The mass spectral analysis of isolated hops A-type proanthocyanidins by electrospray ionization tandem mass spectrometry

Comprehensive mass spectral fragmentation patterns have been established for sequencing chromatographically isolated A-type proanthocyanidins (PAs) using electrospray ionization tandem mass spectrometry (ESI-MSⁿ) in the positive ion mode similar to those used for sequencing previously reported B-type PAs. Sequence-identifying fragmentations for A-type PAs include heterocyclic ring fission (HRF), retro-Diels-Alder (RDA) fission, benzofuran-forming (BFF) fission, and quinone methide (QM) fission. There is commonality in fragmentation patterns between A-type and B-type PAs, but distinguishing features in the mass spectral patterns between the two classes include 2-Da mass differences in the pseudo molecular ions, the propensity for the A-type PAs to undergo QM fissions and yield bis-quinoid ions as opposed to mono-quinoid ions in the upper unit of the sequence, and the reluctance of A-type linkages to undergo RDA, BFF, and BFF/H₂O fissions from the upper unit. The positions of one or more A-type (C2 →O→C7′) ether linkages have been located in sequences of PAs ranging in chain lengths of two to five monomer units using ESI-MSⁿ data. Using the fragmentation information from ESI-MSⁿ experiments, a total of 17 PAs were structurally sequenced by systematic real time ESI-MSⁿ. Among them ten A-type and six B-type hop PAs are reported here for the first time.