Identification and characterization of proanthocyanidins of 16 members of the Rhododendron genus (Ericaceae) by tandem LC–MS

The proanthocyanidins of the leaves of 16 taxa of the Rhododendron genus (Ericaceae) [Rhododendron ‘Catawbiense Grandiflorum’, Rhododendron ‘Cunningham's White’, Rhododendron smirnowii Trautv., Rhododendron calophytum Franch., Rhododendron dichroanthum ssp. scyphocalyx (Balf. f. & Forrest ) Cowan, Rhododendron micranthum Turcz., Rhododendron praevernum Hutch., Rhododendron ungernii Trautv., Rhododendron kaempferi Planch., Rhododendron degronianum ssp. heptamerum var. hondoense ( Nakai ) H. Hara, Rhododendron fortunei Lindl., Rhododendron ponticum L., Rhododendron galactinum Balf. f. ex Tagg., Rhododendron oreotrephes W. W. Sm., Rhododendron brachycarpum ssp. brachycarpum D. Don ex G. Don, and Rhododendron insigne Hemsl. & E. H. Wilson ] were investigated qualitatively by liquid chromatography–mass spectrometry in series. Twenty‐nine dimeric proanthocyanidins based on (epi)catechin and (epi)gallocatechin were detected and characterized on the basis of their unique fragmentation pattern in the negative ion mode tandem mass spectrometry spectra. All of them were extracted for the first time from these sources, and ten of them were not reported previously in nature. The position of the galloyl residue was assigned on the basis of the retro‐Diels–Alder fragmentation and the dehydrated retro‐Diels–Alder fragmentation; it resulted from the loss of gallic acid as a neutral loss in the negative ion mode. Furthermore, four caffeoylquinic acids, six p‐coumaroylquinic acids, epigallocatechin, gallocatechin, catechin, epicatechin, epigallocatechin gallate, catechin gallate, epicatechin gallate, gallocatechin gallate, two quercetin‐O‐hexosides, quercetin‐O‐galloyl‐hexoside, quercetin‐O‐pentoside, quercetin‐O‐rhamnoside, quercetin‐O‐pentoside‐O‐hexoside, quercetin‐O‐rhamnoside‐O‐hexoside, quercetin‐O‐feruloyl‐hexoside, quercetin‐O‐(p‐hydroxy)benzoyl‐hexoside, taxifolin‐O‐pentoside, myricetin‐O‐rhamnoside, two myricetin‐O‐pentosides, three myricetin‐O‐hexosides, and two myricetin‐O‐galloyl‐hexosides were detected and shown to possess characteristic tandem mass spectrometry spectra and were tentatively assigned on the basis of their retention time. Copyright © 2012 John Wiley & Sons, Ltd.