Direct enantiomeric TLC resolution of dl-penicillamine using (R)-mandelic acid and l-tartaric acid as chiral impregnating reagents and as chiral mobile phase additive

Chiral Materials: Progress, Applications, and Prospects

Chirality is a universal phenomenon in molecular and biological systems, denoting an asymmetric configurational property where an object cannot be superimposed onto its mirror image by any kind of translation or rotation, which is ubiquitous on the scale from neutrinos to spiral galaxies. Chirality plays a very important role in the life system. Many biological molecules in the life body show chirality, such as the "codebook" of the earth's biological diversity-DNA, nucleic acid, etc. Intriguingly, living organisms hierarchically consist of homochiral building blocks, for example, l-amino acids and d-sugars with unknown reason. When molecules with chirality interact with these chiral factors, only one conformation favors the positive development of life, that is, the chiral host environment can only selectively interact with chiral molecules of one of the conformations. The differences in chiral interactions are often manifested by chiral recognition, mutual matching, and interactions with chiral molecules, which means that the stereoselectivity of chiral molecules can produce changes in pharmacodynamics and pathology. Here, the latest investigations are summarized including the construction and applications of chiral materials based on natural small molecules as chiral source, natural biomacromolecules as chiral sources, and the material synthesized by design as a chiral source.

'Ab Ovo' Chiral Phases and Chiral Reagents for Liquid Chromatographic Separation and Isolation of Enantiomers

The de-novo approach of mixing chirally pure reagents or Cu(II)-L-amino acid complexes in the slurry of silica gel for preparing TLC plates was reported from author's laboratory and was successful for separation and isolation of enantiomers. Using high molar absorptivity molecules, e. g., 1,5-difluoro-2,4-dinitrobenzene and cyanuric chloride, more than 38 new chiral derivatizing reagents were synthesized in our laboratory by straightforward nucleophilic substitution with simple chiral auxiliaries. Besides, (S)-naproxen, (S)-ketoprofen, and (S)-levofloxacin were used as chiral platforms. A conceptual approach using both achiral phases in chromatography for enantioseparation was also adopted. ¹ H NMR and DFT based software were used to explain structures of non-covalent and covalent diastereomeric pairs and determination of configuration and separation mechanism. The methods can be easily used to determine and control enantiomeric purity with advantages over a variety of commercial chiral phases.

Analysis and Enantioseparation of Amino Acids by Liquid Chromatography

Enantioseparation studies of proteinogenic, non-proteinogenic, and dansyl amino acids are described herein by using liquid chromatographic techniques, i.e., HPLC and TLC. A researcher who wants to perform amino acid (AA) analysis or separate enantiomers of AAs by HPLC or TLC can follow the method. Figures included represent the actual experiments.Synthesis and application of chiral derivatizing reagents (CDRs) based on cyanuric chloride (CC) and difluorodinitrobenzene (DFDNB) have been described for AA analysis and enantioseparation by indirect approach. The methods represent pre-column derivatization of AAs and represent a good and less expensive substitute of AA analyzer. The application of commercial "Chiralplate" and use of erythromycin and L-tartaric acid have been described as chiral selector either as impregnating reagent in the stationary phase or as an additive in the mobile phase for direct enantioseparation by TLC. Application of the homemade TLC plates has also been described; the methods are successful in obtaining the native enantiomer as well.

Separation and determination of chiral composition in penicillamine tablets by capillary electrophoresis in a broad pH range

A chiral capillary electrophoretic method with nearly full pH window was explored for the separation and determination of dl-penicillamine. A facile one-pot labeling technique was coupled in the method for introduction of chromophore and charge groups onto the analytes to facilitate the electromigration and sensitive detection. By using simply a cost-effective neutral β-cyclodextrin as chiral selector, baseline separation of the dl-penicillamine was achieved from pH 2.0 to over pH 10. Quantification of standard d- and l-penicillamines was demonstrated by taking pH 4.5, 7.4, and 9.7 as the representatives of acidic, neutral, and basic conditions. The working curves were constructed between peak area and concentration, having linear ranges of 8.56-8.56 × 10(2) μg/mL for pH 4.5 and 8.56-1.71 × 10(3) μg/mL for pH 7.4 and 9.7, with correlation coefficients all better than 0.999. The limit of detection (S/N = 3) was 2.58 μg/mL in acidic and neutral conditions or 1.41 μg/mL in basic condition. The method was further validated by assaying the commercial penicillamine tablets, applicable to quantification of the effective enantiomer and the trace impurity of l-penicillamine at a content of down to 0.2, 0.6, and 2.0% for pH 9.7, 4.5, and 7.4, respectively. The recovery determined by spiking technique was in a range from 93.1 to 105 %. The method is easily extendable to the analysis of other chiral amines or amino acids.

Metabolic engineering of the L-phenylalanine pathway in Escherichia coli for the production of S- or R-mandelic acid

Background

Mandelic acid (MA), an important component in pharmaceutical syntheses, is currently produced exclusively via petrochemical processes. Growing concerns over the environment and fossil energy costs have inspired a quest to develop alternative routes to MA using renewable resources. Herein we report the first direct route to optically pure MA from glucose via genetic modification of the L-phenylalanine pathway in E. coli.

Results

The introduction of hydroxymandelate synthase (HmaS) from Amycolatopsis orientalis into E. coli led to a yield of 0.092 g/L S-MA. By combined deletion of competing pathways, further optimization of S-MA production was achieved, and the yield reached 0.74 g/L within 24 h. To produce R-MA, hydroxymandelate oxidase (Hmo) from Streptomyces coelicolor and D-mandelate dehydrogenase (DMD) from Rhodotorula graminis were co-expressed in an S-MA-producing strain, and the resulting strain was capable of producing 0.68 g/L R-MA. Finally, phenylpyruvate feeding experiments suggest that HmaS is a potential bottleneck to further improvement in yields.

Conclusions

We have constructed E. coli strains that successfully accomplished the production of S- and R-MA directly from glucose. Our work provides the first example of the completely fermentative production of S- and R-MA from renewable feedstock.

Improved spectrofluorimetric methods for determination of penicillamine in capsules

Two simple, sensitive and specific fluorimetric methods have been developed for the determination of Penicillamine (PNC), a sulphur containing compound. Method (I) involves the reaction of PNC with 2′,7′-bis(acetoxymercuri)-fluorescein (AMF) in the presence of Kolthoff’s buffer, pH 8.2, with subsequent measurement of fluorescence spectra at 520 nm (λEx 497 nm). Method (II) is based on PNC being oxidized into penicillaminic acid using Cerium (IV) in an acidic medium. Method sensitivity has been improved using sodium triphosphate which enhances the luminescence intensity of Ce(III). Fluorescence spectra were then measured at 348 nm (λEx 293 nm). The reaction conditions and the fluorescence spectral properties have been investigated for both methods. Under the described conditions, the proposed methods were applicable over the concentration ranges 0.0048 − 0.0288 µg mL−1 and 0.096 − 0.288 µg mL−1 with mean percentage recoveries 99.95 ± 1.29 and 100.04 ± 1.10 for methods I and II, respectively. The proposed methods were validated in terms of accuracy, precision, LOD and LOQ and robustness and then were successfully applied to the determination of PNC in bulk powder and in capsules as well as in the presence of the related disulphide. The results obtained were determined to be in good agreement with those obtained using a previously reported method.

Chiral separations

The main goal of this review is to provide a brief overview of chiral separations to researchers who are versed in the area of analytical separations but unfamiliar with chiral separations. To researchers who are not familiar with this area, there is currently a bewildering array of commercially available chiral columns, chiral derivatizing reagents, and chiral selectors for approaches that span the range of analytical separation platforms (e.g., high-performance liquid chromatography, gas chromatography, supercritical-fluid chromatography, and capillary electrophoresis). This review begins with a brief discussion of chirality before examining the general strategies and commonalities among all of the chiral separation techniques. Rather than exhaustively listing all the chiral selectors and applications, this review highlights significant issues and differences between chiral and achiral separations, providing salient examples from specific classes of chiral selectors where appropriate.