Enantioseparation of d- and l-amino acids by a coupled system consisting of an ion-exchange column and a chiral column and determination of d-aspartic acid and d-glutamic acid in soy products

A coupled system, consisting of a conventional ion-exchange column and a chiral column, was used for separation of the d- and l-enantiomers as constituents of an amino acid mixture. The ion-exchange column was connected to an amino acid analyser and the eluents necessary for the chiral column were delivered by an HPLC pump. With this on-line system the d- and l-forms of eight of the proteinogenic amino acids were separated. An alternative method was also investigated, in which single amino acids or groups of amino acids were fed to the chiral column via a three-way divider, in order to subject the column to less aggressive conditions. By using aspartic acid and/or glutamic acid as an index of racemization and, if necessary, increasing the number of peaks investigated, a simple and fast method for the determination of the degree of racemization was achieved. The method was applied to three soy products. The products contain 3.4–5.8% d-aspartic acid and 1.4–2.4% d-glutamic acid