Structures and reactions of compounds involved in pink discolouration of onion

► Cepathiolanes and valine formed a pigment precursor involved in pinking of onion. ► The pigment precursors and formaldehyde yielded two colourless and two pink compounds. ► They consisted of two to three Pigment Precursor molecules linked via formaldehyde. ► The trimeric pigment structure should be the first to be reported in the literature. In “pinking” of onion, E-(+)-S-(1-propenyl)-l-cysteine sulfoxide is first cleaved by alliinase to yield colour developers (CDs), which react with amino acids, such as valine, to form pigment precursors (PPs). The PPs react with naturally occurring carbonyls (NOCs) to form pigments. By inducing a PP from previously isolated cepathiolanes and l-valine, it was confirmed that cepathiolanes constitute at least a part of the CDs. From the PP and formaldehyde as a NOC, two colourless and two pink compounds were derived. The structure of one of the colourless compounds was established as 2-(2-(1-(1-carboxy-2-methylpropyl)-3,4-dimethyl-1H-pyrrol-2-yl)methyl-3,4-dimethyl-1H-pyrrol-1-yl)-3-methylbutanoic acid. The structures of the other colourless compound and the pink pigments were predicted based on their molecular formula and the MSn spectral data. A trimeric pigment structure was predicted for one of the pink pigments, which was believed to be the first to be reported in the literature. With these, a new reaction scheme for “pinking” of onion is proposed.