Abstract
An alternative spectrophotometric ferric reducing activity power (FRAP) method for quantification of total reducing capacity (TRC) was developed. The method is based on the reduction of FeIII to FeII by antioxidant compounds containing 2,2’-bipyridine (bipy) in aqueous solution. Absorbance values recorded at 521 nm, characteristic of the Fe(bipy)32+ complex formed, were used to determine the TRC of some plants-derived beverages. For the teas samples, the TRC values obtained with the proposed method and cupric reducing antioxidant capacity (CUPRAC) reagent had an excellent agreement (adjusted correlation coefficient (r2) = 0.951). Concerning herbs samples, the TRC values obtained with the proposed FRAP method correlated very well with values obtained using the 2,2’-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS•+) method (adjusted r2 = 0.975). It can be inferred from these results that other beverages derived from plants (e.g., beers, wines, and fruits juices) could also be analyzed with this new FRAP assay. In addition, the reducing capacity of 21 phenolic derivatives was determined with the proposed method in order to elucidate their structure-reactivity relationship. As expected, the phenolic derivative structure changes greatly the TRC values obtained with this proposed FRAP assay.
Highlights
The acronym for ferric reducing activity power (FRAP) was employed to designate the ferric reducing ability of plasma, an assay designed to measure the antioxidant power of this biological sample
This spectrophotometric test was developed based on the reduction reaction of FeIII to FeII in aqueous solution containing the 2,4,6-tripyridyl-s‐triazine (TPTZ) ligand, being the absorbance measurements of the FeII/TPTZ complex formed related to the reducing capacity of these biological samples.[1]
Bipy is partially protonated in aqueous solutions in pH < 4.0,9-11 and ferric hydroxo complexes (e.g., FeOH2+ and Fe(OH)2+) may be present in unbuffered solutions in pH > 3.5.22 in the present study, the pH was maintained at 4.6 with acetate buffer solution, which has been used with the same reduction reaction of FeIII to FeII in a solution containing bipy, in a recently proposed method for the quantification of the polyphenolic content in medicinal plants.[12]
Summary
The acronym for ferric reducing activity power (FRAP) was employed to designate the ferric reducing ability of plasma, an assay designed to measure the antioxidant power of this biological sample. This spectrophotometric test was developed based on the reduction reaction of FeIII to FeII in aqueous solution (pH 3.6; acetate buffer) containing the 2,4,6-tripyridyl-s‐triazine (TPTZ) ligand, being the absorbance measurements (at 593 nm) of the FeII/TPTZ complex formed related to the reducing capacity of these biological samples.[1]. These iron complexes, as far as we know, have not yet been applied to determine the reducing capacity in any sample of plant origin
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