Gas—liquid chromatography of amino acid derivatives: The effect of substituents

Abstract

The effect of the substituent groups in amino acid derivatives on their behavior in gas—liquid chromatography has been further investigated. As in our previous studies on glycine derivatives, in this work leucine and alanine were used as standard units. Acyl esters of leucine and alanine have been synthesized and chromatographed on both polar and non-polar columns at isothermal temperatures. The additive effects of the substituent groups on the N and C terminals of the amino acid were confirmed in the leucine series, which showed that the sum of the slopes of N-acylleucine methyl esters and of N-acetylleucine alkyl esters equaled the slope of N-acylleucine alkyl esters in graphs of the logarithm of the retention time versus the number of carbon atoms in the substituent. The same effects were observed for the alanine series.In variations of the substituent R of the amino acid with the substitutions at N and C terminals of the amino acid derivatives, the sum of the slopes of the three monosubstituted acylamino acid esters: acylalanine methyl esters, acetylalanine alkyl esters and acetylamino acid methyl esters, equals the slope of the trisubstituted acylamino acid alkyl esters. The result appears to confirm further the additive effect of the substituting groups of amino acid derivatives.An alternative expression of the effect of the substituent groups is the differential of the logarithm of the retention times of any two adjacent members of a series, log Cx+1 − log Cx = ΔC, where ΔC is a constant. ΔC is affected little by the nature of the column or by other factors under the experimental conditions used, with the exception of the column temperature.