Preparation of a permethylated ß-cyclodextrin chiral stationary phase by one-pot hydrosilylation and immobilization at the C2 position for chiral high-performance liquid chromatography.

A novel cyclodextrin intermediate, mono-2(A)-allylcarbamido-2(A)-deoxy-permethylated ß-cyclodextrin, was synthesized by reacting allylamine and newly prepared mono-2(A)-azido-2(A)-deoxy-permethylated ß-cyclodextrin by the Staudinger reaction and anchored onto porous silica beads by a one-pot hydrosilylation and immobilization procedure to afford a novel chiral stationary phase. This stationary phase acts as a new member of the previous chiral stationary phase series immobilized on the cyclodextrin C2 position. This stationary phase depicted enantiomeric separation abilities toward a series of bicyclic and tricyclic racemates under reversed-phase conditions. The resolutions for hesperetin and naringenin achieved on the current phase reached 3.91 and 1.11, respectively, much higher than the previous permethylated ß-cyclodextrin with the linkage at the C6 position.

Evaluation of perphenylcarbamated cyclodextrin clicked chiral stationary phase for enantioseparations in reversed phase high performance liquid chromatography.

In this study, perphenylcarbamated cyclodextrin clicked chiral stationary phase (CSP) was developed with high column efficiency. The characteristics of the column were evaluated in terms of linearity, limit of detection and limit of quantification. The enantioselectivity of the as-prepared clicked CSP was explored with 26 recemates including aryl alcohols, flavanoids and adrenergic drugs in reversed phase high-performance liquid chromatography. The effect of separation parameters including flow rate, column temperature, organic modifier and buffer on the enantioselectivity of the clicked CSP was investigated in detail. The correlation study of the analytes structure and their chiral resolution revealed the great influence of analytes' structure on the enantioseparations with cyclodextrin CSP. Methanol with 1% of triethylammonium acetate buffer (pH 4) was proved to be the best choice for the chiral separation of basic enantiomers.

Hydroxyethylammonium monosubstituted cyclodextrin as chiral selector for capillary electrophoresis.

A novel cationic cyclodextrin, mono-6(A)-(2-hydroxyethyl-1-ammonium)-6(A)-ß-cyclodextrin chloride (HEtAMCD) has been successfully synthesized and applied as chiral selector in capillary electrophoresis. The NMR study revealed this chiral selector has three recognition sites: ß-CD, ammonium cation and hydroxy group in the sidearm to contribute three corresponding driving forces including inclusion complexation, electrostatic interaction and hydrogen bonding. The effect of buffer pH and HEtAMCD concentration (2.5-10 mM) on enantioselectivity, chiral resolution as well as effective mobility of analytes was investigated. This elegantly designed CD exhibits outstanding enantioselectivities toward the studied hydroxyl acids and ampholytic racemates in CE with the aid of extra hydrogen bonding. Under optimum pH 6.0, chiral resolutions over 5 can be readily obtained for hydroxy acids with CD concentration below 5mM. The comparison study between HEtAMCD and our earlier reported ammonium CDs indicates the hydroxyethylammonium group of HEtAMCD significantly increased the enantioselective capability.

Hydrophilic interaction chromatography coupled matrix assisted laser desorption/ionization mass spectrometry for molecular analysis of organic compounds in medicines, tea, and coffee.

Natural occurring organic compounds from food, natural organic matter, as well as metabolic products have received intense attention in current chemical and biological studies. Examination of unknown compounds in complex sample matrices is hampered by the limited choices for data readout and molecular elucidation. Herein, we report a generic method of hydrophilic interaction chromatography (HILIC) coupled with matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the rapid characterization of ingredients in pharmaceutical compounds, tea, and coffee. The analytes were first fractionated using a cationic HILIC column prior to MALDI-MS analyses. It was found that the retention times of a compound arising from different samples were consistent under the same conditions. Accordingly, molecules can be readily characterized by both the mass and chromatographic retention time. The retention behaviors of acidic and basic compounds on the cationic HILIC column were found to be significantly influenced by the pH of mobile phases, whereas neutral compounds depicted a constant retention time at different pH. The general HILIC-MALDI-MS method is feasible for fast screening of naturally occurring organic compounds. A series of homologs can be determined if they have the same retention behavior. Their structural features can be elucidated by considering their mass differences and hydrophilic properties as determined by HILIC chromatogram.

Enantioselective separation of dansyl-DL-amino acids and some racemates on "click" functionalized native α-cyclodextrin based sub-2 µm columns.

The current work demonstrates that native α-cyclodextrin, anchored onto sub-2 µm silica particles via "click" reactions and packed into a 5 cm column, was found to be effective for the resolution of 11 pairs of dansyl-DL-amino acids (DAAs) using ultra-high performance liquid chromatography (UHPLC). All DAAs were completely or partially separated on the column and the resolution achieved for 7 pairs of DAAs was significantly greater than 1.5. It was found that the buffer type exerted a profound impact on the separation. The effects of analyte substituents adjacent to the chiral center of analytes as well as operation conditions with respect to the separation efficiency were discussed. Five racemic compounds with single or double rings also got resolved on this short α-CD column to some extent.

HPLC enantioseparation on cyclodextrin-based chiral stationary phases.

Cyclodextrin-bonded silica material is one of the most commonly used chiral stationary phases in liquid chromatography for pharmaceutical analysis and enantioseparations. The approaches for immobilization of cyclodextrins onto the silica surface influence both, the stability of the chiral stationary phase and the enantioselectivity. In this chapter, we describe an established example of the preparation of a cyclodextrin-based chiral stationary phase via "click chemistry" and their application for enantioseparations of racemic compounds by HPLC.

Recent development of cyclodextrin chiral stationary phases and their applications in chromatography.

The current article reviews the development and applications of novel cyclodextrin chiral stationary phases (CD-CSPs) in liquid chromatography (LC), capillary electrochromatography (CEC), gas chromatography (GC) and supercritical fluid chromatography (SFC) over the period of January 2007 to March 2012. The synthetic routes of CD-CSPs, as well as the presence of selective functional groups in effecting inclusion complexation and molecular interactions have been found to exert profound influence in the enantioseparation process. In this article, various synthetic and functional groups immobilization strategies of novel CD-CSPs, and their applications in chiral resolution using different chromatography techniques are discussed. After introducing the topic in Section 1, Section 2 describes novel CD-CSPs in LC applications, where the CSPs are classified according to its coating approaches (physical and chemical manners) for ease of readership. Section 3 discusses recent development of CD-CSPs in open tubular CEC (OT-CEC), packed-bed CEC (P-CEC), pseudostationary phase CEC (PSP-CEC) and monolithic CEC. The last part illustrates novel CD-CSPs in gas chromatography (GC) and supercritical fluid chromatography (SFC).

Preparation and chiral separation of a novel immobilized cellulose-based chiral stationary phase in high-performance liquid chromatography.

The chiral selector 6-azido-2, 3-di(p-chlorophenylcarbamoylated) cellulose was synthesized and further chemically immobilized onto 5-µm amino functionalized spherical porous silica gel. It was used as chiral stationary phase in high-performance liquid chromatography. Thirty racemates were successfully separated into enantiomers in either normal phase mode or reversed-phase mode. Good reproducibility and stability of the chiral stationary phase have been demonstrated.

Cationic cyclodextrins chemically-bonded chiral stationary phases for high-performance liquid chromatography.

Two covalently bonded cationic ß-CD chiral stationary phases (CSPs) prepared by graft polymerization of 6(A)-(3-vinylimidazolium)-6-deoxyperphenylcarbamate-ß-cyclodextrin chloride or 6(A)-(N,N-allylmethylammonium)-6-deoxyperphenylcarbamoyl-ß-cyclodextrin chloride onto silica gel were successfully applied in high-performance liquid chromatography (HPLC). Their enantioseparation capability was examined with 12 racemic pharmaceuticals and 6 carboxylic acids. The results indicated that imidazolium-containing ß-CD CSP afforded more favorable enantioseparations than that containing ammonium moiety under normal-phase HPLC. The cationic moiety on ß-CD CSPs could form strong hydrogen bonding with analytes in normal-phase liquid chromatography (NPLC) to enhance the analytes' retention and enantioseparations. In reversed-phase liquid chromatography (RPLC), the analytes exhibited their maximum retention when the pH of mobile phase was close to their pK(a) value. Inclusion complexation with CD cavity and columbic/ionic interactions with cationic substituent on the CD rim would afford accentuated retention and enantioseparations of the analytes.

Chemically bonded cationic ß-cyclodextrin derivatives and their applications in supercritical fluid chromatography.

Cationic ß-cyclodextrin (CD) perphenylcarbamoylated derivatives were chemically bonded onto vinylized silica using a radical co-polymerization reaction. The derived materials were used as chiral stationary phases (CSP) in supercritical fluid chromatography (SFC). Enantioseparations were successfully demonstrated on 14 racemates encompassing flavanones, thiazides and amino acid derivatives. The electrostatic force between the analytes and the cationic moiety on ß-CD derivative was found to be important for retention and enantioseparation of the racemates. Aromatic cationic moiety on ß-CD enabled better enantioseparations than aliphatic cationic moiety. It was also found that the presence of acid additives would result in lower retention of the analytes but often assist the chiral resolutions.

Sub-2 µm porous silica materials for enhanced separation performance in liquid chromatography.

Fully or partially sub-2 µm porous silica materials have garnered strong interests as column packing materials in separation and analytical technologies due to the promise of rapid separation, enhanced efficiency and separation resolution. Silica support materials of different morphology and sub-2 µm size have been developed to improve separation performances in liquid chromatography (LC) and capillary electrochromatography (CEC). The current review highlights the recent development of sub-2 µm fully/partially porous silica materials and the demonstrations of their enhanced performance in achiral and chiral chromatography.

Novel cyclodextrin chiral stationary phases for high performance liquid chromatography enantioseparation: effect of cyclodextrin type.

Three novel chiral stationary phases (CSPs) were prepared by regioselective chemical immobilization of mono(6(A)-N-allylamino-6(A)-deoxy)perphenylcarbamoylated (PICD) α-, ß-, and γ-cyclodextrins (CDs) onto silica support via hydrosilylation. Their enantioseparation properties in high performance liquid chromatography (HPLC) were evaluated with a large spectrum of racemates including flavanone compounds, ß-adrenergic blockers, amines and non-protolytic compounds. The effect of CD's cavity size on enantioseparation abilities was studied and discussed. The results indicated that CD's surface loading at silica support played an important role in the enantioseparation on these CSPs under normal-phase conditions while inclusion phenomena contributed the major driving force under reverse-phase conditions. As expected, α-PICD demonstrated the best resolutions towards flavonone and most aromatic alcohols under normal-phase conditions with the highest surface loading; while Fujimura's competitive inclusion model can be applied to explain the better enantioseparations towards ß-adrenergic blockers, amines and non-protolytic compounds with α- and ß-PICD CSPs. γ-PICD CSP showed superior enantioseparation ability for sterically encumbered analytes like flavanone compounds under both normal-phase and reversed phase conditions.

Preparation of cyclodextrin chiral stationary phases by organic soluble catalytic 'click' chemistry.

We describe an effective and simple protocol that uses click chemistry to attach native ß-cyclodextrin (ß-CD) to silica particles, resulting in a chiral stationary phase (CCNCSP) that can be used for the enantioseparation of chiral drugs by high-performance liquid chromatography (HPLC). Starting from ß-CD, the CCNCSP is prepared in several steps: (i) reaction of ß-CD with 1-(p-toluenesulfonyl)-imidazole to afford mono-6-toluenesulfonyl-ß-CD; (ii) azidolysis of mono-6-toluenesulfonyl-ß-CD in dimethylformamide to give mono-6-azido-ß-CD (N(3)-CD); (iii) reaction of cuprous iodide with triphenylphosphine to form an organic soluble catalyst CuI(PPh(3)); (iv) preparation of alkynyl-modified silica particles; and (v) click chemistry immobilization of N(3)-CD onto alkynyl-modified silica to afford the desired chiral stationary phase. Synthesis of the stationary phase and column packing takes ∼1 week.

Resonance energy transfer (RET)-Induced intermolecular pairing force: a tunable weak interaction and its application in SWNT separation.

This paper explores evidence of an optically mediated interaction that is active in the separation mechanism of certain selective agents through consideration of the contrasting selective behaviors of two conjugated polymers with distinct optical properties. The involvement of a RET-induced intermolecular pairing force is implied by the different illumination response behaviors. The magnitude of this interaction scales with the external stimulus parameter, the illumination irradiance (I), and thus is tunable. This suggests a facile technique to modify the selectivity of polymers toward specific SWNT species by altering the polymer structure to adjust the corresponding intermolecular interaction. This is the first experimental verification and application of a RET-induced intermolecular pairing force to SWNT separation. With this kind of interaction taken into account, reasonable interpretation of some conflicting data, especially PLE maps, can be easily made. The above conclusion can be applied to other substances as long as they are electrically neutral and there is photon-induced RET between them. The significant magnitude of this interaction makes direct manipulation of molecules/particles possible and is expected to have applications in molecular engineering.

Novel ß-cyclodextrin chiral stationary phases with different length spacers for normal-phase high performance liquid chromatography enantioseparation.

Cyclodextrin and its derivatives are widely used as selectors of chiral stationary phases (CSPs) for high performance liquid chromatography (HPLC) due to their unique molecular structure and resolution capability. Three mono(6(A)-N-(ω-alkenylamino)-6(A)-deoxy)perphenylcarbamoylated ß-cyclodextrin (PICD) based CSPs with different length spacers have been prepared, with their enantioseparation abilities evaluated with 10 model racemates including aromatic alcohols, flavanone compounds, amine and non-protolytic compounds under normal-phase conditions. The effect of spacer length and surface loading on the enantioseparation performance of CSPs is investigated herewith. The results indicate that higher surface loading 6C-PICD displays the best enantioselectivities towards selected racemates under normal-phase conditions.

Enantioseparation of dansyl amino acids by ultra-high pressure liquid chromatography using cationic ß-cyclodextrins as chiral additives.

This work reports the application of ultra-high pressure liquid chromatography (UHPLC) for reasonably fast enantiorecognition of some dansyl amino acids by employing three cationic ß-cyclodextrins (ß-CDs) as chiral additives. Good resolutions were obtained on an Agilent C18 column (2.1 mm i.d.; 1.8 µm; 50 mm length) with 1% (v/v) triethylammonium acetate buffered at pH 4.7 and acetonitrile as the mobile phase. Most of the analytes could be baseline resolved within 10 min. Increased cationic CD concentration or acetonitrile proportion in the mobile phase results in a decreased retention factor but accentuated selectivity. Furthermore, molecular mechanics calculation was performed and found to be consistent with the experimental results.

"Click" preparation of hindered cyclodextrin chiral stationary phases and their efficient resolution in high performance liquid chromatography.

This communication reports the preparation of two new cyclodextrin (CD) chiral stationary phases (CSPs): heptakis(6-deoxy-6-azido)-ß-CD and heptakis(6-deoxy-6-azido-phenylcarbamoylated)-ß-CD CSPs that perform quite differently to our previously reported "click" immobilized CD-CSPs. These CSPs are sterically congested at the narrow mouth of the CD and exhibit chiral discrimination between over 40 pairs of enantiomers in high performance liquid chromatography. The free hydroxyl CSP afforded better separation of indoprofen, ketoprofen, Tröger's base, hydroxyl, carboxylic and dansyl amino acids than did the phenylcarbamoylated CSP, while the latter was better at resolving aryl alcohols, flavonoids, ß-blockers and ß-agonists. The current work shows that enantiodiscrimination achieved with different CSPs for different classes of analyte may be correlated with CD accessibility and peripheral functionality.

Sub-1-micron mesoporous silica particles functionalized with cyclodextrin derivative for rapid enantioseparations on ultra-high pressure liquid chromatography.

Mesoporous silica particles of relatively uniform sub-1-micron size (0.6-0.9 µm) were successfully prepared by a modified synthesis strategy and applied in chiral separation in an ultra-high pressure liquid chromatography system. These particles were prepared via a ternary surfactant system (Pluronic P123, F127 and hexadecyltrimethyl-ammonium bromide) and subsequently derivatized with perphenylcarbamoylated-ß-cyclodextrin moieties. The mesoporous silica particles, despite their submicron size, enabled low back-pressure operation on an ultra-high pressure liquid chromatography system at a maximum flow rate of 2 ml/min. In addition, the particles possessed high surface area (480 m(2)/g) and thus afforded high cyclodextrin derivative loading (32 µmol/g), demonstrating rapid enantioseparation and good resolution of 6 basic and neutral racemates.

"Click" immobilized perphenylcarbamated and permethylated cyclodextrin stationary phases for chiral high-performance liquid chromatography application.

Two cyclodextrin-based chiral stationary phases have been prepared by immobilization of functionalized mono-6-azido-beta-CD derivatives to alkynyl modified silica via "click" chemistry and applied to the HPLC enantioseparation of various chiral compounds. The perphenylcarbamated CD CSP (CCP-CSP) exhibited excellent chiral recognition of a wide range of analytes including racemic aryl alcohols, flavonoids, bendroflumethiazide, atropine and some beta-blockers. Methanol proved to be a better organic modifier than acetonitrile for most of the analytes with the exception of bendroflumethiazide. The "click" chemistry immobilized permethylated CD CSP (CCM-CSP) afforded poor chiral recognition for most analytes, but could resolve non-aromatic ionone derivatives which were not separated on CCP-CSP. These results suggest that resolution with cyclodextrin derived CSPs depend on a complex interplay of 'host'-'guest' inclusion, hydrogen bonding, pi-pi and hydrophobic interactions.

Chiral capillary electrophoresis with cationic pyrrolidinium-beta-cyclodextrin derivatives as chiral selectors.

New single-isomer, cationic beta-cyclodextrins, including mono-6-deoxy-6-pyrrolidine-beta-cyclodextrin chloride (pyCDCl), mono-6-deoxy-6-(N-methyl-pyrrolidine)-beta-cyclodextrin chloride (N-CH(3)-pyCDCl), mono-6-deoxy-6-(N-(2-hydroxyethyl)-pyrrolidine)-beta-cyclodextrin chloride (N-EtOH-pyCDCl), mono-6-deoxy-6-(2-hydroxymethyl-pyrrolidine)-beta-cyclodextrin chloride (2-MeOH-pyCDCl) were synthesized and used as chiral selectors in capillary electrophoresis for the enantioseparation of carboxylic and hydroxycarboxylic acids and dansyl amino acids. The unsubstituted pyCDCl exhibited the greatest resolving ability. Most analytes were resolved over a wide range of pH from 6.0 to 9.0 with this chiral selector. In general, increasing pH led to a decrease in resolution. The effective mobilities of all the analytes were found to decrease with increasing CD concentration. The optimal concentration for most carboxylic acids and dansyl amino acid was in the range 5-7.5 mM and >15 mM for hydroxycarboxylic acids. (1)H NMR experiments provided direct evidence of inclusion in the CD cavity.