A kinetic approach in the evaluation of radical-scavenging efficiency of sinapic acid and its derivatives

A kinetic approach was used to determine the radical scavenging activities of sinapic acid and its derivatives: sinapine, 4-vinylsyringol, syringic acid, syringaldehyde, and ethyl, propyl and butyl sinapate. The responses were expressed as rates of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH$^·$) scavenging (R$_S$), superoxide radical (O$_2$$^·$$^−$) scavenging (R$_F$$_F$), and β-carotene bleaching in the emulsion system (R$_B$). For R$_S$ and R$_B$, the esters of sinapic acid showed the highest responses while, for R$_F$$_F$, this was seen for syringic acid. The effectiveness of the selected compounds for scavenging these free radicals was also determined at a fixed endpoint. The early response parameters were demonstrated to be good discriminators in assessing differences for antioxidants with comparable fixed endpoint activity. The primary feature that ranks the kinetic data and the endpoint determinations is interpreted in terms of the mechanisms of the reactions involved in each of the assays conducted.