Cyclodextrin inclusion interferes with trolox oxygen radical scavenging capacity measurement

The interference of cyclodextrin solubilization with the measurement of oxygen radical scavenging capacity was investigated. Cyclodextrin (CD) that can solubilize water-insoluble compounds has been used in the oxygen radical scavenging capacity assay called oxygen radical absorbance capacity (ORAC) method. A vitamin E analog, trolox (2-hydroxy-2,5,7,8-tetramethylchroman-2- carboxylic acid) has been employed as a standard compound in ORAC methods and the results were often expressed in the trolox equivalent unit. We found that trolox ORAC values measured with electron spin resonance-based ORAC method, were markedly dependent on the CD concentration, i.e., it decreased by 50% when [CD]/[Trolox]=100 was present. 2D ROESY NMR study of trolox/CD inclusion complex revealed that trolox resided within the CD cavity. The reactive phenoxyl group in trolox is shielded from the attack by the oxygen radical, suggesting that this hindrance was causal for the decrease in ORAC values.