Analysis of problems encountered in the determination of amino acid enantiomeric ratios by gas chromatography.

A previously described procedure for determining the enantiomeric ratios of amino acids has produced inconsistent results when determining relatively low (less than or equal to 0.110) D/L ratios. The method involves synthesis of diastereomeric N-trifluoroacetyl-L-prolyl-D/L-amino acid ester dipeptides which are resolved by gas chromatography (GC). We have found that triethylamine, which is added to maintain a basic pH during the coupling reaction, racemizes the chiral reagent N-trifluoroacetyl-L-prolyl chloride (TPC). Coupling of partially racemized TPC to D/L-amino acid esters results in the formation of four dipeptides (two pairs of enantiomers) instead of the expected two diastereomeric dipeptides. The enantiomeric dipeptides coelute on an achiral GC column, resulting in erroneous D/L ratios. More accurate D/L ratios are obtained by preparing the volatile N-trifluoroacetyl-D/L-amino acid isopropyl ester derivative which can be separated into its enantiomers on a chiral GC column such as the Chirasil-Val III (registered trademark of Applied Science Laboratories).