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,
•O
2−-scavenging, 1,1-diphenyl-2-picrylhydrazyl radical (DPPH
•
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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,
•O
2−-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 Fe
3+-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 Fe
3+-reducing) percentages. However, galangin always gave lower IC
50 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.
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