High-speed counter-current chromatographic separation and thermodynamic mechanism of an antioxidant from Morus alba leaves

Morus alba leaves have a long history as a plant medicine and possess many biological activities. Antioxidant activity is the basis of its many biological activities. In order to explore the antioxidant effect of Morus alba leaves, antioxidant and its mechanism of scavenging free radicals have been investigated in this study. As results, rutin with the EC 50 of 27.94 ± 1.18 μg/mL and 25.86 ± 0.09 μg/mL to DPPH· and ABTS· + respectively was enriched by liquid-liquid extraction, and isolated by high-speed counter-current chromatography and identified by NMR, and was recognized as the main antioxidant in Morus alba leaves. The results of reaction thermodynamics and frontier molecular orbital theory indicated that 4′-OH in rutin was the most active group, which scavenged free radicals by hydrogen atom transfer in non-polar media and single electron transfer followed by proton transfer in polar media. Polar solvents were more helpful for rutin to exert its antioxidant effect. This study not only explored the antioxidant basis of Morus alba leaves, but also revealed its mechanism of scavenging free radicals.