Mass spectrometric profiling of valepotriates possessing various acyloxy groups from Valeriana jatamansi

Valepotriates, plant secondary metabolites of the family Valerianaceae, contain various acyloxy group linkages to the valepotriate nucleus and exhibit significant biological activities. Identification of valepotriates is important to uncover potential lead compounds for the development of new sedative and antitumor drugs. However, making their structure elucidation by nuclear magnetic resonance (NMR) experiments is too difficult to be realized because of the overlapped carbonyl carbon signals of acyloxy groups substituted at different positions. Thus, the mass spectrometric profiling of these compounds in positive ion mode was developed to unveil the exact linkage of acyloxy group and the core of valepotriate. In this study, electrospray ionization tandem multistage mass spectrometry (ESI-MS/MS(n)) in ion trap and collision-induced dissociation tandem MS were used to investigate the fragmentation pathways of four types of valepotriates in Valeriana jatamansi, including 5-hydroxy-5,6-dihydrovaltrate hydrin (5-hydroxy-5,6-dihydrovaltrate chlorohydrin), 5,6-dihydrovaltrate hydrin (5,6-dihydrovaltrate chlorohydrin), 5-hydroxy-5,6-dihydrovaltrate and valtrate hydrin (valtrate chlorohydrin). The high-resolution mass spectrum (HRMS) data of all the investigated valepotriates from quadrupole time-of-flight MS/MS were used as a supportive of the fragmentation rules we hypothesized from ion-trap stepwise MS(n). As a result, the loss sequence of acyloxy groups and the abundance of key product ions, in combination with the characteristic product ions corresponding to the valepotriate nucleus, could readily differentiate the four different types of valepotriates. The summarized fragmentation rules were also successfully exploited for the structural characterization of three new trace valepotriates from V. jatamansi. The results indicated that the developed analytical method could be employed as a rapid, effective technique for structural characterization of valepotriates, especially for the trace compounds that could not be identified by NMR techniques. This study may also arouse interest for further structural analysis of other valepotriate-containing type herbal medicines.