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.

Department of Biophysics Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University tř 17 listopadu 12 771 46 Olomouc Czech Republic

Department of Biophysics Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University tř 17 listopadu 12 771 46 Olomouc Czech Republic; INSERM UMR 850 Univ Limoges School of Pharmacy 2 rue du Docteur Marcland 87025 Limoges France

Department of Medical Chemistry and Biochemistry Faculty of Medicine and Dentistry Palacký University Hněvotínská 3 775 15 Olomouc Czech Republic

Faculty of Physical Chemistry University of Belgrade Studentski trg 12 16 11158 Belgrade Serbia

INSERM UMR 850 Univ Limoges School of Pharmacy 2 rue du Docteur Marcland 87025 Limoges France; Department of Physical Chemistry Regional Centre of Advanced Technologies and Materials Faculty of Science Palacký University tř 17 listopadu 12 771 46 Olomouc Czech Republic

Institute of Microbiology Laboratory of Biotransformation Czech Academy of Sciences Vídeňská 1083 142 20 Prague Czech Republic

J Heyrovský Institute of Physical Chemistry Czech Academy of Sciences Dolejškova 3 182 23 Prague 8 Czech Republic

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