IR ion spectroscopy in a combined approach with MS/MS and IM-MS to discriminate epimeric anthocyanin glycosides (cyanidin 3-O-glucoside and -galactoside)

Abstract

Anthocyanins are widespread in plants and flowers, being responsible for their different colouring. Two representative members of this family have been selected, cyanidin 3-O-β-glucopyranoside and 3-O-β-galactopyranoside, and probed by mass spectrometry based methods, testing their performance in discriminating between the two epimers. The native anthocyanins, delivered into the gas phase by electrospray ionization, display a comparable drift time in ion mobility mass spectrometry (IM-MS) and a common fragment, corresponding to loss of the sugar moiety, in their collision induced dissociation (CID) pattern. However, the IR multiple photon dissociation (IRMPD) spectra in the fingerprint range show a feature particularly evident in the case of the glucoside. This signature is used to identify the presence of cyanidin 3-O-β-glucopyranoside in a natural extract of pomegranate. In an effort to increase any differentiation between the two epimers, aluminum complexes were prepared and sampled for elemental composition by FT-ICR-MS. CID experiments now display an extensive fragmentation pattern, showing few product ions peculiar to each species. More noteworthy is the IRMPD behavior in the OH stretching range showing significant differences in the spectra of the two epimers. DFT calculations allow to interpret the observed distinct bands due to a varied network of hydrogen bonding and relative conformer stability.