Direct chiral resolution of tartaric acid in food products by ligand exchange capillary electrophoresis using copper(II)-D-quinic acid as a chiral selector

Nanopore analysis of cis-diols in fruits

Natural fruits contain a large variety of cis-diols. However, due to the lack of a high-resolution sensor that can simultaneously identify all cis-diols without a need of complex sample pretreatment, direct and rapid analysis of fruits in a hand-held device has never been previously reported. Nanopore, a versatile single molecule sensor, can be specially engineered to perform this task. A hetero-octameric Mycobacterium smegmatis porin A (MspA) nanopore modified with a sole phenylboronic acid (PBA) adapter is prepared. This engineered MspA accurately recognizes 1,2-diphenols, alditols, α-hydroxy acids and saccharides in prune, grape, lemon, different varieties of kiwifruits and commercial juice products. Assisted with a custom machine learning program, an accuracy of 99.3% is reported and the sample pretreatment is significantly simplified. Enantiomers such as DL-malic acids can also be directly identified, enabling sensing of synthetic food additives. Though demonstrated with fruits, these results suggest wide applications of nanopore in food and drug administration uses.

Versatile capillary electrophoresis method for the direct chiral separation of aliphatic and aromatic α-hydroxy acids, β-hydroxy acids and polyhydroxy acids using vancomycin as chiral selector

Hydroxy acids (HAs) are ubiquitous in nature and play significant roles in various industrial and biological processes. Most HAs harbor at least one chiral center, therefore the development of efficient chiral analysis techniques for HA stereoisomers is of crucial importance across a wide range of fields. A capillary electrophoresis (CE) method was developed for the chiral analysis and quantification of aliphatic and aromatic α‑hydroxy acid (AHA) enantiomers, aliphatic β‑hydroxy acid (BHA) enantiomers and aliphatic polyhydroxy acid (PHA) stereoisomers. Using a modified partial filling-counter current method with indirect UV detection, high resolution (Rs) was achieved with vancomycin as a chiral selector added to the background electrolyte composed of 10 mM of benzoic acid/L-histidine at pH 5 using a polyacrylamide-coated capillary. This method could be readily applied to the determination of the enantiomers of 12 aliphatic AHAs, 4 aromatic AHAs, 3 aliphatic BHAs, as well as to the determination of the stereoisomers of tartaric acid, 2,3-dihydroxybutanoic acid, 2,3,4,5-tetrahydroxypentanoic acid, and 2,3,4,5,6-pentahydroxyhexanoic acid without the need for sample derivatization. Finally, our study provides a robust and versatile strategy for the chiral and stereoselective analysis of a broad range of hydroxy acid compounds.

Ample dietary fat reduced the risk of primary vesical calculi by inducing macrophages to engulf budding crystals in mice

Although primary vesical calculi is an ancient disease, the mechanism of calculi formation remains unclear. In this study, we established a novel primary vesical calculi model with d,l-choline tartrate in mice. Compared with commonly used melamine and ethylene glycol models, our model was the only approach that induced vesical calculi without causing kidney injury. Previous studies suggest that proteins in the daily diet are the main contributors to the prevention of vesical calculi, yet the effect of fat is overlooked. To assay the relationship of dietary fat with the formation of primary vesical calculi, d,l-choline tartrate-treated mice were fed a high-fat, low-fat, or normal-fat diet. Genetic changes in the mouse bladder were detected with transcriptome analysis. A high-fat diet remarkably reduced the morbidity of primary vesical calculi. Higher fatty acid levels in serum and urine were observed in the high-fat diet group, and more intact epithelia in bladder were observed in the same group compared with the normal- and low-fat diet groups, suggesting the protective effect of fatty acids on bladder epithelia to maintain its normal histological structure. Transcriptome analysis revealed that the macrophage differentiation-related gene C–X–C motif chemokine ligand 14 (Cxcl14) was upregulated in the bladders of high-fat diet-fed mice compared with those of normal- or low-fat diet-fed mice, which was consistent with histological observations. The expression of CXCL14 significantly increased in the bladder in the high-fat diet group. CXCL14 enhanced the recruitment of macrophages to the crystal nucleus and induced the transformation of M2 macrophages, which led to phagocytosis of budding crystals and prevented accumulation of calculi. In human bladder epithelia (HCV-29) cells, high fatty acid supplementation significantly increased the expression of CXCL14. Dietary fat is essential for the maintenance of physiological functions of the bladder and for the prevention of primary vesical calculi, which provides new ideas for the reduction of morbidity of primary vesical calculi.

Re-evaluation of l(+)-tartaric acid (E 334), sodium tartrates (E 335), potassium tartrates (E 336), potassium sodium tartrate (E 337) and calcium tartrate (E 354) as food additives

The EFSA Panel on Food Additives and Flavourings (FAF) provides a scientific opinion on tartaric acid-tartrates (E 334-337, 354) when used as food additives. The Scientific Committee for Food (SCF) in 1990 established an acceptable daily intake (ADI) of 30 mg/kg body weight (bw) per day, for l(+)-tartaric acid and its potassium and sodium salts. The metabolism of l(+)-tartaric acid and its potassium sodium salt was shown to be species dependent, with a greater absorption in rats than in humans. No toxic effects, including nephrotoxicity, were observed in toxicological studies in which the l(+)-form was tested. There was no indication for a genotoxic potential of tartaric acid and its sodium and potassium salts. In a chronic study in rats, no indication for carcinogenicity of monosodium l(+)-tartrate was reported at the highest dose tested (3,100 mg/kg bw per day). The available studies for maternal or developmental toxicity did not report any relevant effects; no studies for reproductive toxicity were available; however, no effects on reproductive organs were observed in the chronic toxicity study. The Panel concluded that the data on systemic availability were robust enough to derive a chemical-specific uncertainty factor instead of the usual default uncertainty factor of 100. A total uncertainty factor of 10 was derived by applying a total interspecies uncertainty factor of 1 instead of 10, based on data showing lower internal exposure in humans compared to rats. The Panel established a group ADI for l(+)-tartaric acid-tartrates (E 334-337 and E 354) of 240 mg/kg bw per day, expressed as tartaric acid, by applying the total uncertainty factor of 10 to the reference point of 3,100 mg sodium tartrate/kg bw per day, approximately to 2,440 mg tartaric acid/kg bw per day. The exposure estimates for the different population groups for the refined non-brand-loyal exposure scenario did not exceed the group ADI of 240 mg/kg bw per day, expressed as tartaric acid. Some recommendations were made by the Panel.

Addressing the pitfalls when designing intervention studies to discover and validate biomarkers of habitual dietary intake

INTRODUCTION

Dietary exposure monitoring within populations is reliant on self-reported measures such as Food Frequency Questionnaires and diet diaries. These methods often contain inaccurate information due to participant misreporting, non-compliance and bias. Urinary metabolites derived from individual foods could provide additional objective indicators of dietary exposure. For biomarker approaches to have utility it is essential that they cover a wide-range of commonly consumed foods and the methodology works in a real-world environment.

OBJECTIVES

To test that the methodology works in a real-world environment and to consider the impact of the major sources of likely variance; particularly complex meals, different food formulations, processing and cooking methods, as well as the dynamics of biomarker duration in the body.

METHODS

We designed and tested a dietary exposure biomarker discovery and validation strategy based on a food intervention study involving free-living individuals preparing meals and collecting urine samples at home. Two experimental periods were built around three consecutive day menu plans where all foods and drinks were provided (n = 15 and n = 36).

RESULTS

The experimental design was validated by confirming known consumption biomarkers in urinary samples after the first menu plan. We tested biomarker performance with different food formulations and processing methods involving meat, wholegrain, fruits and vegetables.

CONCLUSION

It was demonstrated that spot urine samples, together with robust dietary biomarkers, despite major sources of variance, could be used successfully for dietary exposure monitoring in large epidemiological studies.

Enantiomeric Tartaric Acid Production Using cis-Epoxysuccinate Hydrolase: History and Perspectives

Tartaric acid is an important chiral chemical building block with broad industrial and scientific applications. The enantioselective synthesis of l(+)- and d(−)-tartaric acids has been successfully achieved using bacteria presenting cis-epoxysuccinate hydrolase (CESH) activity, while the catalytic mechanisms of CESHs were not elucidated clearly until very recently. As biocatalysts, CESHs are unique epoxide hydrolases because their substrate is a small, mirror-symmetric, highly hydrophilic molecule, and their products show very high enantiomeric purity with nearly 100% enantiomeric excess. In this paper, we review over forty years of the history, process and mechanism studies of CESHs as well as our perspective on the future research and applications of CESH in enantiomeric tartaric acid production.

Anomalous circular dichroism in high harmonic generation of stereoisomers with two chiral centers

When a molecule has more than one chiral center, it can be either a chiral molecule or a meso isomer. High harmonic generation (HHG) of stereoisomers with two chiral centers driven by circularly polarized (CP) laser pulses is investigated. Counterintuitively, it is found that the HHG exhibits prominent circular dichroism for the meso isomer, while the harmonic spectra with left and right CP laser pulses are nearly the same for the chiral isomers. We show that the anomalous circular dichroism is attributed to the characteristic recollision dynamics of HHG. This feature makes the HHG a promising tool to discriminate the meso isomer and racemic mixture, where no optical activity can be found in both cases. Similar dichroism responses are also found by applying the counter-rotating bicircular laser pulses.

Chiral separation using capillary electromigration techniques based on ligand exchange principle

Over the last couple of decades, researchers have developed diverse chiral separation methods emerged from a few chiral separation principles. This review article is primarily focused on the application of chiral ligand-exchange (CLE) principle in capillary electromigration techniques, such as capillary electrophoresis (CE) and capillary electrochromatography (CEC). First, the most commonly used CLE-CZE separation mode by using different kinds of central ions, such as Cu(II), Zn(II), borate ion, and other metal ions, has been introduced. Meanwhile, several kinds of surfactants have been applied as the micelle-forming agents in the CLE micellar electrokinetic chromatography mode. The highlight of recent research of CLE-CEC is the exploitation of novel columns for chiral separation. Then, two kinds of capillary columns, packed capillary and monolithic capillary column, have been briefly described. Finally, the effective application of these chiral separation methods has been presented, including the application in life science and food analysis area.

Enantioseparation of α-hydroxy acids by chiral ligand exchange CE with a dual central metal ion system

Using two kinds of central metal ions in a background electrolyte, ligand exchange CE was investigated for the simultaneous enantioseparation of dl-malic, dl-tartaric, and dl-isocitric acids. Ligand exchange CE with 100 mM d-quinic acid as a chiral selector ligand and 10 mM Cu(II) ion as a central metal ion could enantioseparate dl-tartaric acid but not dl-malic acid or dl-isocitric acid. A dual central metal ion system containing 0.5 mM Al(III) ion in addition to 10 mM Cu(II) ion in the background electrolyte enabled the simultaneous enantioseparation of the three α-hydroxy acids. These results suggest that the use of a dual central metal ion system can be useful for enantioseparation by ligand exchange CE.

A novel chiral ligand exchange capillary electrophoresis system with amino acid ionic liquid as ligand and its application in screening D-amino-acid oxidase inhibitors

A novel amino acid ionic liquid (AAIL) with L-ornithine (L-Orn) as anion was successfully synthesized, and subsequently applied as an available chiral ligand coordinated with Zn(II) in a chiral ligand exchange capillary electrophoresis (CLE-CE) system for the enantioseparation of dansyl amino acids (Dns-D,L-AAs). The influence of key parameters, such as buffer pH, concentration ratio of Zn(II) to ligand and complex concentration, was investigated in detail. Eleven pairs of Dns-D,L-AAs enantiomers were baseline separated and three pairs were partly separated under the optimum conditions. For exploring its potential application, the quantitative features of this proposed method were studied. Good linearity (r(2) = 0.999) and favorable repeatability (RSD ≤ 3.4%) were obtained by using Dns-D,L-Met as the test analyte. Finally, this method was employed to investigate the inhibition efficiency of d-amino acid oxidase (DAAO) inhibitors, which may pave a new way for the high-throughput screening of enzyme inhibitors and relevant drug discovery.

Chiral metal-ion complexes for enantioseparation by capillary electrophoresis and capillary electrochromatography: a selective review

This review gives an overview about chiral separation by capillary electrophoresis and capillary electrochromatography using different chiral metal-ion complexes. The topic enantioseparation is still of big interest for chiral drugs and natural compounds. Regarding chiral drugs it is often the case that the enantiomers differ in activity. The chiral separation principle of ligand-exchange (LE) can be enabled for liquid chromatography, capillary electrophoresis as well as for capillary electrochromatography. Ligand-exchange can be applied particularly for chiral amino acids, amino alcohols or α-hydroxy acids. Examples and applications are given along with the latest developments.

Mechanism of change in enantiomer migration order of enantioseparation of tartaric acid by ligand exchange capillary electrophoresis with Cu(II) and Ni(II)-D-quinic acid systems

The mechanism of change in the enantiomer migration order (EMO) of tartarate on ligand exchange CE with Cu(II)- and Ni(II)-D-quinic acid systems was investigated thoroughly by circular dichroism (CD) spectropolarimetry. The (13) C NMR spectra of solutions containing D-quinate (pH 5.0) with Cu(II) or Ni(II) revealed the coordination of carboxylate and hydroxyl groups on D-quinate. The D-quinic acid concentration dependence of the CD spectra at a fixed Cu(II) concentration at pH 5.0 indicates that the 1:1, 1:2 and 1:3 Cu(II)-D-quinate complexes were formed with an increase in the concentration of D-quinic acid. The CD spectral behavior revealed that D-tartarate is selectively coordinated to the 1:1 complex to give the 1:1:1 Cu(II)-D-quinate-D-tartarate ternary complex while L-tartarate is selectively bound to the 1:2 and 1:3 complexes to form the 1:2:1 ternary complex. In the Ni(II)-D-quinic acid system, it became apparent that the 1:2 Ni(II)-D-quinate complex is mainly formed in the wide range of D-quinic acid concentration at pH 5.0 and D-tartarate is selectively coordinated to the 1:2 complex to form the 1:2:1 ternary complex. The change in EMO of tartarate on ligand exchange CE was explainable by the change in coordination selectivity for D- and L-tartarates in the Cu(II)- and Ni(II)-D-quinic acid systems depending on the compositions of the complexes formed in BGE.

Optimization of culture conditions for production of cis-epoxysuccinic acid hydrolase using response surface methodology

Response surface methodology, which allows for rapid identification of important factors and optimization of them to enhance enzyme production, was employed here to optimize culture conditions for the production of cis-epoxysuccinic acid hydrolase from Bordetella sp. strain 1-3. In the first step, a Plackett-Burman design was used to evaluate the effects of nine variables (yeast extract, cis-epoxysuccinic acid, KH(2)PO(4), K(2)HPO(4).3H(2)O, MgSO(4).7H(2)O, trace minerals solution, culture volume, initial pH and incubation time) on the enzyme production. Yeast extract, cis-epoxysuccinic acid and KH(2)PO(4) had significant influences on cis-epoxysuccinic acid hydrolase production and their concentrations were further optimized using central composite design and response surface analysis. A combination of adjusting the concentration of yeast extract to 7.8 g/l, cis-epoxysuccinic acid to 9.8 g/l, and KH(2)PO(4) to 1.12 g/l would favor maximum cis-epoxysuccinic acid hydrolase production. An enhancement of cis-epoxysuccinic acid hydrolase production from 5.6 U/ml to 9.27 U/ml was gained after optimization.

Chiral separation of amino acids and glycyl dipeptides by chiral ligand-exchange capillary electrophoresis comparing Cu(II), Co(II), Ni(II) and Zn(II) complexes of three different sugar acids

This paper deals with comparative studies on the use of copper(II), cobalt(II), nickel(II) and zinc(II) complexes of d-gluconic acid, d-saccharic acid and l-threonic acid as chiral selectors for the enantioseparation of aromatic amino acids and glycyl dipeptides using the principle of ligand-exchange capillary electrophoresis. Although copper(II) is the most frequently used central ion in ligand-exchange capillary electrophoresis, in the case of d-gluconic acid cobalt(II) was shown to be an alternative for the enantioseparation of amino acids. Glycyl dipeptides, however, were resolved only with copper(II) complexes. Zn(II) as a central ion was not effective in all cases and with Ni(II) only some partial separations were achieved.

Mechanism of enantioseparation ofDL-pantothenic acid in ligand exchange capillary electrophoresis using a diol–borate system

Borate complexes formed in the ternary system at pH 9.2 containing borate, (S)-3-amino-1,2-propanediol (SAP), and DL-pantothenic acid (DL-PTA) were identified by 13C and 11B NMR, and it is confirmed that the binary complexes, [B(OH)2(SAP)], [B(SAP)2]+ [B(OH)2(D- or L-PTA)]2-, and [B(D- or L-PTA)2]3- (including [B(D-PTA)(L-PTA)]3-), and the ternary complexes, [B(SAP)(D- or L-PTA)]-, coexist at equilibrium in the ternary system. Thermodynamic experiments by variable-temperature 11B NMR revealed that the ternary complex, [B(SAP)(D-PTA)]-, is entropically more stable than [B(SAP)(L-PTA)]-. Because two geometrical isomers are possible for the respective ternary complexes, semi-empirical molecular orbital calculations were performed by PM5, PM3, and AM1 methods in order to obtain the optimized structures. It is indicated from the calculated heats of formation and experimentally obtained thermodynamic parameters that the (S)-isomer is more probable for the respective ternary complexes with D- and L-PTA. In the optimized structure of (S)-[B(SAP)(D-PTA)]- in water, the SAP and D-PTA ligands were oppositely oriented to form a rather linear structure, while the diastereomer, (S)-[B(SAP)(L-PTA)]-, had a folded structure. Because such a difference in the solvated structure of the ternary complexes can give a different electrophoretic velocity in CE, the enantioseparation of DL-PTA in CE is reasonably attributed to a difference in the observed electrophoretic mobility for the equilibrated ternary systems containing the respective ternary complexes.

Chiral ligand exchange micellar electrokinetic chromatography using borate anion as a central ion

Three compounds having 1,2-diol structure (1-phenyl-1,2-ethanediol, 3-phenoxy-1,2-propanediol, and 3-benzyloxy-1,2-propanediol) were enantioseparated by ligand exchange MEKC using (5S)-pinanediol (SPD) as a chiral selector and borate anion as a central ion together with SDS. When (S)-1,2-propanediol, (S)-1,2,4-butanetriol, or (S)-3-tert-butylamino-1,2-propanediol were used as the chiral ligand instead of SPD, these three compounds were not enantioseparated. When borate was replaced with 2-aminoethane-1-sulfonate or N-cyclohexyl-3-aminopropanesulfonate, no chiral separation was achieved. Therefore, the hydrophobic interaction between the chiral selector and the chiral analytes within the transient diastereomeric complex may play an important role in the enantioseparation achieved by the proposed method.

Chiral separation of halogenated amino acids by ligand-exchange capillary electrophoresis

The chiral separation of halogenated amino acids by ligand-exchange CE is described. Halogenated amino acids attracted increasing interest in recent years because of their physiological activities. Different chiral selectors, as there are L-4-hydroxyproline, L-histidine, and N-alkyl derivatives of L-4-hydroxyproline in form of their copper(II) complexes, are compared for their chiral recognition ability for halogenated amino acids. The influence of various parameters, such as selector concentration, pH, organic modifier, and field strength, on the resolution was investigated. All halogenated amino acids investigated were baseline-separated under optimized conditions.

Analytical approaches to expanding the use of capillary electrophoresis in routine food analysis

Capillary Electrophoresis (CE) is becoming an ever more powerful analytical technique for the separation, identification, and quantification of a wide variety of compounds of interest in many application fields. Particularly in food analysis this technique can offer interesting advantages over chromatographic techniques because of its greater simplicity and efficiency. Nevertheless, CE needs to advance with regard to compatibility with sample matrices, sensitivity, and robustness of the methodologies in order to gain even wider acceptance in food analysis laboratories, specially for routine work. This article presents various approaches to expanding the analytical usefulness of CE in food analysis, discussing their advantages over conventional CE. These approaches focus on sample screening, automated sample preparation with on-line CE arrangements, and the automatic integration of calibration in routine analytical work with CE.

Recent advances in chiral separation principles in capillary electrophoresis and capillary electrochromatography

This review summarizes recent developments in chiral separation in capillary zone electrophoresis (CZE), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC) covering literature published since the year 2000. New chiral selectors and innovative approaches for CE and CEC are introduced. Recent progress in column technology for CEC is highlighted and the development of new chiral stationary phases is discussed. This review is not dedicated to list applications but will focus on new developments.

Polystyrene monolithic column functionalized with copper-iminodiacetate complex as a stationary phase for open tubular capillary electrochromatography

Polystyrene-divinylbenzene copolymer functionalized with copper-iminodiacetate complex was prepared in situ for the capillary electrochromatographic separation of amino acids and oligopeptides. The steps included silanization of the fused-silica capillary column, functionalization, polymerization, hydrolysis and complexation. For having a homogeneous polymerization system in the selected porogen, the functional monomer was obtained from the reaction of 4-vinylbenzyl chloride and diethyl iminodiacetate. Scanning electron microscopy showed that the polymeric monolith column was an open tubular column with a thickness of 0.3 microm. Various modes of hydrolysis were investigated via the electroosmatic flow measurement to find the optimization condition prior to introducing the copper ion. The influence of pH, composition and concentration of mobile phase as well as the organic modifier were investigated. Judging from our results, the recognition performance of the prepared stationary phase might be integrated from the mechanisms of ligand exchange, electrophoretic mobility, hydrophobic interaction, and hydrogen bonding.

Coordination properties of 3-functionalized β-cyclodextrins. Thermodynamic stereoselectivity of copper(ii) complexes of the A,B-diamino derivative and its exploitation in LECE

The bis-amino AB derivative of beta-cyclodextrin on the secondary rim was synthesised and spectroscopically characterised by different techniques. Its binary systems both with protons and copper(II) were thermodynamically characterised by pH-metric potentiometry. In addition the ternary systems with each of the enantiomers of tryptophan and alanine were investigated. A thermodynamic stereoselectivity was observed for the tryptophan enantiomers and this was exploited to separate them by capillary electrophoresis through a ligand exchange mechanism (LECE). LECE separation of racemates of phenylalanine and tyrosine was also obtained.