A Procedure for Assessment of the Reducing Capacity of Plants-Derived Beverages Based on the Formation of the FeII/2,2’-Bipyridine Complex

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.本研究开发了一种用于定量总还原能力（Total Reducing Capacity, TRC）的替代分光光度法——铁还原抗氧化能力（Ferric Reducing Antioxidant Power, FRAP）法。该方法基于水溶液中含2,2'-联吡啶（2,2’-bipyridine, bipy）的抗氧化化合物将三价铁（FeIII）还原为二价铁（FeII）的反应。以生成的特征络合物三(2,2'-联吡啶)合铁(II)（Fe(bipy)₃²⁺）在521 nm波长处的吸光度值，测定若干植物源饮品的总还原能力。对于茶叶样品，本方法所得总还原能力值与铜还原抗氧化能力（CUPRAC）试剂测得的结果具有极佳的一致性（校正相关系数（r²）= 0.951）。针对草药样品，本FRAP法所得总还原能力值与采用2,2'-联氮-双-3-乙基苯并噻唑啉-6-磺酸自由基（ABTS•+）法测得的结果相关性极佳（校正r² = 0.975）。由上述结果可推知，其他植物源饮品（如啤酒、葡萄酒及果汁）也可采用该新型FRAP分析法进行检测。此外，本研究还采用该方法测定了21种酚类衍生物的还原能力，以阐明其结构-反应活性关系。正如预期，酚类衍生物的结构对本FRAP法所得的总还原能力值具有显著影响。