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 Activity Power, FRAP）法。该方法的原理为：在水溶液中，含2,2'-联吡啶（2,2’-bipyridine, bipy）的抗氧化化合物可将三价铁（FeIII）还原为二价铁（FeII）。通过记录所生成的Fe(bipy)₃²+络合物在521 nm处的特征吸光度值，即可测定部分植物源饮品的总还原能力。针对茶叶样品，本方法与铜还原抗氧化能力（Cupric Reducing Antioxidant Capacity, CUPRAC）试剂测得的总还原能力值具有极佳的一致性（校正相关系数（r²）=0.951）。对于草药样品，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（2,2’-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid, 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）同样可采用该新型FRAP检测法进行分析。此外，本研究采用所提方法测定了21种酚类衍生物的还原能力，以阐明其结构-反应性关系。正如预期，酚类衍生物的结构对本FRAP法所得总还原能力值具有显著影响。