Insights into Genipin Dye Mechanisms and Colour Origins

Genipin is a natural compound that forms colours when reacted with primary amines. Curiously, it contains a large amount of uncertainty surrounding its dye structure. It was previously believed that genipin reacts with primary amines to first form a red intermediate dye that polymerizes to form a blue dye when exposed to oxygen. We have determined that the dye colour can be altered by reacting genipin with aromatic amines, producing green dyes instead of blue dyes. In addition, we have found that these dyes are not polymers at all, and their characteristic blue colour is instead the result of a persistent radical, supported by NMR spectroscopy, MS, and EPR spectroscopy. Furthermore, we have shown that the oxidation of the allylic alcohol functionality can form the oxetane derivative via a 2,3-epoxyalcohol rearrangement that can alter the redox properties of the dye, and thereby halting the dye formation at its red intermediate.