Preparation and antioxidant activity of tyrosol and hydroxytyrosol esters

A series of lipophilic esters of tyrosol and hydroxytyrosol, two naturally occurring phenols with interesting biological properties, have been prepared by an enzymatic synthesis and saponification procedure, using lipase from Candida antarctica as the catalyst. Their antioxidant activities were evaluated by the 2,2-diphenyl-1-picrylhydrazyl (DPPH), human low-density lipoprotein (LDL) oxidation, and DNA strand scission assays. All methods used for evaluating antioxidant activity revealed that tyrosol and its derivatives were less active than hydroxytyrosol and its analogues due to the influence of the catechol structure of the latter. For hydroxytyrosol, the introduction of the lipid moiety decreased its antioxidant activity. Meanwhile, an unsaturation-dependent antioxidant trend was observed for tyrosol and hydroxytyrosol esters in DNA strand scission assay, and for tyrosol esters in DPPH procedure. However, in LDL oxidation assay, possible oxidation of the polyunsaturated fatty acid moiety of tyrosol esters should also be considered.