On-column labeling technique and chiral CE of amino acids with mixed chiral selectors and UV detection

Precursor Quantitation Methods for Next Generation Food Production

Food is essential for human survival. Nowadays, traditional agriculture faces challenges in balancing the need of sustainable environmental development and the rising food demand caused by an increasing population. In addition, in the emerging of consumers' awareness of health related issues bring a growing trend towards novel nature-based food additives. Synthetic biology, using engineered microbial cell factories for production of various molecules, shows great advantages for generating food alternatives and additives, which not only relieve the pressure laid on tradition agriculture, but also create a new stage in healthy and sustainable food supplement. The biosynthesis of food components (protein, fats, carbohydrates or vitamins) in engineered microbial cells often involves cellular central metabolic pathways, where common precursors are processed into different proteins and products. Quantitation of the precursors provides information of the metabolic flux and intracellular metabolic state, giving guidance for precise pathway engineering. In this review, we summarized the quantitation methods for most cellular biosynthetic precursors, including energy molecules and co-factors involved in redox-reactions. It will also be useful for studies worked on pathway engineering of other microbial-derived metabolites. Finally, advantages and limitations of each method are discussed.

Past, present, and future developments in enantioselective analysis using capillary electromigration techniques

Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis.

Chiral Micellar Electrokinetic Chromatography

The potential of Micellar Electrokinetic Chromatography to achieve enantiomeric separations is reviewed in this article. The separation principles and the most frequently employed separation strategies to achieve chiral separations by Micellar Electrokinetic Chromatography are described. The use of chiral micellar systems alone or combined with other micellar systems or chiral selectors, as well as of mixtures of achiral micellar systems with chiral selectors is discussed together with the effect of different additives present in the separation medium. Indirect methods based on the derivatization of analytes with chiral derivatizing reagents and the use of achiral micelles are also considered. Preconcentration techniques employed to improve sensitivity and the main approaches developed to facilitate the coupling with Mass Spectrometry are included. The most recent and relevant methodologies developed by chiral Micellar Electrokinetic Chromatography and their applications in different fields are presented.

Chiral capillary electrophoresis with UV-excited fluorescence detection for the enantioselective analysis of 9-fluorenylmethoxycarbonyl-derivatized amino acids

The potential of capillary electrophoresis (CE) with ultraviolet (UV)-excited fluorescence detection for sensitive chiral analysis of amino acids (AAs) was investigated. DL-AAs were derivatized with 9-fluorenylmethoxycarbonyl chloride (FMOC)-Cl to allow their fluorescence detection and enhance enantioseparation. Fluorescence detection was achieved employing optical fibers, leading UV excitation light (< 300 nm) from a Xe-Hg lamp to the capillary window, and fluorescence emission to a spectrograph equipped with a charge-coupled device (CCD). Signal averaging over time and emission wavelength intervals was carried out to improve the signal-to-noise ratio of the FMOC-AAs. A background electrolyte (BGE) of 40 mM sodium tetraborate (pH 9.5), containing 15% isopropanol (v/v), 30 mM sodium dodecyl sulfate (SDS), and 30 mM β-cyclodextrin (β-CD), was found optimal for AA chemo- and enantioseparation. Enantioresolutions of 1.0 or higher were achieved for 16 proteinogenic DL-AAs. Limits of detection (LODs) were in the 10-100-nM range (injected concentration) for the D-AA enantiomers, except for FMOC-D-tryptophan (536 nM) which showed intramolecular fluorescence quenching. Linearity (R² > 0.997) and repeatability for peak height (relative standard deviations (RSDs) < 7.0%; n = 5) and electrophoretic mobility (RSDs < 0.6%; n = 5) of individual AA enantiomers were established for chiral analysis of DL-AA mixtures. The applicability of the method was investigated by the analysis of cerebrospinal fluid (CSF). Next to L-AAs, endogenous levels of D-glutamine and D-aspartic acid could be measured in CSF revealing enantiomeric ratios of 0.35 and 19.6%, respectively. This indicates the method's potential for the analysis of low concentrations of D-AAs in presence of abundant L-AAs.

In-capillary derivatization with (-)-1-(9-fluorenyl)ethyl chloroformate as chiral labeling agent for the electrophoretic separation of amino acids

An original micellar electrokinetic chromatography (MEKC) method using in-capillary derivatization with a chiral labeling reagent was developed for the separation of amino acid (AA) derivatives. The potential of (-)-1-(9-fluorenyl)-ethyl chloroformate (FLEC) as in-capillary derivatization agent is described for the first time. Several parameters for in-capillary derivatization and subsequent MEKC separation were systematically investigated using experimental designs. Firstly experimental conditions for in-capillary derivatization were optimized using face-centered central composite design (FCCD). Mixing voltage and time as well as concentration of the labeling solution were investigated. Efficient labeling was achieved by sequential injection of AAs and FLEC labeling solution followed by the application of a voltage of 0.2 kV for 570 s. The background electrolyte (BGE) composition was then optimized in order to achieve selectivity. A FCCD was performed with two factors, namely the sodium dodecyl sulfate (SDS) concentration and the percentage of propan-2-ol (IPA). The separation of 12 pairs of derivatized AA (FLEC-AA) diastereomers was achieved with resolution values comprised between 3 and 20. Furthermore, an efficient derivatization and separation of 29 FLEC-AA derivatives were achieved in a single run using a buffer made up of 40 mM sodium tetraborate, 21 mM SDS and 8.5% IPA. The method was successfully applied to the analysis of spiked artificial cerebrospinal fluid (aCSF) sample.

On-line sample preconcentration by sweeping and poly(ethylene oxide)-mediated stacking for simultaneous analysis of nine pairs of amino acid enantiomers in capillary electrophoresis

This study proposes a sensitive method for the simultaneous separation and concentration of 9 pairs of amino acid enantiomers by combining poly(ethylene oxide) (PEO)-based stacking, β-cyclodextrin (β-CD)-mediated micellar electrokinetic chromatography (MEKC), and 9-fluoroenylmethyl chloroformate (FMOC) derivatization. The 9 pairs of FMOC-derivatized amino acid enantiomers were baseline separated using a discontinuous system, and the buffer vials contained a solution of 150 mM Tris-borate (TB), 12.5% (v/v) isopropanol (IPA), 0.5% (w/v) PEO, 35 mM sodium taurodeoxycholate (STDC), and 35 mM β-CD, and the capillary was filled with a solution of 1.5 M TB, 12.5% (v/v) IPA, 35 mM STDC, and 35 mM β-CD. Based on the difference in viscosity between the sample zone and PEO solution and because of the STDC sweeping, the discontinuous system effectively stacked 670 nL of the 9 pairs of FMOC-derivatized amino acid enantiomers without losing chiral resolution. Consequently, the limits of detection for the 9 pairs of FMOC-derivatized amino acid enantiomers were reduced to 40-60 nM. This method was successfully used to determine d-Tryptophan (Trp), l-Trp, d-Phenylalanine (Phe), l-Phe, d-Glutamic acid (Glu), and l-Glu in various types of beers.

Chemo- and enantio-selective method for the analysis of amino acids by capillary electrophoresis with in-capillary derivatization

A novel dual chiral CE method was developed for the separation of l- and d-amino acids (AAs), using in-capillary derivatization with 9-fluoroenylmethyl chloroformate (FMOC). Firstly, using pre-column derivatization, the enantioseparation of FMOC-AAs was optimized according to the nature of cyclodextrins (CD). A background electrolyte (BGE) composed of 30 mM β-CD, 30 mM octakis(2,3-dihydroxy-6-O-sulfo)-γ-CD (OS-γ-CD), 40 mM tetraborate and 15% isopropanol (IPA) was selected and led to 17 baseline resolved pairs (R(s)=1.7-5.8) and two partially resolved pairs (Lys, R(s)=0.5 and Arg, R(s)=1.2). Experimental conditions for in-capillary derivatization were then optimized. Several parameters, such as mixing voltage and time, concentration of labeling solution and the length of the spacer plug were studied. The optimal conditions for in-capillary derivatization procedure were obtained using successive hydrodynamic injections (30 mbar) of AAs for 2s, borate buffer for 4s and 10mM FMOC solution for 6s, followed by a mixing at 3 kV for 72 s and wait time of 1 min. Moreover, a particular attention was paid to improve separation chemoselectivity. The effect on stereoselectivity and chemoselectivity of different factors, such as decrease of pH and tetraborate concentration and the addition of sodium dodecyl sulfate (SDS), was investigated using the in-capillary derivatization procedure. The best separation of a standard mixture of ten AA racemates was observed using a BGE containing 30 mM β-CD, 30 mM OS-γ-CD, 25 mM SDS, 40 mM sodium tetraborate and 17% IPA.

Combined use of chiral ionic liquid and cyclodextrin for MEKC: Part I. Simultaneous enantioseparation of anionic profens

The enantiomers of five profen drugs were simultaneously separated by MEKC with the combined use of 2,3,6-tri-O-methyl-beta-cyclodextrin and chiral cationic ionic liquid, N-undecenoxy-carbonyl-L-leucinol bromide, which formed micelles in aqueous buffers. Enantioseparations of these profen drugs were optimized by varying the chain length and concentration of the IL surfactant using a standard recipe containing 35 mM 2,3,6-tri-O-methyl-beta-cyclodextrin, 5 mM sodium acetate at pH 5.0. The batch-to-batch reproducibility of N-undecenoxy-carbonyl-L-leucinol bromide was tested and found to have no significant impact in terms of enantiomeric resolution, efficiency, and migration time. Finally, this method was successfully applied for the quantitative determination of ibuprofen in pharmaceutical tablets.

1,3,5,7-Tetramethyl-8-aminozide-difluoroboradiaza-s-indacene as a new fluorescent labeling reagent for the determination of aliphatic aldehydes in serum with high performance liquid chromatography

A BODIPY-based fluorescent derivatization reagent with a hydrazine moiety, 1,3,5,7-tetramethyl-8-aminozide-difluoroboradiaza-s-indacene (BODIPY-aminozide), has been designed for aldehyde labeling. An increased fluorescence quantum yield was observed from 0.38 to 0.94 in acetonitrile when it reacted with aldehydes. Twelve aliphatic aldehydes from formaldehyde to lauraldehyde were used to evaluate the analytical potential of this reagent by high performance liquid chromatography (HPLC) on C(18) column with fluorescence detection. The derivatization reaction of BODIPY-aminozide with aldehydes proceeded at 60 degrees C for 30min to form stable corresponding BODIPY hydrazone derivatives in the presence of phosphoric acid as a catalyst. The maximum excitation (495nm) and emission (505nm) wavelengths were almost the same for all the aldehyde derivatives. A baseline separation of all the 12 aliphatic aldehydes (except formaldehyde and acetaldehyde) is achieved in 20min with acetonitrile-tetrahydrofuran (THF)-water as mobile phase. The detection limits were obtained in the range from 0.43 to 0.69nM (signal-to-noise=3), which are better than or comparable with those obtained by the existing methods based on aldehyde labeling. This reagent has been applied to the precolumn derivatization followed with HPLC determination of trace aliphatic aldehydes in human serum samples without complex pretreatment or enrichment method.

Cyclodextrins in capillary electrophoresis enantioseparations--recent developments and applications

Capillary EKC has been established as a versatile and robust CE method for the separation of enantiomers. Within the chiral selectors added to the BGE CDs continue as the most widely used selectors due to their structural variety and commercial availability. This is reflected in the large number of practical applications of CDs to analytical enantioseparations that have been reported between January 2006 and January 2008, the period of time covered by this review. Most of these applications cover aspects of life sciences such as drug analysis, bioanalysis, environmental analysis, or food analysis. Moreover, new CD derivatives have been developed in an attempt to achieve altered enantioselectivities and to further broaden the application range. Finally, efforts will be summarized that aim at an understanding of the molecular level of the chiral recognition between CDs and the analytes.

Sample preparation

A panorama of sample preparation methods has been composed from 481 references, with a highlight of some promising methods fast developed during recent years and a somewhat brief introduction on most of the well-developed methods. All the samples were commonly referred to molecular composition, being extendable to particles including cells but not to organs, tissues and larger bodies. Some criteria to evaluate or validate a sample preparation method were proposed for reference. Strategy for integration of several methods to prepare complicated protein samples for proteomic studies was illustrated and discussed.