Antioxidant potential of humic substances measured by Folin-Ciocalteu, CUPRAC, QUENCHER-CUPRAC and ESR methods
Attila Csicsor, Etelka Tombácz, Petra Kulcsár
Abstract
An increasing proportion of the population in the Western world suffers from one of the oxidative stress related diseases. Although the radical-scavenging and therapeutic potential of humic substances (HSs) are known, little is known about their antioxidant properties and structure-property relationships. The aim of the research is to reveal the antioxidant potential of different HS samples and to compare them with already well-known antioxidants like gallic acid and Trolox. We determined the total phenolic content (TPC) of the samples using Folin-Ciocalteu (FC) method because it is generally associated with the antioxidant effect. We also applied the CUPRAC (CUPric Reducing Antioxidant Capacity) and QUENCHER-CUPRAC methods to achieve a comprehensive picture about the antioxidant potential of HSs. Furthermore, we tested the HS samples using electron spin resonance (ESR) method as a new tool to characterize the antioxidant properties. According to the results of FC method given in gallic acid equivalent antioxidant capacity (GAEAC) the HA fractions contain the most phenolic moieties (300–470 mg GAE/g) which is followed by the HY (150–175 mg GAE/g) and then FA (80–155 mg GAE/g). The antioxidant capacities of HS samples were related to Trolox (Trolox equivalent antioxidant capacity (TEAC)) in addition to gallic acid (GAEAC), namely 5–35 µmol GAE/g, 50–310 µmol TE/g and 12–42 µmol GAE, 175–530 µmol TE /g obtained by CUPRAC and QUENCHER-CUPRAC methods, respectively. The ESR results supported that the tested HS samples have antioxidant properties. Moreover, we compared the results of our fractions with the standard samples of the IHSS, in terms of origin and extraction methods. We could conclude that, in addition to the source of raw materials for the fractions, the method of their extraction also greatly influences their antioxidant effect.