Antioxidant and Antidiabetic Activities of Flavonoid Derivatives from the Outer Skins of <i>Allium cepa</i> L.

Abstract

The onion, known as the bulb onion or common onion, is not only a key ingredient in many tasty and healthy vegetarian meals but also many traditional medicines. Nine new flavonoids [cepaflavas A, B (<b>5</b>, <b>6</b>), cepadials A-D (<b>7</b>-<b>9</b> and <b>14</b>), and cepabiflas A-C (<b>10</b>-<b>12</b>)] and six known compounds (<b>1</b>-<b>4</b>, <b>13</b>, <b>15</b>) were obtained from the outer skins of <i>Allium cepa</i> L. Among them, compounds <b>5</b>, <b>6</b>, and <b>9</b> might be artificial products formed during extraction and isolation. New compounds were structurally elucidated using various spectroscopy/spectrometry techniques, including NMR and HRMS, and computational methods. Their absolute configurations were determined using time-dependent density functional theory calculations, combined with ECD spectroscopy, optical rotation calculation, and statistical procedures (CP3 and DP4 analysis). The free radical scavenging assays revealed that the new compounds <b>10</b>-<b>12</b> possessed considerable antioxidant activities with IC₅₀ values of 4.25-8.88 and 7.12-8.14 μM against DPPH and ABTS^•+, respectively. Compounds <b>13</b>-<b>15</b> showed substantial inhibitory activities against both α-glucosidase and protein tyrosine phosphatase 1B (PTP1B), with IC₅₀ values of 0.89-6.80 and 1.13-6.82 μM, respectively. On the basis of molecular docking studies, <b>13</b> and <b>15</b> were predicted to have high binding capacity and strong affinity toward the active site of PTP1B.