Evaluation of enantioselective gas chromatography for the determination of minute deviations from racemic composition of α‐amino acids with emphasis on tyrosine: Accuracy and precision of the method

Abstract

Whereas the determination of high enantiomeric fractions (EF) of chiral compounds is very well established, the accurate determination of small deviations from racemic compositions has not yet received much attention despite its relevance to studies dealing with the origin of homochirality, where only small initial enantiomeric bias is expected. Racemic samples of representative alpha-amino acids were derivatized as N-(O,S)-trifluoroacetyl/ethylesters and analyzed by enantioselective gas chromatography (GC) on fused silica capillaries coated with the chiral stationary phases (CSPs) Chirasil-D-Val, Chirasil-L-Val, and Lipodex E with GC/FID and GC/MS detection. The validation (accuracy and precision) of the determination of the enantiomeric fraction EF of the D-enantiomer in racemic or near-racemic compositions for 10 DL-alpha-amino acids obtained from commercial sources has been carried out. Emphasis is given to DL-tyrosine, the enantiomers of which have recently been claimed to show different crystallization properties. Values of EF obtained from GC measurements using CSPs were compared with those from CE using chiral mobile phase additives. While the precision of the GC method is generally better than 0.08% for all DL-alpha-amino acids studied, accuracy (trueness) of determination of amino acids with polar side chains is poorer than expected from the precision as a result of systematic errors. The accuracy determined relied on measurements on two oppositely configurated CSPs.

Gas chromatographic enantioseparation of derivatized α-amino acids on chiral stationary phases—Past and present

“…The enantiomeric purity of the chiral reagents R-and S-1-amino-2-methoxymethylpyrrolidine (RAMP and SAMP) obtained from Rand S-proline were determined by enantioselective GC on ChirasilVal 4 [140] and on an XE-60-l-valine-(S)-␣-phenylethylamide polymer of König et al [141]. Many applications are related to the topic of ˛-amino acids and the asymmetry of life [142], in astrobiology [143,144] and investigations related to the origin of homochirality involving the validation of minute but important deviations from the true racemic composition (vide infra) [145].…”

Salient Features of Enantioselective Gas Chromatography: The Enantiomeric Differentiation of Chiral Inhalation Anesthetics as a Representative Methodological Case in Point

“…. .when true 1:1 ratios of racemic mixtures, and small deviations therefrom, are quantified with high precision [84]. The mean enantiomeric enrichment (52.1 AE 0.1%) of (+)-(S)-isoflurane observed in vivo in the clinical study on day 2 as compared with the racemic isoflurane controls (50.0 AE 0.1%) was considered to be statistically significant [82].…”

Photochirogenesis: Photochemical Models on the Origin of Biomolecular Homochirality

“…The latter requires several conditions for its use in accurate ee determination investigations including quantitative enantiomeric purity of the derivatization auxiliary, complete absence of racemization of both reactants during derivatization, and identical reaction kinetics of both enantiomers with the chiral agents. Therefore, the use of the achiral derivatization approach combined with the separation on a chiral stationary phase should be applied preferably due to the absence of the above mentioned factors [142].…”