Amino acid enantiomers, chiral

As separation materials, the obvious advantage offered by MIPs is a relatively straightforward and predetermined selectivity. Based on molecular imprinting, some difficult separations, particularly enantiomer separations, have been solved. Amino acid enantiomers, chiral dipeptide, and racemic naproxen are examples. In particular, molecular imprinting is probably currently the only choice where no suitable biomolecule is available. When coupling with appropriate transducers such as electro-, photo- or magnetochemical transducers, MIPs may be applied as the monitors in several sys-... 

The principle of this method depends on the formation of a reversible diastereomeric complex between amino acid enantiomers and chiral addends, by coordination to metal, hydrogen bonding, or ion—ion mutual action, in the presence of metal ion if necessary. L-Proline (60), T.-phenylalanine (61),... 

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

One of the most useful applications of chiral derivatization chromatography is the quantification of free amino acid enantiomers. Using this indirect method, it is possible to quantify very small amounts of enantiomeric amino acids in parallel and in highly complex natural matrices. While direct determination of free amino acids is in itself not trivial, direct methods often fail completely when the enantiomeric ratio of amino acid from protein hydrolysis must be monitored in complex matrices. 

S Einarsson, B Josefsson, P Moller, D Sanchez. Separation of amino acid enantiomers and chiral amines using precolumn derivatization with (+)-l-(9-fluorenyl)ethyl chlo-roformate and reversed phase liquid chromatography. Anal Chem 59, 1191, 1987. 

Ilisz, I., Berkecz, R., and Peter, A., HPLC separation of amino acid enantiomers and small peptides on macrocyclic antibiotic-based chiral stationary phases a review, J. Sep. ScL, 29, 1305, 2006. 

Peyrin, E. et al., Dansyl amino acid enantiomer separation on a teicoplanin chiral stationary phase effect of eluent pH, J. Chromatogr. A, 923, 37, 2001. 

Amino acid enantiomers can be separated on a chiral stationary phase after derivatization with chloroformates (Abe et al., 1996). The derivatization procedure is quite simple and rapid, but the derivatizing reagent must be synthesized, which complicates the assay. Another method for the analysis of amino acid enantiomers uses N,0-pentafluoropropionyl isopropyl derivatives and a chiral column with NPD detection (Hashimoto et al., 1992). 

Abe I, Fujimoto N, Nishiyama T, Terada K, Nakahara T. 1996. Rapid analysis of amino acid enantiomers by chiral-phase capillary gas chromatography. J Chromatogr A 722 221. Ahuja S. 1976. Derivatization in gas chromatography. J Pharm Sci 65 163. 

Analysis using a CMPA is usually resolved on a nonchiral column. A transient diastereomeric complex is formed between the enantiomer and the chiral component in the mobile phase, similar to the complexes formed with chiral stationary phases. A review by Liu and Liu (2002) cites several papers where addition of CPMAs has been used in analyzing amphetamine-related compounds. Some CPMAs include amino acid enantiomers, metal ions, proteins, and cyclodextrins. Advantages of this method of analysis include the use of less expensive columns and more flexibility in the optimization of chiral separation (Misl anova and Hutta, 2003). 

OPA in combination with chiral thiols is one method used to determine amino acid enantiomers. A highly fluorescent diastereomeric isoindole is formed and can be separated on a reverse-phase column. Some of these chiral thiols include N-acetyl-L-cysteine (NAC), N-tert-butyloxy-carbonyl- L-cysteine (Boc-L-Cys), N-isobutyryl- L-cysteine (IBLC), and N-isobutyryl- D -cysteine (IBDC). Replacing OPA-IBLC with OPA-IBDC causes a reversal in the elution order of the derivatives of D- and L-amino acids on an ODS column (Hamase et al., 2002). Nimura and colleagues (2003) developed a novel, optically active thiol compound, N-(tert-butylthiocarbamoyl)- L-cysteine ethyl ester (BTCC). This reagent was applied to the measurement of D-Asp with a detection limit of approximately 1 pmol, even in the presence of large quantities of L-ASP. 

CSPs and chiral mobile phase additives have also been used in the separation of amino acid enantiomers. Another technique that should be mentioned is an analysis system employing column-switching. D-and L- amino acids are first isolated as the racemic mixture by reverse-phase HPLC. The isolated fractions are introduced to a second column (a CSP or a mobile phase containing a chiral selector) for separation of enantiomers. Long et al. (2001) applied this technique to the determination of D- and L-Asp in cell culture medium, within cells and in rat blood. 

Three approaches can be employed to separate peptide stereoisomers and amino acid enantiomers separations on chiral columns, separations on achiral stationary phases with mobile phases containing chiral selectors, and precolumn derivatization with chiral agents [111]. Cyclodextrins are most often used for the preparation of chiral columns and as chiral selectors in mobile phases. Macrocyclic antibiotics have also been used as chiral selectors [126]. Very recently, Ilsz et al. [127] reviewed HPLC separation of small peptides and amino acids on macrocyclic antibiotic-based chiral stationary phases. 

Novel methods for reversed-phase, pressurized liquid-chromatographic resolution of nonesterified amino acid enantiomers by the formation of diastereoisomers using two chiral reagents, namely, 2,3,4,6-tetra-0-acetyl-/3-D-glucopyranosyl isothiocyanate (2) and 2,3,4-tri-O-acetyl-a-D-arabinopyranosyl isothiocyanate (30), have been reported.96,97... 

H Brucker, M Langer, M Lupke, T Westhauser, H Godel. Liquid chromatographic determination of amino acid enantiomers by derivatization with o-phthalaldehyde and chiral thiols. Applications with reference to food science. J Chromatogr A 697 229-245, 1995. 

N Nimura, T. Kinoshita. o-Phthalaldehyde-A -acetyl-t.-cystcine as a chiral derivatization reagent for liquid chromatographic optical resolution of amino acid enantiomers and its application to conventional amino acid analysis. J Chromatogr 352 169-177, 1986. 

A. S. Bommarius, K. Drauz, U. Groeger, and C. Wandrey, 1992, Membrane bioreactors for the production of enantiomer ically pure a-Amino acids, in Chirality in Industry, A. N. Collins, G. N. Sheldrake, and J. Crosby (eds.), Wiley Sons Ltd., London, Chapter 20, pp. 371-397. 

Frank H, G. J. Nicholson and E. Bayer, Rapid gas chromatographic separation of amino acid enantiomers with a novel chiral stationary phase , J. Chromatogr. Sci. 15 174-176(1977). 

Chirality of derivatized cyclodextrin was used for recognition of stereoisomers. Phenylazobenzoyl modified y-cyclodextrin was anchored onto silica gel used as stationary phase in HPLC and photoresponsive chromatographic behavior of dansyl amino acid enantiomers was studied [64],... 

Liu, J. Volk, K. J. Mata, M. J. Kems, E. H. Lee, M. S. 1997. Analysis of amino acid enantiomers derived from antitumor antibiotics using chiral capillary electrophoresis. J. Pharm. Biomed. Anal., 15,1729-1739. 

Analytical Properties Separation of amino acid enantiomers good chiral recognition of A/-acetyl amino acid methyl esters depends on hydrogen bond interactions hexane with isopropanol modifier has been used as the liquid phase usually prepared on LiChrosorb (10 pm)... 

Chiral separation of FITC-labeled amino acid enantiomers was performed on a glass chip using fluorescent detection. Analysis time ranged from 75 s for the most basic amino acids to 160 s for the most acidic ones. y-CD was used as the chiral selector [627]. Chiral separation of amino acids in extraterrestrial samples or meteorites were also performed [610,628],... 

R. Wernicke, Separation of underivatized amino acid enantiomers by means of a chiral solvent-generated phase, J. Chromatogr., 318 117(1985). 

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