LC-MS and NMR characterization of the purple chromophore formed in the o-aminobenzaldehyde assay of dihydrodipicolinate synthase

The enzyme dihydrodipicolinate synthase (DHDPS) has been widely investigated as a target for new antibiotics. The o-aminobenzaldehyde (o-ABA) assay is routinely used as a highly specific, if qualitative, tool for DHDPS purification, whereby fractions containing active DHDPS appear purple upon addition of o-ABA. The purple adduct absorbs in the visible region (540 nm) but has never been characterized in the 50 years since it was first reported. Structural characterization of this purple compound has been performed by UV spectrophotometry, NMR spectroscopy and tandem mass spectrometry. The extinction coefficient of this chromophore was also determined.