The protective constituents of
silymarin, an extract from Silybum marianum fruits, have been extensively studied in terms of their
antioxidant and hepatoprotective activities. Here, we explore the electron-donor properties of the major
silymarin flavonolignans.
Silybin (SB),
silychristin (SCH),
silydianin (SD) and their respective 2,3-dehydroderivatives (DHSB, DHSCH and DHSD) were oxidized electrochemically and their antiradical/
antioxidant properties were investigated. Namely,
Folin-Ciocalteau reduction, DPPH and
ABTS(+) radical scavenging, inhibition of microsomal lipid peroxidation and cytoprotective effects against
tert-butyl hydroperoxide-induced damage to a human
hepatocellular carcinoma HepG2 cell line were evaluated. Due to the presence of the highly reactive C3-OH group and the C-2,3 double bond (ring C) allowing electron delocalization across the whole structure in the 2,3-dehydroderivatives, these compounds are much more easily oxidized than the corresponding
flavonolignans SB, SCH and SD. This finding was unequivocally confirmed not only by experimental approaches, but also by density functional theory (DFT) calculations. The hierarchy in terms of ability to undergo electrochemical oxidation (DHSCH~DHSD>DHSB>>SCH/SD>SB) was consistent with their antiradical activities, mainly DPPH scavenging, as well as in vitro cytoprotection of HepG2 cells. The results are discussed in the context of the
antioxidant vs. prooxidant activities of
flavonolignans and molecular interactions in complex
biological systems.