Evaluation of the antioxidant activity and capacity of some natural N6-substituted adenine derivatives (cytokinins) by fluorimetric and spectrophotometric assays

•Cytokinins are N6-derivatives of adenine endowed with hormonal activity in plants.•Cytokinins show biological activities related to antioxidant properties.•The antioxidant activity of four cytokinins has been evaluated by different assays.•Each cytokinin shows a specific antioxidant activity, depending on N6-substitution.•The antioxidant activity can contribute to the biological role of cytokinins. Four natural N6-substituted adenine derivatives (cytokinins) were evaluated for the first time in vitro for they antioxidant capacity by using fluorimetric and spectrophotometric assays, i.e., the oxygen radical absorbance capacity (ORAC), trolox equivalence antioxidant capacity (TEAC) and the 2-deoxyribose degradation (2-DRA) assays. The results from the TEAC assay show that only N6-(4-hydroxybenzyl)adenine (p-topolin) shows an electron transfer capacity due to the presence of a phenolic moiety in the N6-position. The results from the ORAC test show that the antioxidant activity of N6-furfuryladenine (kinetin, K) is the highest up to a concentration of 1μM, whereas at concentrations higher than 1μM p-topolin is the most efficient antioxidant. Analysis of the kinetic data suggests that, compared to the other cytokinins, more sites of the molecular structure of p-topolin are available for the quenching of peroxyl radicals. The hydroxyl radical scavenger ability, as measured by the 2-DRA assay, showed that all tested cytokinins react in this test and that N6-(Δ2-isopentenyl)adenine is slightly more potent, probably because of the allylic methylene group present in the N6-isopentenyl moiety. Our data suggest that a part of the biological activity of the evaluated cytokinins is likely to be related to an intrinsic antioxidant capacity.