Structural elucidation and antioxidant activities of proanthocyanidins from Chinese bayberry (Myrica rubra Sieb. et Zucc.) leaves

Abstract

Proanthocyanidins in Chinese bayberry leaves (PCBLs) were qualitatively analyzed. NMR data suggest that PCBLs are mostly composed of (epi)gallocatechin gallate units. Matrix-assisted laser desorption time-of-flight MS data indicate 95 possible prodelphinidin structures, ranging from dimers to tridecamers. Preparative normal-phase HPLC and further analysis by reverse-phase HPLC together with electrospray ionization MS enabled detection of 20 compounds, including seven newly identified compounds in Chinese bayberry leaves. The antioxidant capacity of PCBLs was evaluated by (1,1-diphenyl-2-picryl-hydrazyl), ferric-reducing antioxidant power, and oxygen radical absorption capacity assays. The EC50 of DPPH radical scavenging activities (as 50% decrease in the initial DPPH concentration) were 7.60 µg. The FRAP and ORAC values were 8859.33 ± 978.39 and 12991.61 ± 1553.34 µmol Trolox equivalents per gram, respectively. The results indicate the high antioxidant potency of PCBLs.

Figure 1. Basic structure of proanthocyanidins.

R1, R2, R3 = H, propelargonidins; R1, R3 = H, R2 = OH, procyanidins; R1, R2 = OH, R3 = H, prodelphinidins; R3 = galloyl, galloylated proanthocyanidins.

Figure 2. ¹³C NMR spectrum of Chinese bayberry leafs proanthocyanidins.

Samples were dissolved in methanol-d ₄.

Figure 3. Normal-phase HPLC chromatograms and negative and positive ESI-MS spectra of Chinese bayberry leaf proanthocyanidins.

N1 to N10 referred to fractions collected from preparative normal-phase HPLC according to the collecting time in Table 3.

Figure 4. Retro-Diels–Alder (RDA) fragmentation pattern of gallocatechin.

Figure 5. Reversed-phase HPLC chromatograms of fractions collected from preparative normal-phase HPLC.

N1 to N6 referred to fractions collected from preparative normal-phase HPLC according to the collecting time in Table 3.