Antioxidative Indole Diketopiperazine Alkaloids from Endophytic Fungi Aspergillus sp. JXC-5

Three previously undescribed indole diketopiperazine alkaloids and seventeen known compounds were characterized by Aspergillus sp. JXC-5 by solid fermentation. Their structures were elucidated by spectroscopic methods and high-resolution electrospray ionization mass spectrometry, and the absolute configurations were further confirmed by electronic circular dichroism (ECD), induced CD spectra, and ML_J_DP4 methods. In the in vitro bioassays, compound 5 exhibits potent 2,2-diphenyl-1-picrylhydrazyl and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)·+ free radicals radical scavenging activities with half-maximal inhibitory concentration values of 19.54 ± 0.39 and 26.39 ± 0.09 µM, respectively. Compounds 10-12, 14, and 15 exhibited potent ABTS·+ free radicals radical scavenging activity.