Metabolite profiling and free radical scavenging activity studies of alkaloids from Erythrina crista-galli twigs through in vitro and in silico analysis

Erythrina crista-galli is among 130 species of genus Erythrina which widely distributed in Indonesia, Australia, Argentina, Uruguay, Paraguay, and Brazil. Among the botanical parts used for the exploration of alkaloids from E. crista-galli (cockspur coral, ceibo, corticeira), the twig being one that rarely explored. This study aims to explore the alkaloid content from the twig of E. crista-galli as well as to evaluate their free radical scavenging activities through in vitro and in silico studies. The metabolite profile of the ethanol extract from Erythrina crista-galli twigs was characterized using Ultra-Performance Liquid Chromatography coupled with Tandem Mass Spectrometry (UPLC-MS/MS). The structures of the isolated alkaloids were determined through Nuclear Magnetic Resonance (NMR) and Mass Spectrometry (MS). Their free radical scavenging activities were evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging assay, while their electronic properties were assessed via Density Functional Theory (DFT). Twelve erythrina type alkaloids were tentatively identified from the twig of E. crista-galli. Four among these alkaloids, namely crystamidine (1), 8-oxoerythraline (2), erythrinine (3), erythraline (4) were isolated and their structures were confirmed through NMR and MS spectroscpy. Among the isolated alkaloids, erythraline (4) show highest free radical scavenging activity with IC₅₀ of 182.5 ± 5.3 µg/mL, whereas 8-oxoerythraline (1) show the lowest free radical scavenging activity with IC₅₀ of 868.2 ± 0.26 µg/mL. Structure-Activity relationship study shows that any modification in erythraline base skeleton, would decrease the antioxidant activity of erythraline, Furthermore, DFT calculation revealed that erythrinine (3), erythraline (4) tend to act as electron donors, whereas crystamidine (1), 8-oxoerythraline (2) tend to act as electron acceptors.