5-S-Lipoylhydroxytyrosol and its polysulfides: synthesis and assessment of the antioxidant properties by in vitro and cellular assays

Hydroxytyrosol occupies a prominent position among natural polyphenols because of its antioxidant potency and the wide range of biological properties. Several efforts have therefore been directed toward the preparation of hydroxytyrosol derivatives with improved antioxidant and pharmacological activities and different solubility properties, particularly enhanced lipophilicity. Along this line 5-S-lipoylhydroxytyrosol (1) was prepared by conjugation of hydroxytyrosol with dihydrolipoic acid. The expedite synthetic procedure involves regioselective oxidation of tyrosol with 2-iodoxybenzoic acid to hydroxytyrosol o-quinone, followed by addition of dihydrolipoic acid. Further aim of the study was the preparation of polysulfide derivatives of 1, as polyfunctional compounds combining the potential of the catechol moiety with that of the polysulfide functionality, typically associated to remarkable chemical, biological, and pharmacological properties. Specific conditions to obtain each polysulfide, namely the disulfide 2, the trisulfide 3 and the tetrasulfide 4, were developed relying on a fine tuning of the reaction parameters such as the absence or presence of sulfur in different solvents. All the polysulfides 2-4 were found to have stronger hydrogen donor ability than Trolox in the DPPH assay. In the FRAP assay, 1 exhibited the best reducing activity. All compounds 1-4 acted as efficient hydroxyl radical scavengers at concentration as low as 10 M in a Fenton reaction inhibition assay. The antioxidant activity of compound 1, disulfide 2 and tetrasulfide 4 was also tested in human hepatocarcinoma cell line (HepG2). Direct treatment of cells with the compounds induced significant changes in cellular intrinsic antioxidant status, reducing ROS generation. Moreover, pretreatment of cells with the compounds counteracted cell damage induced by t-BOOH by decreasing ROS generation. All the compounds proved more active than the parent hydroxytyrosol.