HPLC enantiomeric resolution of phenyl isothiocyanated amino acids on teicoplanin-bonded phase using an acetonitrile-based mobile phase: A structural consideration

A variety of alpha-amino acids are enantioresolved on a teicoplanin bonded chiral phase, using the acetonitrile-based mobile phase after their pre-column derivatization with phenyl isothiocyanate (PHES) in alkaline medium. The resolution is considered to be much better, as compared to that for a given amino acid in N-benzoylated or N-carbobenzyloxylated form under the same chromatographic conditions, and found reversed in the elution order as the amino acid is benzyl isothiocyanated. The resolution is sensitive to the size of the analyte, and enhanced due to the re-location of the hydrogen receptor site from sulfur to nitrogen on the isothiocyanyl fragment of the derivatizing reagent, which in turn alters the selectivity The stereogenic center of analyte nearing the aromatic moiety of the tagging reagent is important as well. The resolution is either not observed, or unsatisfactory, in the reversed- or normal phase mode for most of the phenyl isothiocyanated amino acids examined in this study. Finally, the enantiomer of amino acids is found resistant to racermizeation after being phenyl isothiocyanated at room temperature.