A Fragmentation Study on Four Unusual Secoiridoid Trimers, Swerilactones H-K, by Electrospray Tandem Mass Spectrometry.

Swerilactones H-K (1-4) as four unprecedented secoiridoid trimers represent a new type of natural product, which has attracted much interest of natural chemists due to their novel skeletons and promising bioactivity. In order to well understand their MS fragmentation behaviors, they were investigated by electrospray ionization ion-trap time-of-flight multistage product ion mass spectrometry (ESI-IT-TOF-MS (n) ) for the first time. The protonated molecules ([M+H](+)) of swerilactones J and K, and deprotonated molecules ([M-H](-)) of swerilactones H, J and K were readily observed in the conventional single-stage mass spectra (MS); however only the [M+Cl](-) ion for swerilactone I was obtained in negative mode. Based on the MS (n) study, the fragmentation pathways of swerilactones H and I in negative mode, and swerilactones J and K in both positive and negative modes were proposed. The neutral losses of H2O, CO, CO2 and C2H4O moieties are the particular elimination from the precursor ions due to the presence of hydroxyl, δ-lactone and 1-O-ethyl moieties in their structures, of which the retro-Diels-Alder cleavage was the most particular dissociation. The fragment ions at m/z 341 and 291 in negative mode can be considered as the diagnostic ions for secoiridoid trimers. This investigation will provide valuable information for their fast characterization from complicated natural mixtures and extensive understanding their structural architectures.