The dried plant of Tadehagi triquetrum is called Hulu Tea in traditional Chinese medicinal (TCM). The study attempted to analyze the antioxidant products of the interaction with cellular reactive oxygen species. LAEH, a lyophilized aqueous extract of Hulu Tea, was prepared and analyzed using high-performance liquid chromatography. Eight phytophenols were found, namely rutin, isoquercitrin, p-coumaric acid, p-hydroxybenzoic acid (pHBA), kaempferol, protocatechuic acid, afzelin, and daidzein. The antioxidant levels of the phytophenols were evaluated using 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO•) trapping, 1,1-diphenyl-2-picryl-hydrazl (DPPH•) trapping, Fe3+-reducing, and Cu2+-reducing assays. The reaction products of DPPH• with phytophenol were further analyzed by ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UPLC-ESI-Q-TOF-MS/MS). A series of molecular ion and fragment peaks were observed, which corresponded to seven phytophenol-DPPH adducts, four phytophenol-phytophenol homodimers, and one p-coumaric acid-pHBA heterodimer. Seven possible heterodimer linking sites were further calculated by computational chemistry based on density functional theory with the methanol solvation model at the B3LYP (or UB3LYP)-D3/6-311+G(d,p) level. However, the 2,2’-linking heterodimer possessed the lowest enthalpy change (∆H) value (-69.54 kcal/mol); the 2,2’-bond presented a Laplacian bond order value (1.083) and 148.9-pm bond length. In conclusion, eight antioxidant phytophenols can produce three types of antioxidant products: phytophenol-radical adduct, phytophenol-phytophenol homodimer, and phytophenol-phytophenol heterodimer. The phytophenol-phytophenol heterodimer refers to the p-coumaric acid-pHBA 2,2’-linking heterodimer rather than to the 2-8’ benzofuran p-coumaric acid/pHBA heterodimer.
