Structure-resolution relationship. I. The effect of the alkylamide side-chain of aspartyl derivatives on the resolution of amino acid enantiomers

The resolution of amino acid enantiomers in liquid chromatography using chiral eluents is examined in light of the three "points" association model. The resolution is found to be a function of the alkyl chain length of the resolving agent, which is an alkyl amide derivative of aspartic acid complexed with copper(II). For example, mobile phases with l-aspartylethylamide copper(II) chiral reagent gave lower resolution than with l-aspartyl-n-hexylamide copper(II). The pH also influences the resolution. In general, k′ values and selectivity factors increase with increasing pH. A mobile phase containing a derivative of glutamic acid could not resolve enantiomers. Analysis of the results shows that the three "points" contact is between the α-carboxy and α-amino groups of the amino acid and the copper(II) and the side-chain of the amino acid and the alkylamide moiety of the aspartyl derivative.