Modification of Natural Proanthocyanidin Oligomers and Polymers Via Chemical Oxidation under Alkaline Conditions

Abstract

We tested the susceptibility of 102 proanthocyanidin (PA)-rich plant extracts to oxidation under alkaline conditions and the possibility to produce chemically modified PAs via oxidation. Both the nonoxidized and the oxidized extracts were analyzed using group-specific ultrahigh-performance liquid chromatography–diode array detection–tandem mass spectrometry (UHPLC–DAD–MS/MS) methods capable of detecting procyanidin (PC) and prodelphinidin (PD) moieties along the two-dimensional (2D) chromatographic fingerprints of plant PAs. The results indicated different reactivities for PCs and PDs. When detected by UHPLC–DAD only, most of the PC-rich samples exhibited only a subtle change in their PA content, but the UHPLC–MS/MS quantitation showed that the decrease in the PC content varied by 0–100%. The main reaction route was concluded to be intramolecular. The PD-rich and galloylated PAs showed a different pattern with high reductions in the original PA content by both ultraviolet (UV) and MS/MS quantitation, accompanied by the shifted retention times of the chromatographic PA humps. In these samples, both intra- and intermolecular reactions were indicated.

Characterization of Natural and Alkaline-Oxidized Proanthocyanidins in Plant Extracts by Ultrahigh-Resolution UHPLC-MS/MS

“…For example, galloylated (epi)catechin at m/z 441, galloylated dimeric PC at m/z 729, and galloylated trimeric PC at m/z 1017 exhibit well-separated peaks in extracted ion chromatograms (EICs) in Figure 2C-E, and they can be easily detected based on their UV and mass spectra, including exact masses and the corresponding molecular formula. The results obtained here were consistent with previous results obtained by MS/MS, showing that Ruprechtia salicifolia leaves contained 24 mg/g of PAs, of which only 3% were PD-containing PAs, some of them were galloylated, and the mDP was found to be 7 [13].…”

“…We selected 55 PA-rich plant samples containing interesting but complex mixtures of natural PAs based on our previous study [13]. These plant samples were similarly oxidized by aerial oxidation under alkaline conditions as in [13], and after the oxidation, PAs were even more complex. These natural and modified PAs were studied here by UHPLC-DAD connected to ultrahigh-resolution Q-orbitrap MS/MS in order to detect the sophisticated changes in PA structures.…”

“…The result suggested that plant PAs could be modified at high pH, and the presence of PD groups significantly enhanced the probability of modification reactions. The main reaction route was concluded to be intramolecular, but for PD-rich and galloylated PAs, both intra-and intermolecular reactions were indicated [13]. In addition, other studies have shown that oxidation of PAs includes both intramolecular and intermolecular reactions [14,15,24,25].…”

“…Instead, the behavior of PA mixtures in the plant extracts is not known, and this is most probably due to difficulties in the analyzing of structurally different initial PAs, which is challenged even further after oxidation. In our previous study, we tested whether different types of natural PAs could be chemically modified to produce new types of PAs and studied the effects of nonspecific alkaline oxidation mimicking the often-used alkaline extraction process for bark waste [19][20][21][22][23] on 102 PA-containing plant extracts [13]. The results indicated different reactivities for PCs and PDs.…”

“…However, even waste or byproducts from handling and processing fruits, vegetables and forest resources from the above listed industrial applications could be a potential source for natural PAs [8][9][10][11]. The PAs could also be further modified in order to enhance their usability and bioactivities [12,13].…”