Analysis of rotenone degradation products by electrospray mass spectrometry

Rotenone is a natural phenolic compound inhibiting the mitochondrial respiratory chain that was formerly used for its insecticidal activity. Its toxicity linked with oxidative stress and dopaminergic neuronal degeneration causes many features of Parkinson's disease in animal models [1] and may also contribute to this disease in man. This compound is found in several plants among which seeds of yam bean (Pachyrhizus sp.) which are also rich in macro- and micronutrients [2]. As rotenone is somewhat unstable, we decided to decrease its initial content of the seeds by thermal degradation. HPLC-HR-(+)ESI-MSⁿ was used to analyse the mixtures formed upon thermal degradation of rotenone, and the MS fragmentation of rotenone and its major degradation products.

First, based on HR-(+)ESI-MS, MS² and MS³ spectra, we examined in detail the fragmentation behavior of rotenone and were able to propose structures for the seven main product ions of the protonated molecule. Then, based on the MS fragmentation behaviors, UV absorption spectra and abundance of the compounds according to pH, we identified the three main degradation products formed when rotenone is heated at 100°C in methanol. The major degradation product was identified as a rotenone epimer and the epimerisation mechanism is also proposed.