Multivariate curve resolution - alternating least squares (MCR-ALS) has been applied to data collected from UV/Vis spectrophotometric analysis of the autoxidation process of pyrogallol in weakly alkaline aqueous solutions. The MCR-ALS analysis was able to explain the autoxidation kinetics of pyrogallol at pH 7.4 and 8.0, allowing deduction of the pure spectra and concentration changes of different species present throughout the entire process. The autoxidation process at pH 7.4 was found to follow a first-order reaction model, with formation of purpurogallin as the sole and terminal product. Changing the pH to 8.0 not only accelerated autoxidation of pyrogallol to purpurogallin but also introduced a further autoxidation of purpurogallin. At pH 8.0 the process fits a model of two consecutive first-order reactions. The first step is formation of purpurogallin, which reacts in a further autoxidation to form a yellow colored substance, most probably purpurogallin polymer. [...]
The detection of antioxidant activity in plant extracts or in pure compounds can be performed by a large number of methods with different reaction mechanisms, however, the criteria for choosing comparative standards are still not consensual. Thus, the present work intends to compare the antioxidant efficiency of nine substances, namely, gallic acid (GA), pyrogallol (PyG), propyl gallate (nPG), tannic acid (TA), quercetin (Qtn), rutin (Rut), ascorbic acid (Asc), Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) and butyl hydroxytoluene (BHT) using methods, (1) Ferric Reducing Antioxidant Power (FRAP) Assay, (2) Ferric Reducing Power (FRP), (3) Ferric-Ferrozine Antioxidant Capacity (FFAC), (4) Total Phosphomolybdenum Antioxidant Capacity (TAC) and (5) Radical Cation Elimination Assay 2,2' Azinobis(3 Ethylbenzothiazoline 6 Sulfonic Acid) (ABTS). Antioxidant efficacy by the 1,1-diphenyl-2-picrylhydrazine free radical scavenging method was previously described in a preliminary study. The results show that the maximum effectiveness was exhibited by PyG in the ABTS (0.425 ± 0.005 µM) and TAC (0.872 ± 0.075 µM) methods, Qtn in the FRP (5.776 ± 0.020 µM) and FFAC (20.390 ± 0.291 µM) methods and GA in the FRAP (6.765 ± 0.086 µM) and DPPH (1.105 ± 0.003 µM) methods. The results found in this study reveal that the effectiveness of a standard depends on the method applied, and the antioxidant activity of the same standard may present differences between the methods, which suggests that the selection of a comparative standard for the antioxidant activity tests of the extracts of plants or functional foods must be made according to the method to be applied.
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