Identification of Procyanidins in the Pericarps of Different Litchi Cultivars and Their in Vitro Antioxidant Activities

To screen eminent litchi varieties suitable for the development of natural antioxidant products, the total phenol, total proanthocyanidin, total flavonoid contents and A-type oligomeric procyanidin fractions in the pericarps of seven diverse litchi cultivars were analyzed. The in vitro antioxidant activities including DPPH, ABTS⁺ free radical scavenging and FRAP were determined. The results demonstrated that the total phenols, total proanthocyanidins, total flavonoids as well as total A-type oligomeric procyanidins contained in the pericarp extract of 'Hemaoli' were the highest. In addition, the primary and secondary mass spectra of the contained substances were analyzed and identified by electrospray liquid-mass spectrometry with quadrupole time-of-flight mass spectrometry (HPLC-Q-TOF-MS/MS). The mass spectrometry results demonstrated that the oligomeric procyanidin fractions contained in litchi pericarps were mainly epicatechin, procyanidin A₂, procyanidin A₄, A-type trimer and tetramer. Moreover, the content of A-type oligomeric procyanidins in litchi pericarp varied significantly (P<0.05) among diverse cultivars. The pericarp extract of 'Hemaoli' possessed potent free radical scavenging activity (DPPH∙: 1144.60 μmol Trolox/g DW, ABTS⁺∙: 2905.56 μmol Trolox/g DW, FRAP: 868.84 μmol Trolox/g DW). The contents of A-type procyanidin monomer, dimer and trimer were significantly correlated with antioxidant activity by Pearson's correlation analysis (P<0.05). In summary, a comprehensive analysis indicated that the pericarp extract of 'Hemaoli' contained the highest content of A-type oligomeric procyanidins as well as remarkable antioxidant activity compared to other cultivars. Therefore, 'Hemaoli' could be utilized as a novel potential material for the development of natural antioxidant products.