A kinetic-based stopped-flow DPPH center dot method

The reaction kinetics of antioxidants with free radicals is crucial to screen their functionality. However, studying antioxidant-radical interactions is very challenging for fast electron-donor substances, such as ascorbic acid, because the reaction ends in a few seconds. Accordingly, this work proposes a rapid and sensitive method for the determination of the absolute rate constant of the reaction between fast antioxidants and DPPH center dot. The method consists of a stopped-flow spectrophotometric system, which monitors the decay of DPPH center dot during its interaction with antioxidants. A kinetic-based reaction mechanism fits the experimental data. Kinetic parameters include a second order kinetics (k(1)) and, depending on the type of antioxidant, a side reaction (k(2)). Ascorbic acid was the fastest antioxidant (k(1) = 21,100 +/- 570 M-1 s(-1)) in comparison with other eleven phenols, showing k(1) values from 45 to 3070 M-1 s(-1). Compounds like catechin, epicatechin, quercetin, rutin, and tannic, ellagic and syringic acids presented a side reaction (k(2) from 15 to 60 M-1 s(-1)). Among seven fruit juices, strawberry was the fastest, while red plum the slowest. Overall, the proposed kinetic-based DPPH center dot method is simple, rapid, and suitable for studying the activity and capacity of different molecules, and food samples rich in fast antioxidants, like fruit juices.

Automated summary

This study proposes a rapid and sensitive method for determining the absolute rate constant of the reaction between fast antioxidants and DPPH center dot. The method involves a stopped-flow spectrophotometric system that monitors the decay of DPPH center dot during its interaction with antioxidants. Kinetic parameters including second order kinetics and side reactions were determined for various antioxidants. Overall, this method is simple, rapid, and suitable for studying the activity and capacity of different molecules, as well as food samples rich in fast antioxidants like fruit juices.