Bioactive's Characterization, Biological Activities, and In Silico Studies of Red Onion (Allium cepa L.) Skin Extracts

Abstract

This study aimed to investigate the thermal stability and biological activities of the phytochemicals from the red onion skins extract, which are a rich source of anthocyanins. Eight anthocyanins were identified in the extract by high-performance liquid chromatography, the most abundant ones being cyanidin 3-O-laminaribioside and cyanidin 3-O-(6″-malonoyl-laminaribioside). The study also involved the assessment of the thermal degradation kinetics of anthocyanins and antioxidant activity in the 75-155 °C temperature range. The thermal degradation kinetics was described using the first-order kinetics model. In terms of thermal stability, increasing the temperature resulted in lower half-life values (t_1/2) and higher degradation rate constant values (k) for both anthocyanins and antioxidant activity. The thermodynamic parameters revealed that the phytochemicals' degradation is a non-spontaneous and endothermic reaction. Furthermore, the inhibitory effect of the extract was investigated against the enzymes affiliated with metabolic syndrome, oxidative stress, and inflammatory process diseases. Thus, we also demonstrated that the red onion skins extract exerted inhibitory activity on α-glucosidase, α-amylase, lipase, and lipoxygenase. Considering the high content of bioactives and various biological properties, the red onion skins extract is suitable for multiple applications.

Keywords: anthocyanins; antioxidant activity; biological activity; molecular modeling; red onion skins; thermal stability.

Figure 1

Chromatographic profile of the anthocyanins from the red onion skins extract obtained by ultrasounds-assisted extraction: Peak (1) cyanidin 3-O-glucoside; Peak (2) cyanidin 3-O-laminaribioside; Peak (3) cyanidin 3-O-(3″-malonylglucoside); Peak (4) peonidin 3-O-glucoside; Peak (5) cyanidin 3-O-(6″-malonylglucoside); Peak (6) cyanidin 3-O-(6″-malonyl-laminaribioside); Peak (7) peonidin 3-O-malonylglucoside and Peak (8) cyanidin 3-dimalonylaminaribioside.

Figure 2

Thermal degradation kinetics of TAC (a) and antioxidant activity (b) of red onion skin extract that was treated at different temperatures ( 75 °C, 95 °C, 115 °C, 135 °C, 155 °C); C₀ and C are the TAC or antioxidant activity before and after thermal treatment (mg/g dw).

Figure 3

The results of the molecular docking tests showing the complexes formed by α-amylase (a) α-glucosidase (b), lipase (c), and LOX (d) shown in silver, with cyanidin 3-O-laminaribioside and/or cyanidin 3-O-(6″-malonyl-laminaribioside) represented in Licorice style in blue and red, respectively. The catalytic amino acids establishing contacts with the ligands are represented in purple in Van der Waals style. Images were prepared using VMD software [61].