Development and Validation of an Improved Oxygen Radical Absorbance Capacity Assay Using Fluorescein as the Fluorescent Probe

Abstract
An improved method of oxygen radical absorbance capacity (ORAC) assay has been developed and validated using fluorescein (3‘,6‘-dihydroxyspiro[isobenzofuran-1[3H],9‘[9H]-xanthen]-3-one) as the fluorescent probe. Our results demonstrate that fluorescein (FL) is superior to B-phycoerythrin. The oxidized FL products induced by peroxyl radical were identified by LC/MS, and the reaction mechanism was determined to follow a classic hydrogen atom transfer mechanism. In addition, methodological and mechanistic comparison of ORACFL with other widely used methods was discussed. It is concluded that, unlike other popular methods, the improved ORACFL assay provides a direct measure of hydrophilic chain-breaking antioxidant capacity against peroxyl radical. Keywords: Fluorescein; ORAC; TEAC; FRAP; chain-breaking antioxidant; free radical; hydrogen atom transfer; single electron transfer