To clarify the role of the B-ring in antioxidant flavonols, we performed a comparative study between galangin with a null B-ring and 3,5,7-trihydroxychromone without a B-ring using five spectrophotometric assays, namely, •O₂--scavenging, 1,1-diphenyl-2-picrylhydrazyl radical (DPPH•)-scavenging, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide radical-scavenging, 2,2'-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) radical-scavenging, and Fe3+-reducing activity. The DPPH•-scavenging reaction products of these assays 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) technology. In the five spectrophotometric assays, galangin and 3,5,7-trihydroxychromone dose-dependently increased their radical-scavenging (or Fe3+-reducing) percentages. However, galangin always gave lower IC50 values than those of 3,5,7-trihydroxychromone. In the UPLC-ESI-Q-TOF-MS/MS analysis, galangin yielded galangin-DPPH adduct MS peaks (m/z 662, 434, 301, 227,196, and 151) and galangin-galangin dimer MS peaks (m/z 538, 385, 268, 239, 211, 195, and 151). 3,5,7-Trihydroxychromone, however, only generated m/z 3,5,7-trihydroxychromone-DPPH adduct MS peaks (m/z 586, 539, 227, 196, and 136). In conclusion, both galangin and 3,5,7-trihydroxychromone could similarly undergo multiple antioxidant pathways, including redox-dependent pathways (such as electron transfer (ET) and ET plus proton transfer (PT)) and a non-redox-dependent radical adduct formation (RAF) pathway; thus, the null B-ring could hardly change their antioxidant pathways. However, it did improve their antioxidant levels in these pathways. Such improvement of the B-ring toward an antioxidant flavonol is associated with its π-π conjugation, which can provide more resonance forms and bonding sites.
Keywords: 3,5,7-trihydroxychromone; B-ring; antioxidant; flavonol; galangin; structure–activity.