The Radical Scavenging Activity of Kinetin: Mechanisms, Kinetics, and the Effects of pH and Solvent Polarity

Abstract Kinetin (N6-furfuryladenine, KNT) is a naturally occurring cytokinin that has been widely used as an active ingredient in anti-aging skin care products because of its antioxidant and cytoprotective properties; however, its direct radical scavenging activity, particularly toward hydroxyl (HO•) and hydroperoxyl (HOO•) radicals remains poorly understood. Herein, a combined experimental and quantum chemical investigation was performed to elucidate the radical scavenging activity of KNT. The calculated overall rate constant for the neutral KNT species, which predominates at pH 7, was 9.09 × 109 M–1 s–1, in good agreement with the experimental value of (4.95 ± 0.08) × 109 M–1 s–1, supporting the reliability of the computational approach. The HO•-scavenging activity of KNT was found to be strongly dependent on pH and temperature, with alkaline conditions significantly enhancing the overall reaction rate because of the increased fraction of reactive deprotonated species. In both aqueous and lipid media, KNT exhibited rapid reactions with HO• radicals through the radical adduct formation (RAF) and formal hydrogen transfer (FHT) mechanisms. Under physiological conditions the HO•-scavenging activity of KNT was comparable to that of well-established antioxidants, including syringic acid, gallic acid, melatonin, edaravone, caffeic acid, rosmarinic acid, and Trolox, whereas its HOO•-scavenging activity was moderate. These findings provide new mechanistic insights into the HO•/HOO• radical scavenging activity of KNT and highlight its potential as an efficient hydroxyl radical scavenger.