Structural studies on stereoisomerism of a natural dye miraxanthin I

Chemical structure of a yellow dye present in Mirabilis jalapa L., miraxanthin I, was characterized by NMR spectroscopy. The extract of M. jalapa was analysed by a high-performance liquid chromatographic system (LC-DAD-FL-ESI-MS/MS) and the presence of miraxanthin I among other betaxanthins was confirmed. This dye was synthesized from previously generated methionine-betaxanthin by oxidation with 10% H2O2. E/Z stereoisomers of miraxanthin I were found by NMR analysis to occur in a 50:33:10:7 ratio in aqueous solution (7E,9E : 7Z,9E : 7E,9Z : 7Z,9Z configurations, respectively). Comparison of NMR data with chemical shifts obtained from quantum chemical calculations strongly suggests a presence of intramolecular hydrogen bonds, that may favour a more rigid structure of the dye. This explains the highest fluorescence quantum yield among betaxanthins. Observed changes in 1H NMR spectra during the measurements indicate on miraxanthin I hydrolysis for which a mechanism is proposed. The first step during the hydrolysis process is protonation of the nitrogen atom within the central bridge bond system which is observed in the ZE stereoisomer by NMR.