Recognition of enantiomers by Chirasil-Val and oligopeptide analogues as studied by gas-phase calorimetry and 1H NMR spectroscopy in solution

A series of oligopeptides, L-Valn-NHtBu (n = 1-4), were linked to poly(beta-methyl)siloxy-alpha-methylpropanoic acid copolymer; the resulting chirally modified polydimethylsiloxanes were used for the resolution of enantiomers by gas chromatography. Chiral recognition proved most effective for n = 1 (known as Chirasil-Val), as judged from the resolution factors (alpha) and the thermodynamic parameters delta delta H, delta delta S and chi = delta delta H/delta H', where delta H' = delta HChir -delta HSE30. Likewise, 1H NMR spectroscopy in carbon tetrachloride solution revealed a maximum chemical shift non-equivalence of the amide N-H signal of racemic N-TFA-amino acid methyl esters on addition of the chiral polymer, for n = 1. From circular dichroism spectroscopy of L-Valn-NH'Bu (n = 2, 3, 4 and 6) and polyoxyethylene-bound pivaloyl-L-Valn-Gly-NH-POE3000 (n = 2-8), it is concluded that the peptide moiety of the stationary phase mostly adopts the unfavourable random coil conformation, whereas the beta-sheet structure was only partially found and only for n > or = 6.

Resolution of stereoisomers of dipeptides by gas chromatography on Chirasil-Val

Several dipeptides have been separated into stereoisomers (enantiomers, diastereomers, sequential isomers) by gas chromatography on Chirasil-Val. Sufficiently volatile N-trifluoroacetyl dipeptide methyl esters were formed at ambient temperature, thus avoiding racemization (or epimerization, respectively, and cleavage of the peptide bond. This method is suitable for ascertaining the stereoisomeric composition of dipeptides in biological samples.

Direct enantiomeric resolution of mephenytoin and its N-demethylated metabolite in plasma and blood using chiral capillary gas chromatography

A gas chromatographic method was developed for the determination of the R- and S- enantiomers of the anticonvulsant, mephenytoin, and its N-demethylated metabolite, 5-phenyl-5- ethylhydantoin ( PEH ), in plasma and blood. Direct enantiomeric separation of mephenytoin and its internal standard was obtained using a chiral capillary column ( Chirasil -Val) followed by nitrogen specific detection. However, resolution of the enantiomers of PEH and its internal standard required propylation at the 3-position of the hydantoin ring prior to analysis. Similar linear and reproducible standard curves were obtained from both plasma and blood over the concentration range 50 ng/ml to 5 micrograms/ml, and above 100 ng/ml the reproducibility was less than 8% (coefficient of variation). Pronounced stereoselective differences in the plasma concentration--time curves for both mephenytoin and PEH were observed in a normal subject who received a single oral dose of 300 mg racemic mephenytoin. The peak plasma level of S-mephenytoin was only one-fifth that of the R-enantiomer and its elimination half-life was less than 3 h compared to over 70 h for R-mephenytoin. Similarly, S- PEH levels were barely detectable whereas concentrations of R-metabolite steadily increased over 4-6 days before slowly declining.

Interaction between asymmetric solutes and solvents. Diamides derived from L-valine as stationary phases in gas-liquid partition chromatography

The resolution of enantiomers of (+/-)-a-amino acid derivatives on optically active diamides derived from L-valine, R1CONHCH[CH(CH3)2]CONHR2, as gas chromatographic stationary phases was studied. The effects of the structure of R1 and R2 in the stationary phases, the structure of the racemic a-amino acid derivatives, temperature, etc., on the separation factors are reported. In the light of these and related results, a more probable mechanism of resolution is presented.

Present status of enantiomeric analysis by gas chromatography

The development of selective chiral solute-solvent systems showing separation of enantiomers by gas chromatography is reviewed. The enantiomeric analysis of alpha-amino-acids and its application to various problems, including notably the analysis of meteorites, is discussed. Some recent results with the highly efficient diamide phases, which permit resolution on packed columns, are presented. Some new ideas on the mechanism of resolution are briefly mentioned.

Chromatographic resolution of enantiomers selective review

Recent research has produced a notable increase in knowledge useful for chromatographic enantiomer resolution. Some major advances have emerged in the understanding of asymmetric solute-solvent interactions, and many successful separations by gas and by liquid chromatography have been reported. This review presents a selective discussion of the major advances, with primary emphasis on the use of chiral stationary phases. The latter offer advantages (over indirect techniques) such as generally easier sample preparation, decreased analysis time, and simultaneous chemical as well as optical purity analysis.

Gas chromatography of amino acids

This review summarizes all papers that have appeared on the gas chromatography of amino acids (including the iodoamino acids) and their enantiomers in the period 1956-mid-1974. It has been found that the methods used for analysis of amino acids can be divided into three classes: (1) degradative procedures and techniques for converting the amino acid into another chemical compound; (2) procedures based on esterification of the carboxyl group and derivatization of the a-amino and other reactive groups in at least two steps; and (3) procedures based on a simultaneous derivatization of the carboxyl and a-amino groups in one reaction medium. For the treatment of the amino acid or its alkyl ester, three approaches can be distinguished for the two latter cases, i.e., acylation, alkylation (including silylation) and condensation. Of the procedures used for the resolution of optical antipodes, two methods are discussed, namely analysis of diastereoisomers on optically inactive stationary phases and separation of enantiomers on optically active stationary phases.

Gas chromatographic resolution of enantiomeric amphetamines and related amines. I. Structural effects on some diastereomer separations

Eight structurally related enantiomeric primary amines including several amphetamines were resolved as distereomers by gas chromatography. Optically active acylated amino acid chlorides were reacted with the amine racemates to yield volative diastereomers which were chromatographed on polar and on nonpolar columns. Selectivity of diastereomeric pair separations was found to be dependent on substitution at the chiral centers of both the amino acids and the amines. Baseline resolution was achieved on a 6-ft. diethylene glycol succinate column at 200 degrees for seven of the eight amines. The polar stationary phases yielded higher relative separation factors than the nonpolar phase in all cases for the same chromatographic conditions. The effects of varying the perfluoroacyl group of the amino acid resolving agent from trifluoroacetyl to pentafluoropropionyl to heptafluorobutyryl was to significantly decrease the retention times without significantly decreasing these separation factors.

A technique for the determination of enantiomeric amino acids in biological samples

Racemic amino acids can be separated into their enantiomers by means of gas-liquid chromatography. The most applied technique, today, is the conversion of chiral compunds into diastereoisomers with optically active reagents and subsequent chromatography on conventional optically inactive stationary phases. In previous studies it has been realized that this technique is associated with various problems. We studied the use of optically active stationary phases for separating enantiomers directly via a diastereoisomeric association complex. The optically acitve stationary phases employed are N- and C-terminal substituted dipeptides of the type N-trifluoroacetyl-dipeptide-cyclohexyl esters and have been synthesised by the I-hydroxibenztriazole dicyclohexylcarbodiimide method. The quality of these phases with respect to separation factors, resolution factors, and thermodynamical properties have been evaluated. All synthetic phases show excellent properties; however, when attempting separation of mixtures of naturally occurring amino acids extensive overlap in the elution diagram was detected. Only one phase - N-TFA-L-chi-amino-n-butyryl-L-chi-amino butyric acid cyclohexyl ester - gave complete resolution of the naturally occurring amino acids alanine, valine, glycine, threonine, leucine, isoleucine, serine and proline on a 400 ft x 0.02 in capillary column. Less volatile amino acids such as aspartic acid, phenylalanine, methionine, glutamic acid, tyrosine, arginine, and tryptophan can be resolved at a 100 ft x 0.02 in column.