π-π Conjugation Enhances Oligostilbene's Antioxidant Capacity: Evidence from α-Viniferin and Caraphenol A

alpha-Viniferin and caraphenol A, the two oligostilbenes, have the sole difference of the presence or absence of an exocyclic double bond at the pi-pi conjugative site. In this study, the antioxidant capacity and relevant mechanisms for alpha-viniferin and caraphenol A were comparatively explored using spectrophotometry, UV-visible spectral analysis, and electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UPLC-ESI-Q-TOF-MS/MS) analysis. The spectrophotometric results suggested that caraphenol A always gave lower IC50 values than alpha-viniferin in cupric ion-reducing antioxidant capacity assay, ferric-reducing antioxidant power assay, 1,1-diphenyl-2-picryl-hydrazl radical (DPPH center dot)-scavenging, and 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide radical-scavenging assays. In UV-visible spectra analysis, caraphenol A was observed to show enhanced peaks at 250-350 nm when mixed with Fe-2(+), but alpha-viniferin exhibited no similar effects. UPLC-ESI-Q-TOF-MS/MS analysis revealed that alpha-viniferin mixed with DPPH center dot produced radical adduct formation (RAF) peak (m/z = 1070-1072). We conclude that the antioxidant action of alpha-viniferin and caraphenol A may involve both redox-mediated mechanisms (especially electron transfer and H+-transfer) and non-redox-mediated mechanisms (including Fe-2(+)-chelating or RAF). The pi-pi conjugation of the exocyclic double bond in caraphenol A can greatly enhance the redox-mediated antioxidant mechanisms and partially promote the Fe-2(+)-chelating mechanism. This makes caraphenol A far superior to alpha-viniferin in total antioxidant levels.