Structural discrimination of isomeric tetrahydrofuran lignan glucosides by tandem mass spectrometry

Due to the possible role in human health, the number of analytical studies on lignans aimed at their quali- and quantitative analysis in plant extracts, biological fluids and foods is continuously increasing. However, helpful systematic mass spectrometric investigations on these compounds are few and rather limited to specific lignan sub-classes. To increase the comprehension of the previously outlined picture of the gas-phase properties of furofuran lignans, we extended the study to tetrahydrofuran lignans and here we reported the collision-activated dissociation (CAD) fragmentation patterns of the alkali metal cation adducts, [M+Alk](+), and [M-H](-) ions of three isomeric tetrahydrofuran lignans, (+)-8'-hydroxylariciresinol 4'-O-beta-D-glucopyranoside (1), (+)-7'-hydroxylariciresinol 7'-O-beta-D-glucopyranoside (2) and 4-O-beta-D-glucopyranosyloxy-3,3'-dimethoxy-7,9'-epoxylignan-5',8',9-triol (3) investigated by electrospray ionization triple quadrupole mass spectrometry (ESI-TQMS). Hydrogen/deuterium (H/D) solution exchange experiments, allowing the selective H/D exchange of all the acidic hydrogen atoms, proved to be a very effective tool to obtain information on the nature of fragments generated during TQ/CAD processes. The [M+Na](+) CAD mass spectra of the three isomeric tetrahydrofurans revealed four different pathways involving the loss of the glucose moiety, which allowed the assignment of the glycosylation site. In the negative ion mode, the main fragmentation channel of the [M-H](-) ions of O-glucosylated lignans at the phenolic oxygen atoms is represented by the loss of 162 Da. When the sugar is bound to a benzylic OH group the loss of the sugar as a 180 Da unit occurs eventually following the loss of a water molecule involving both the C(9)H(2)OH chain and the sugar.