A thorough study was done to test the reactivity of the Folin-Ciocalteu (F-C) reagent towards various compound classes. Over 80 compounds were tested. Compound classes included phenols, thiols, vitamins, amino acids, proteins, nucleotide bases, unsaturated fatty acids, carbohydrates, organic acids, inorganic ions, metal complexes, aldehydes and ketones. All phenols, proteins and thiols tested were reactive towards the reagent. Many vitamin derivatives were also reactive, as were the inorganic ions Fe+2, Mn+2, I− and SO3−2. Other compounds showing reactivity included the nucleotide base guanine and the trioses glyceraldehyde and dihydroxyacetone. Copper complexation enhanced the reactivity of salicylate derivatives towards the reagent while zinc complexation did not. Several amino acids and sugars which were reported to be reactive towards the F-C reagent in earlier studies were found not to be reactive in our study, at least in the concentrations used. Reaction kinetics of each compound with the F-C reagent were also measured. Most compounds tested showed a biphasic kinetic pattern with half lives under one minute. Trolox and ascorbic acid displayed a rapid monophasic pattern in which the reaction reached endpoint within one minute. In summary, our study has shown that the F-C reagent is significantly reactive towards other compounds besides phenols. As other investigators have suggested, the F-C assay should be seen as a measure of total antioxidant capacity rather than phenolic content. Since phenolics are the most abundant antioxidants in most plants, it gives a rough approximation of total phenolic content in most cases.

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