Abstract
Guided by a probe-based molecular networking strategy, five undescribed cycloheptapeptides, phakefusins A−E (1−5), were isolated from the marine sponge Phakellia fusca. Compounds 1 and 2 contain the nonproteinogenic amino acid residues of dioxindolyalanine (Dioia) and β-3-oxindolylalanine (Oia), respectively. Compound 3 possesses a unique methionine sulfoxide, whereas compound 5 includes a glutamic acid ethyl ester unit. Their structures were elucidated through NMR spectroscopy, HR-MS/MS analysis, and the advanced Marfey's method. By synthesizing the (S, S/R)-Oia standard through tryptophan oxidation, we determined the configuration of this amino acid in compound 2 using the advanced Marfey's method. These cycloheptapeptides were evaluated for their antitumor, antibacterial, and antioxidant activities. Compound 1 showed moderate cytotoxicity against MCF-7 and PC9 cells, with IC50 values of 6.8 and 9.6 μM, respectively, while compounds 2−5 demonstrated potential antioxidant effects by upregulating HO-1, NQO1, and SOD2 levels, as well as inducing Nrf2 activation.
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