Abstract
In this study, we analyzed the proanthocyanidin (PA) composition of 55 plant extracts before and after alkaline oxidation by ultrahigh-resolution UHPLC-MS/MS. We characterized the natural PA structures in detail and studied the sophisticated changes in the modified PA structures and the typical patterns and models of reactions within different PA classes due to the oxidation. The natural PAs were A- and B-type PCs, PDs and PC/PD mixtures. In addition, we detected galloylated PAs. B-type PCs in different plant extracts were rather stable and showed no or minor modification due to the alkaline oxidation. For some samples, we detected the intramolecular reactions of PCs producing A-type ether linkages. A-type PCs were also rather stable with no or minor modification, but in some plants, the formation of additional ether linkages was detected. PAs containing PD units were more reactive. After alkaline oxidation, these PAs or their oxidation products were no longer detected by MS even though a different type and/or delayed PA hump was still detected by UV at 280 nm. Galloylated PAs were rather stable under alkaline oxidation if they were PC-based, but we detected the intramolecular conversion from B-type to A-type. Galloylated PDs were more reactive and reacted similarly to nongalloylated PDs.
Highlights
Teresa Escribano-BailónProanthocyanidins (PAs, syn. condensed tannins) are oligomers and polymers consisting of flavan-3-ol monomeric units (Figure 1)
Connected to ultrahigh-resolution Q-orbitrap MS/MS in order to detect the sophisticated changes in PA structures
We propose that the ions at m/z 451 and 411 could correspond to the heterocyclic ring fission (HRF) of the heterocyclic C-ring of the middle flavan-3-ol unit supporting the location of A-type linkage between the extension units according to Figure 5
Summary
Teresa Escribano-BailónProanthocyanidins (PAs, syn. condensed tannins) are oligomers and polymers consisting of flavan-3-ol monomeric units (Figure 1). Condensed tannins) are oligomers and polymers consisting of flavan-3-ol monomeric units (Figure 1). The diversity of PA structures derives mainly from their hydroxylation patterns, the sequential order of the flavan-3-ol units and the degree of polymerization (DP), in addition to differences in stereochemistry at C2 and. C3 and variation in the location and stereochemistry of interflavanoid bonds [1]. The PA structures presented in the figure have the C4→C8 linkages. PAs can be linked by C4→C6 bonds, and we cannot separate these two linkages from each other by mass spectrometry. PAs are the most commonly available subgroup of plant tannins, responsible for nearly 90% of the world’s overall market for industrial tannins (>multiple 100-kilo tons annually) and are chemically and economically more appealing as a bio-polymer [2,3,4,5]
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