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.

Figure 1

Dimeric PA, prodelphinidin B3, consisting of the gallocatechin extension unit and the catechin terminal unit.

Figure 2

Examples of the UHPLC–DAD profiles (λ = 280 nm) before and after the oxidation of PA containing plant samples from the four categories: (A) nonmodified PAs without a clear loss of PA concentration in the Heritiera solomonensis leaves (51), (B) nonmodified PAs with a clear loss of PA concentration in Coffea arabica leaves (82), (C) modified PAs without a clear loss of PA concentration in the Newtonia buchananii leaflets (46), and(D) modified PAs with a clear loss of PA concentration in Aeonium arboreum flowers (17). The upper panels show the nonoxidized samples, and the lower panels show the oxidized samples. Sample numbers refer to Table 1.

Figure 3

Examples of UHPLC–DAD–MS/MS fingerprints of the studied plant samples in categories A and B before and after oxidation obtained using the Engström method. The solid red lines are PC units detected before oxidation, red dashed lines are PC units detected after oxidation, solid blue lines are PD units detected before oxidation, and blue dashed lines are PD units detected after oxidation. Shown plant species are as follows: (A) Mandevilla splendens leaves (2), (B) Osteomeles schweriniae flowers (80), (C) Pavonia cauliflora flowers (52), (D) Alpinia purpurata leaves (101), (E) Polystichum proliferum leaves (33), (F) Camellia japonica leaves (93), (G) Encephalartos ferox leaflets (98), (H) Aglaonema crispum leaves (4), (I) Coffea arabica leaves (82), (J) Acca sellowiana leaves (56), (K) Psidium cattleianum leaves (60), and(L) Pelargonium odoratissimum leaves (48). The sample numbers refer to Table 1.

Figure 4

Some examples of the modified UHPLC–DAD–MS/MS fingerprints of polymeric PA extracts before and after oxidation and their combined patterns obtained using the Engström method. The solid red lines are the nonoxidized PC units, the red dashed lines are the oxidized PC units, the solid blue lines are the nonoxidized prodelphinidin (PD) units, and the blue dashed lines are the oxidized PD units. Shown plant species are as follows: (A) Callisia gentlei leaves (14), (B) Acacia melanoxylon leaflets (40), (C) Newtonia buchananii leaflets (46), (D) Nepenthes maxima pitcher (63), (E) Cyclamen africanum leaves (74), (F) Nephelium connatum leaves (87), (G) Aeonium arboretum flowers (17), (H) Kalanchoë manginii flowers (20), (I) Tetraclinis articulate pieces (27), (J) Diospyros mespiliformis leaves (34), (K) Cissus javana leaves (95), and(L) Leea guineense leaves (96) . The sample numbers refer to Table 1.

Figure 5

UHPLC–MS/MS galloyl fingerprints obtained before (red line) and after (blue line) oxidation using the Engström method for (A) Acacia melanoxylon leaflets (40), (B) Newtonia buchananii leaflets (46), (C) Nepenthes maxima pitcher (63), (D) Aeonium arboretum flowers (17), (E) Kalanchoë manginii flowers (20), (F) Diospyros mespiliformis leaves (34), (G) Cissus javana leaves (95), and (H) Leea guineense leaves (96). The sample numbers refer to Table 1.