Interaction Between Asymmetric Solutes and Solvents: N-trifluoroacetyl-L-phenylalanyl-L-leucine Cyclohexyl Ester as Solvent

Abstract

The interaction of a dipeptidic phase, N-trifluoroacetyl-L-phenylalanyl-L-leucine cyclohexyl ester (II), with solutes, consisting of N-trifluoroacetyl (N-TFA) and N-pentafluoropropionyl (N-PFP) esters of α-amino acids, has been studied at 100–130°C. The difference of behaviour, as judged by the separation factors of enantiomers, between (II) and N-TFA-L-valyl-L-valine cyclohexyl ester (I), studied previously, depends on the compound used for comparison. The following regularities were observed: As for (I), resolution improves as the alkyl residue of the ester function of the solute increases from a primary to a secondary to a tertiary group. The alkyl radical attached to the asymmetric carbon of the solute decreases resolution, when substituted in the β-position (with respect to the carboxylic group), while the reverse is true of substitution of the γ-position. Under the same conditions N-PFP derivatives have throughout about 25% lower retention volumes than corresponding N-TFA esters. The significance of the results for the gas chromatographic resolution of the common protein amino acids is discussed.