Enantioseparation of selected N-tert.-butyloxycarbonyl amino acids in high-performance liquid chromatography and capillary electrophoresis with a teicoplanin chiral selector

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 separation of intact amino acids by capillary electrophoresis-mass spectrometry employing a partial filling technique with a crown ether carboxylic acid

An enantiomeric separation method for underivatized free amino acids (AAs) using a partial filling technique with CE-MS was developed for the determination of D-AAs in vinegars. A typical chiral separation method was performed with different concentrations of (18-crown-6)-2,3,11,12-tetracarboxylic acid (18C6H4) dissolved in water or formic acid as the background electrolyte. Seventeen AAs, excluding proline and asparagine, were separated, showing chiral resolution values (Rs) ranging from 0.5 to 21.0. These results included baseline separations of 11 AAs, the peaks of which were observed as the ions [AA+18C6H4+H]⁺. The migration order of the chiral AAs was also evaluated, and the L-AAs migrated faster than the counterpart D-AAs except for serine, threonine and methionine when using (+)-18C6H4. To reduce contamination of the ESI source by the nonvolatile chiral selector and improve the ionization efficiency in partial filling technique, the separation zone length was adjusted to 70% of the capillary, which was filled with 30 mM 18C6H4 in water. This method showed a similar separation efficiency as the typical method, and the separated AA peaks were observed as free AA ions, [AA+H]⁺. The optimized method provided limits of detection (LODs) ranging from 0.07 to 1.03 μg/mL and good linearity (R² > 0.99) up to 50 μg/mL for DL-AAs. The developed method was utilized to determine DL-AAs in vinegars with a simple pretreatment process. It may be extended to sensitive AA analysis in the determination of minor enantiomeric impurities in the major component.

Development of a Fourier transform infrared spectroscopy coupled to UV-Visible analysis technique for aminosides and glycopeptides quantitation in antibiotic locks

Antibiotic Lock technique maintains catheters' sterility in high-risk patients with long-term parenteral nutrition. In our institution, vancomycin, teicoplanin, amikacin and gentamicin locks are prepared in the pharmaceutical department. In order to insure patient safety and to comply to regulatory requirements, antibiotic locks are submitted to qualitative and quantitative assays prior to their release. The aim of this study was to develop an alternative quantitation technique for each of these 4 antibiotics, using a Fourier transform infrared (FTIR) coupled to UV-Visible spectroscopy and to compare results to HPLC or Immunochemistry assays. Prevalidation studies permitted to assess spectroscopic conditions used for antibiotic locks quantitation: FTIR/UV combinations were used for amikacin (1091-1115cm(-1) and 208-224nm), vancomycin (1222-1240cm(-1) and 276-280nm), and teicoplanin (1226-1230cm(-1) and 278-282nm). Gentamicin was quantified with FTIR only (1045-1169cm(-1) and 2715-2850cm(-1)) due to interferences in UV domain of parabens, preservatives present in the commercial brand used to prepare locks. For all AL, the method was linear (R(2)=0.996 to 0.999), accurate, repeatable (intraday RSD%: from 2.9 to 7.1% and inter-days RSD%: 2.9 to 5.1%) and precise. Compared to the reference methods, the FTIR/UV method appeared tightly correlated (Pearson factor: 97.4 to 99.9%) and did not show significant difference in recovery determinations. We developed a new simple reliable analysis technique for antibiotics quantitation in locks using an original association of FTIR and UV analysis, allowing a short time analysis to identify and quantify the studied antibiotics.

Effective enantiomeric separations of racemic primary amines by the isopropyl carbamate-cyclofructan6 chiral stationary phase

A new chiral stationary phase (CSP) was developed by bonding isopropyl-carbamate functionalized cyclofructan6 (IP-CF6) to the silica gel. It was evaluated by injecting 119 racemic primary amine-containing compounds. This CSP showed pronounced enantioselectivity toward all types of primary amines, separating 93% of all tested compounds. Baseline separation was achieved even for some simple aliphatic racemic amines that contained no other functionality. The polar organic mode was shown to be the effective mobile phase owing to higher efficiency. This new chiral stationary phase showed great potential for preparative-scale separations. It is also interesting that the chiral selector, R-naphthylethyl-carbamate functionalized CF6 (RN-CF6), was found to provide complementary selectivity for the relatively few amine analytes that did not separate on IP-CF6. Thus between the two CSPs, 98% of attempted amine compounds were separated.

Enantiorecognition of profens by capillary electrophoresis using a novel chiral selector eremomycin

The evaluation of a macrocyclic glycopeptide antibiotic, eremomycin, as a chiral selector in capillary electrophoresis (CE) has been performed. The stability of eremomycin in solution and capillary electrolyte, as well as its optical and electrophoretic properties have been discussed. The effect of experimental parameters influencing the enantioseparation of several profens has been studied. Excellent enantioseparation of profens has been achieved and migration order has been validated. Comparison of enantioseparations of profens in CE by using eremomycin-mediated electrolytes and in HPLC with eremomycin immobilized on silica has revealed similar trends for both methods.

HPLC separation of amino acid enantiomers and small peptides on macrocyclic antibiotic-based chiral stationary phases: a review

The search for new and effective chiral selectors capable of separating a wide variety of enantiomeric compounds is an ongoing process. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of HPLC, TLC and electrophoresis. More chiral analytes have been resolved through the use of glycopeptides than with all the other macrocyclic antibiotics combined (ansamycins, thiostrepton, aminoglycosides, etc.). The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. Teicoplanin, vancomycin, their analogs and ristocetin A seem to be the most useful glycopeptide HPLC bonded phases for the enantioseparation of proteins and unusal native and derivatized amino acids. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these antibiotics and their application in the enantioseparations of amino acids. The mechanism of separation, the sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.

Enantiomeric separation using an l-RNA aptamer as chiral additive in partial-filling capillary electrophoresis

In this paper, we report the chiral resolution of arginine using an anti-arginine l-RNA aptamer chiral selector in partial-filling CE. The effects of the capillary temperature, sample load, and aptamer plug length on the enantiomeric separation were assessed. Very high chiral resolving capability was observed at low or moderate capillary temperatures (the target peak being not detected in the separation window), whereas the practical chiral resolution was achieved only at high enough temperatures (50-60 degrees C). Over this high-temperature range, the electrophoretic behavior of the target enantiomer appeared to result from a combination of binding site heterogeneity, slow desorption kinetics, and concentration overload of aptamer binding sites. From additional thermal UV melting experiments, three RNA conformations were identified for the 50-60 degrees C temperatures. It was suggested that the presence of these different RNA conformations was a plausible source of the binding site heterogeneity.

Enantioselective high-performance liquid chromatographic separation of N-methyloxycarbonyl unsaturated amino acids on macrocyclic glycopeptide stationary phases

This paper describes the enantiomeric resolution of a series of unsaturated N-methyloxycarbonyl-alpha-H-alpha-amino acids (N-MOC-alpha-amino acids) on macrocyclic glycopeptide stationary phases by means of high-performance liquid chromatography (HPLC). Three types of glycopeptide phases, i.e. Chirobiotic T, V and R, were evaluated in both reversed-phase (RP) and polar ionic mode (PIM). The best results in terms of enantioselectivity and resolution were obtained on Chirobiotic R phase, with the PIM mobile phase giving the highest resolution per min. Investigation of the pH of the reversed-phase mobile phase in the pH range 4.1-5.9 showed little effect on enantioselectivity. The method was applied for monitoring the conversion and product enantiomeric excess of an enzymatic hydrolysis reaction using N-MOC-alpha-H-alpha-amino acid esters as substrate.

Comparison of vancomycin-based stationary phases with different chiral selector coverage for enantioselective separation of selected drugs in high-performance liquid chromatography

Two vancomycin-based chiral stationary phases (CSPs) with different coverage of the chiral selector vancomycin (Chirobiotic V and Chirobiotic V2) were compared. beta-Blockers and profens, as structurally diverse groups of drugs, were chosen as analytes. Retention and enantioseparation of beta-blockers were studied in reversed-phase (RP) and polar-organic (PO) separation modes. Higher retention and better enantioresolution were obtained on the CSP with higher coverage of vancomycin in the both separation modes. Baseline separation of four beta-blockers (eight enantiomers) in the PO mode was achieved on the Chirobiotic V2 column within 15 min. The enantioseparation of profens did not bring so excellent and easy to interpret results. Higher retention of profens on the Chirobiotic V2 column was not always accompanied by an improvement of their chiral separation in the RP mode. The polar-organic mode was not suitable for these derivatives at all. The most interesting result was obtained with flobufen; its chiral center is further away from the rigid part of the molecule, which mostly causes difficulties in enantioselective recognition. Nevertheless, the enantiomers of flobufen were shown to be much better (baseline) resolved on the CSP with lower coverage of the chiral selector (Chirobiotic V).

Study of tryptophan enantiomer binding to a teicoplanin-based stationary phase using the perturbation technique. Investigation of the role of sodium perchlorate in solute retention and enantioselectivity

The retention of D,L-tryptophan enantiomers on an immobilized teicoplanin column was investigated in relation to the mobile phase sodium perchlorate concentration using the perturbation method to determine the solute distribution isotherms. From the experimental data, it appeared that the bi-Langmuir model was able to describe D- and L-enantiomer retention on the immobilized selector over the salt concentration range. An increase in the apparent enantioselectivity with an increase in sodium perchlorate concentration was observed. The chiral recognition enhancement was governed by (i) an increase in the difference of the adsorption constants for binding to the high-affinity site (aglycone pocket) between the two enantiomers and (ii) enhancement of the number of aglycone chiral regions interacting with D-tryptophan. It is suggested that an ion-pair formation mechanism between perchlorate and solute and/or selector is responsible for this behavior. In addition, this work shows that additional secondary sites on the teicoplanin surface are involved in the apparent enantioselectivity at low sodium perchlorate concentrations.

Separation and control of the elution order of N-t-butyloxycarbonyl amino acids D/L isomers by reversed-phase HPLC using cyclodextrins as chiral selectors for the mobile phase

The enantiomeric resolution of N-t-butyloxycarbonyl (N-t-Boc) amino acids D/L isomers by reversed-phase HPLC was investigated using cyclodextrins (CD's) as chiral selectors for the mobile phase. The use of a low pH (pH<4) for the mobile phase enabled the enantioseparation of N-t-Boc amino acids. The opposite elution order of D/L isomers was observed when hydroxypropyl-derivatized beta-CD was used instead of native beta-CD. A computer simulation of the enantioseparation showed that the ratio of the retention factors of the chiral selector and the sample determined the elution order and the resolution. When the retention factor of the chiral selector is smaller than that of the sample, an isomer having larger complex formation constant eluted faster. However, when the chiral selector had a larger retention factor than the sample, an opposite elution order of the isomers was obtained. The large difference in the retention factors between the chiral selector and the sample led to good enantiomeric separation.