Separation of amino acid enantiomers by gas chromatography



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Twenty-two pairs of DL-isomers belonging to the class of N-trifluoroacetyl amino acid esters have been studied with respect to the separation of enantiomers by gas chromatography. The optically active phase was N-trifluoroacetyl-L-valyl-L-valine-O-cyclohexyl ester and was coated on stainless steel capillary columns having the following dimensions: 200' x 0.02" and 500' x 0.02". The temperature was 110°C and helium was used as the carrier gas at pressures of 30 or 40 psi. The compounds included in the present work are the methyl, isopropyl, n-l-butyl, and 3-pentyl derivatives of serine, threonine, aspartic acid, glutamic acid, phenylalanine, and methionine. All of the derivatives are resolvable to some extent. The nature of the ester group affects strongly both the resolution factors and the retention times. It was found that retention time increased in the order Me < iso Pr < n-bu < 3-pentyl, and that the resolution factor increased in the order Me < Bu < iso Pr < 3-pentyl. The nature of the substituents at the [alpha]-carbon also influences resolution so that structural correlations may be made. A trend was observed indicating that increase of bulkiness up to a certain optimal size favored separation.