 1997 Wiley-Liss, Inc.     

New sorbent for bilirubin removal from human plasma: Cibacron Blue F3GA-immobilized poly(EGDMA–HEMA) microbeads

Cibacron Blue F3GA-immobilized poly(EGDMA–HEMA) microbeads were investigated as a specific sorbent for bilirubin removal from human plasma. The poly(EGDMA–HEMA) microbeads were prepared by a modified suspension copolymerization technique. Cibacron Blue F3GA was covalently coupled to the poly(EGDMA–HEMA) microbeads via the nucleophilic reaction between the chloride of its triazine ring and the hydroxyl groups of the HEMA molecule, under alkaline conditions. Bilirubin adsorption was investigated from hyperbilirubinemic human plasma on the poly(EGDMA–HEMA) microbeads containing different amounts of immobilized Cibacron Blue F3GA, (between 5.0–16.5 μmol/g). The non-specific bilirubin adsorption on the unmodified poly(EGDMA–HEMA) microbeads were 0.32 mg/g from human plasma. Higher bilirubin adsorption values, up to 14.8 mg/g, were obtained with the Cibacron Blue F3GA-immobilized microbeads. Bilirubin molecules interacted with these sorbents directly. Contribution of albumin adsorption on the bilirubin adsorption was pronounced. Bilirubin adsorption increased with increasing temperature.     

Enantiomeric separation by HPLC of 1,4-dihydropyridines with vancomycin as chiral selector

The macrocyclic antibiotics represent a relatively new class of chiral selectors in CE, HPLC, and TLC. We have examined the use of the macrocyclic antibiotic vancomycin as a chiral selector in HPLC for the separation of 1,4-dihydropyridines (DHPs) calcium antagonists (CAs). Chromatographic data of six 1,4-dihydropyridine calcium channel blockers obtained on the vancomycin chiral stationary phase (Chirobiotic V) were compared with those obtained on an alpha(1)-acid glycoprotein (AGP) HPLC stationary phase. Optimization of pH and organic modifier was carried out in order to modulate the retention properties of each system. All chiral neutral DHPs were resolved on the AGP column, whereas on Chirobiotic V only basic DHPs showed a split peak. The analytical chromatographic procedure on Chirobiotic V proved suitable for semipreparative separation, since the separation factor on the analytical column was high enough to obtain pure enantiomers with high yields.     

The use of multilayered capillaries for chiral separation by electrochromatography

Fused-silica capillaries were modified by the successively multiple ionic-polymer layer (SMIL) coating technique for a capillary electrochromatography (CEC) analysis of binaphthyl enantiomers. The SMIL coating capillaries consisting of three different polymers (A(+)-B(-)-C(+) coating) were prepared by the alternative deposition of positively charged chiral or achiral polymers and negatively charged DNA. Previous studies have indicated that DNA-cationic polypeptide or synthetic polymer complexes immobilized onto the inner surface of the capillary worked as the chiral stationary phases for 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNP). In this study, to investigate the chiral recognition mechanism and optimize the CEC separation condition in the DNA-cationic polymer coating, effects of the chirality of the polymer unit, the strand of DNA, and the number of layer pairs on the separation were investigated. It should be noted that, since single stranded DNA (ssDNA) was more suitable to immobilize cationic polymers than double stranded DNA, the ssDNA-cationic polymer immobilized capillaries gave a stable electroosmotic flow and reproducible CEC analyses. As a result, a poly(ethyleneimine)-ssDNA-protamine (Prt) coating provided the best chiral separation of BNP. The high separation performance of the prepared capillary is discussed in terms of DNA/polycations interaction.     

Enantiomer separation of fungicidal triazolyl alcohols by normal phase HPLC on polysaccharide‐based chiral stationary phases

Three fungicidal triazolyl alcohols (triadimenol, hexaconazole, and cis/trans‐1‐4‐chlorophenyl‐2‐1H‐1,2,4‐triazol‐1‐yl‐cycloheptanol) were completely separated into enantiomers by chiral HPLC using polysaccharide‐based chiral stationary phases. A better separation was achieved on cellulose and amylose carbamate phases compared with a cellulose ester phase. Peak shapes were almost symmetrical except for two cases, where tailing of the first eluted enantiomer and unusual symmetric peak broadening were observed. The effect of eluents on enantioseparation was also investigated. Chirality 11:195–200, 1999. © 1999 Wiley‐Liss, Inc.     

Liquid chromatographic separation of chiral diarylcarbinols

The direct HPLC separation of three chiral carbinols of general formula Mesityl-CH(OH)-Aryl has been achieved using Pirkle (R)-DNBPG ionic or covalent columns and, for Aryl = o-tolyl, on a Chiralpak OP(+) phase. It is apparent that steric hindrance and hydrogen bonding play important roles in chiral recognition. Two compounds structurally very similar but lacking the hydroxyl group were not resolved in their enantiomeric pairs. © 1992 Wiley-Liss, Inc.     

Novel chiral ionic liquids stationary phases for the enantiomer separation of chiral acid by high‐performance liquid chromatography

Novel chiral ionic liquid stationary phases based on chiral imidazolium were prepared. The ionic liquid chiral selector was synthesized by ring opening of cyclohexene oxide with imidazole or 5,6‐dimethylbenzimidazole, and then chemically modified by different substitute groups. Chiral stationary phases were prepared by bonding to the surface of silica sphere through thioene “click” reaction. Their enantioselective separations of chiral acids were evaluated by high‐performance liquid chromatography. The retention of acid sample was related to the counterion concentration and showed a typical ion exchange process. The chiral separation abilities of chiral stationary phases were greatly influenced by the substituent group on the chiral selector as well as the mobile phase, which indicated that, besides ion exchange, other interactions such as steric hindrance, π‐π interaction, and hydrogen bonding are important for the enantioselectivity. In this report, the influence of bulk solvent components, the effects of varying concentration, and the type of the counterion as well as the proportion of acid and basic additives were investigated in detail.     

Identification of enantioselective extractants for chiral separation of amines and aminoalcohols

The major obstacle for the introduction of fractional reactive extraction as a chiral separation method in the chemical and pharmaceutical industries is the lack of versatile enantioselective extractants. Therefore, a rational approach is developed to transfer the extensive knowledge of chiral selectors reported in the literature on chiral recognition and other chiral separation techniques to extraction. Based on a similarity in separation mechanisms, it was expected that chiral selectors originating from a technique in which chiral recognition takes place in the liquid phase are most likely to function as enantioselective extractant. Using this approach, a selection of promising extractants was made from the literature and experimentally evaluated for the enantioseparation of aminoalcohols and amines. As a result, four enantioselective extractant systems, namely, dibutyl-L-tartrate with boric acid, N-(2-hydroxydodecyl)-L-hydroxyproline Cu(II) complex, N-dodecyl-L-hydroxyproline Cu(II) complex, and azophenolic crown ether, have been identified. The azophenolic crown ether system performed the best and demonstrated an enantioselectivity between 1.3-5.0 for five out of six test compounds. Identification of the enantioselective extractant systems was highly facilitated by the developed rational transfer approach that, although partially qualitative, appeared capable of reducing more than 50 encountered candidates to only three promising systems for further experimental evaluation. Therefore, it is expected that this approach can be successfully applied to identify enantioselective extractants for other classes of enantiomers as well.     

β-Cyclodextrin-modified covalent organic framework as chiral stationary phase for the separation of amino acids and β-blockers by capillary electrochromatography

Covalent organic frameworks (COFs) as a novel stationary phase have attracted much attention in the field of chromatography owing to their permanent nanoscale porosity, higher surface area, and exceptional stabilities. Here, a novel isocyanate-β-cyclodextrin-modified COF (MDI-β-CD-modified COF) was synthesized using isocyanate-β-cyclodextrin as the chiral selector and imine-based TpPa-1 COF as the matrix by a bottom-up strategy. The reaction condition and the structure of MDI-β-CD-modified COF were optimized and characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectra, nitrogen adsorption/desorption (Brunauer–Emmett–Teller [BET]), and thermogravimetric analysis (TGA). And then the coated open-tubular column (OT column) was prepared using MDI-β-CD-modified COF as chiral stationary phase (CSP) by in situ growth approach, which exhibited excellent stability and repeatability. For seven consecutive runs, the intraday and interday relative standard deviations (RSDs) were in range from 0.35% to 2.21% for the migration time of histidine. The column-to-column reproducibility ranged from 2.39% to 3.08%. Meanwhile, the separation of eight compounds including four amino acids and four β-blockers by capillary electrochromatography sufficiently verified the favorable chiral resolution properties of the MDI-β-CD-modified COF-coated OT column. This strategy of fabricating MDI-β-CD-modified COF-coated OT column expanded the application of imine-based COFs in chromatographic analytical fields.     

Enantiomeric separation of malathion and malaoxon and the chiral residue analysis in food and environmental matrix

Malathion is a widely used chiral phosphorus insecticide, which has a more toxic chiral metabolite malaoxon. In this work, the enantiomers of malathion and malaoxon were separated by high-performance liquid chromatography-mass/mass (HPLC-MS/MS) with chiral columns using acetonitrile/water or methanol/water as mobile phase, and the chromatographic conditions were optimized. Based on the chiral separation, the chiral residue analysis methods for the enantiomers in soil, fruit, and vegetables were set up. Two pairs of the enantiomers were better separated on CHIRALPAK IC chiral column, and baseline simultaneous separations of malathion and malaoxon enantiomers were achieved with acetonitrile/water (40/60, v/v) as mobile phase at a flow rate of 0.5 mL/min. The elution orders were −/+ for both malathion and malaoxon measured by an optical rotation detector. The chiral residue analysis in soil, fruit, and vegetables was validated by linearity, recovery, precision, limit of detection (LOD), and limit of quantification (LOQ). The LODs and LOQs for the enantiomers of malathion were 1 μg/kg and 3–5 μg/kg and 0.08 μg/kg and 0.20–0.25 μg/kg for malaoxon enantiomers. Good linear calibration curves for each enantiomer in the matrices were obtained within the concentration range of 0.02–12 mg/L. The mean recoveries of the enantiomers of malathion and malaoxon ranged from 82.26% to 109.04%, with RSDs of 0.71–8.63%.The results confirmed that this method was capable of simultaneously determining the residue of malathion and malaoxon in food and environmental matrix on an enantiomeric level.