Antioxidant Mechanisms of Echinatin and Licochalcone A

Abstract

Echinatin and its 1,1-dimethyl-2-propenyl derivative licochalcone A are two chalcones found in the Chinese herbal medicine Gancao. First, their antioxidant mechanisms were investigated using four sets of colorimetric measurements in this study. Three sets were performed in aqueous solution, namely Cu2+-reduction, Fe3+-reduction, and 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide radical (PTIO•)-scavenging measurements, while 1,1-diphenyl-2-picrylhydrazyl radical (DPPH•)-scavenging colorimetric measurements were conducted in methanol solution. The four sets of measurements showed that the radical-scavenging (or metal-reduction) percentages for both echinatin and licochalcone A increased dose-dependently. However, echinatin always gave higher IC50 values than licochalcone A. Further, each product of the reactions of the chalcones with DPPH• was determined using electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UPLC-ESI-Q-TOF-MS/MS). The UPLC-ESI-Q-TOF-MS/MS determination for echinatin yielded several echinatin–DPPH adduct peaks (m/z 662, 226, and 196) and dimeric echinatin peaks (m/z 538, 417, and 297). Similarly, that for licochalcone A yielded licochalcone A-DPPH adduct peaks (m/z 730, 226, and 196) and dimeric licochalcone A peaks (m/z 674 and 553). Finally, the above experimental data were analyzed using mass spectrometry data analysis techniques, resonance theory, and ionization constant calculations. It was concluded that, (i) in aqueous solution, both echinatin and licochalcone A may undergo an electron transfer (ET) and a proton transfer (PT) to cause the antioxidant action. In addition, (ii) in alcoholic solution, hydrogen atom transfer (HAT) antioxidant mechanisms may also occur for both. HAT may preferably occur at the 4-OH, rather than the 4′-OH. Accordingly, the oxygen at the 4-position participates in radical adduct formation (RAF). Lastly, (iii) the 1,1-dimethyl-2-propenyl substituent improves the antioxidant action in both aqueous and alcoholic solutions.

Simultaneous Study of Anti-Ferroptosis and Antioxidant Mechanisms of Butein and (S)-Butin

“…The experimental value of two hydrogen atoms (2.0124) had only 0.16% relative deviation from the calculated value (2.01565) [30], suggesting that there were two HAT reactions. On the basis of these experimental data and previous reports [62,63], a HAT reaction may firstly occur at the 4-OH of butein ( Figure 4A) [42,64]. Via the HAT reaction, butein is transformed to the butein-4-O-radical.…”

“…Loss of the exocyclic double bond and loss of molecular planarity reduces the region of π-π conjugation. From resonance theory, saturation of the exocyclic α,β-double bond prevents stabilization of the free radical intermediate after the HAT reaction [62]. We previously demonstrated that these changes greatly decrease the antioxidant level [87,88].…”

“…The DPPH • probe, however, was recently suggested to have some limitations [35]. Nevertheless, DPPH • determination has successfully accumulated a great deal of evidence in computational [58,59] and experimental models [60][61][62]. More importantly, two tested-samples butein and (S)-butin could not be dissolved in aqueous solution.…”

Antioxidant Change in Biosynthesis from Naringenin Chalcone to Flavonoid Apingenin

“…After the propagation stage, NAC‐3‐C • and DPPH • radicals were hypothesized to couple with each other to give the NAC‐DPPH adduct (I) via an RAF pathway. (I) may be further transformed into (II) via keto‐enol tautomerism to restore the aromaticity of the B‐ ring ,. Since neither (I) nor (II) is a free radical (Figure D), it is confirmed that the free radicals have been scavenged and the chain reaction is virtually terminated by the RAF reaction ,.…”

“…Our previous studies have revealed the following: (1) the DPPH • ‐scavenging reaction preferably proceeds via hydrogen atom abstraction, where one DPPH • radical accepts one hydrogen atom to form a DPPH−H molecule;, (2) in the chalcone antioxidant reaction, the 4‐OH group may donate a hydrogen atom prior to the other ‐OH's . On the basis of these two findings, the NAC molecule was hypothesized to transform into an NAC‐4‐O • intermediate radical via hydrogen atom abstraction (Figure A).…”