In vitro evaluation of antioxidant, α-glucosidase inhibitor, and antibacterial activities of frangipani flower and the principal component analysis of its constituents

Frangipani flower (Plumeria rubra) plants are commonly found and grown in tropical and subtropical regions. The present study explored the antioxidant, α-glucosidase inhibitor, and antibacterial activities of Plumeria rubra flower extracts. P. rubra flower was extracted by maceration using hexane, ethyl acetate, methanol, and ethanol. The capability of P. rubra flower extract to scavenge radicals was evaluated using a β-carotene bleaching assay and DPPH (2,2-diphenyl-1-picryl-hydrazyl) assay. The total phenolic content (TPC) was assessed using the Folin-Ciocalteau method. Its antibacterial activity was assessed using the disc diffusion method against Salmonella typhimurium and Escherichia coli. The α-glucosidase inhibitor assay was conducted using α-glucosidase from Saccharomyces cerevisiae. The results revealed that the ethanolic 50 extract of P. rubra flower showed the highest antioxidant activity of 66.49 at 1000 µg/mL in the β-carotene bleaching assay, and the ethanolic 70 extract had the highest antioxidant activity with an IC50 of 93.2 µg/mL in DPPH assay. Meanwhile, for α-glucosidase inhibitory activity, the methanolic extract showed the highest activity of 94.05 at 1000 µg/mL. These results were in accordance with the total phenolic content (TPC) of the extract, in which the highest TPC was obtained from 50 ethanol extract (1405.33 mg gallic acid equivalent (GAE)/g). P. rubra extracts at 5 mg/mL showed good antibacterial activity against E. coli and moderate activity against S. typhimurium with inhibition zones of 8.14 mm and 6.88 mm, respectively. Moreover, the liquid chromatography-high-resolution mass spectrometry (LC-HRMS) analysis suggested that several phenolic compounds were presented in the P. rubra flower extracts. The constituents of the P. rubra flower were analyzed using principal component analysis (PCA). This study revealed that P. rubra flower extract could be used as a natural antidiabetic, antibacterial, and antioxidant agent in food additives, functional food industries, and pharmaceutical industries. © 2023 Elsevier Ltd